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Grounding, etc 1 https://www.youtube.com/watch?v=awrUxv7B-a8 Single Phase Motors 2 The 10 Worst Grounding Mistakes You'll Ever Make Aug 1, 2008David Herres | Electrical Construction and Maintenance Why common errors in residential, commercial, and industrial wiring can lead to fire and electric shock hazards. Proper grounding and bonding prevent unwanted voltage on non-currentcarrying metal objects, such as tool and appliance casings, raceways, and enclosures, as well as facilitate the correct operation of overcurrent devices. But beware of wiring everything to a ground rod and considering the job well done. There are certain subtleties you must follow to adhere to applicable NEC rules and provide safe installations to the public and working personnel. Although ground theory is a vast subject, on which whole volumes have been written, let's take a look at some of the most common grounding errors you may run into on a daily basis. 3 1. Improper replacement of non-grounding receptacles. Dwellings and nondwellings often contain non-grounding receptacles (Photo 1). It's not the NEC's intent to immediately replace all noncompliant equipment with each new edition of the Code. In fact, it's perfectly fine to leave the old “two prongers” in place. But because an intact functioning equipment ground is such an obvious safety feature, most electricians tend to replace these old relics whenever possible. Photo 1. This non-grounding receptacle is typical of those found in older homes across the country. There are several ways you can complete this upgrade, many of which are erroneous and strictly against the Code. For example, never apply the following non-NEC-compliant solutions: 4 Hook up a new grounding receptacle on the theory that this is a step in the right direction. This can lead future electricians and occupants to believe they are fully protected by a non-functioning ground receptacle. Connect the green grounding terminal of a grounded receptacle via a short jumper to the grounded neutral conductor. This practice is totally noncompliant and dangerous because when a load is connected, voltage will appear on both the neutral and ground wires. Therefore, any noncurrent-carrying appliance or tool case will become energized, causing shock to the user, who is typically partially or totally grounded. Run an individual ground conductor from the green grounding terminal of a grounded receptacle to the nearest water pipe or other grounded object. This “floating ground” presents various hazards. It is likely that this ground rod of convenience will have several ohms of ground resistance so that, in case of ground fault within a connected tool or appliance, the breaker will not trip — and exposed metal will remain energized. Run an individual ground conductor back to the entrance panel and connect it to the neutral bar or grounding strip. This solution is somewhat better, but still noncompliant. Any grounding conductor must be within the circuit cable or raceway. One objection is that an individual conductor could be damaged or removed in the course of work taking place in the future. 5 What are the correct ways to handle this type of situation, when you find yourself working with non-grounded receptacles? The best approach is to run a new branch circuit back to the panel, verifying presence of a valid ground. Because this procedure usually involves fishing cable behind walls or, in some cases, removing and then replacing wall finish, it's not always feasible unless a total rewiring job is being performed. Another possibility is to replace the two-prong receptacle with a GFCI. Hook up the two wires and leave the grounding terminal unattached. Included with the GFCI is a sticker that says, “No equipment ground.” This sticker must be in place so that future electricians and users are not misled. The thinking behind this strategy is that even though the tool or appliance case is not grounded, the GFCI will provide enhanced safety. It's important to note that a GFCI functions properly without the presence of a grounding conductor. The device compares current flowing through the hot and neutral conductors and trips if a difference of more than 5 milliamps is detected. Non-grounding receptacles are still manufactured. If replacement is necessary (and acquiring a ground is not feasible), installation of a new non-grounding receptacle is a way to go. 6 2. Installation of a satellite dish, telephone, CATV, or other low-voltage equipment without proper grounding. If you look at a number of satellite dish installations in your neighborhood, a certain percentage will inevitably not be grounded at all. Of those that are grounded, there is still a high probability many are not fully compliant. For example, the grounding electrode conductor could be too long, too small, have unlisted clamps at terminations, have excess bends, or be connected to a single ground rod but not be bonded to other system grounds. For NEC purposes, a satellite dish is an antenna, and installation requirements are found in Chapter 8, Communications Systems. Article 810, Radio and Television Equipment, details the installation requirements. Part II deals with receiving Equipment — Antenna Systems. This type of equipment, which includes the satellite dish, must have a listed antenna discharge unit, which can be either outside the building or inside between the point of entrance of the lead-in conductors and the receiver — and as near as possible to the entrance of the conductors to the building. The antenna discharge unit is not to be located near combustible material and certainly not within a hazardous (classified) location. The antenna discharge unit must be grounded. The grounding conductor is usually copper; however, you can use aluminum or copper-clad aluminum if it's not in contact with masonry or earth. Outside, aluminum or copper-clad aluminum cannot be within 18 inches of the earth. 7 Photo 2. Grounding means for a satellite dish must be located at the point of entrance to the building. In this particular installation, the grounding conductor is integral with the coax from the dish, but the installer did not bond it to other system grounds. 8 The grounding conductor can be bare or insulated, stranded or solid, and must be securely fastened in place and run in a straight line from the discharge unit to the grounding electrode (Photo 2). If the building has an intersystem bonding termination, the grounding conductor is to be connected to it or to one of the following: Grounding electrode system. Grounded interior metal water piping system within 5 feet of point of entrance to the building. Power service accessible grounding means external to the building. Metallic power service raceway. Service equipment enclosure. Grounding electrode conductor or its metal enclosure. 9 If this grounding conductor is installed within a metal raceway, you must bond the metal raceway to it at both ends. For this reason, if raceway is deemed necessary for extra protection, UL-listed PVC (rigid non-metallic conduit) is generally used. The grounding conductor must be no smaller than 10 AWG copper. Where separate electrodes are used, you must connect the antenna discharge unit grounding means to the premises power system grounding system by a 6 AWG copper conductor. Needless to say, grounding a satellite dish goes well beyond simply driving a ground rod at the point of entrance. Grounding for CATV is slightly different. Typically, CATV is brought into the building via coaxial cable, which has a center conductor, insulating spacer, and outer electrical shield. Because of the spacer, capacitive coupling is diminished so that the cable provides a high-quality signal for data, voice, and video transmission. Improper grounding of coaxial cable used for CATV is very common. There is no antenna discharge unit as required for satellite dish installation. Instead, the shield of the coaxial cable is connected to an insulated grounding conductor that is limited to copper but may be stranded or solid. The grounding conductor is 14 AWG minimum so that it has current-carrying capacity approximately equal to the outer shield of the coaxial cable. 10 The major distinguishing characteristic is that for one- and two-family homes the grounding conductor cannot exceed 20 feet in length and should preferably be shorter. If a grounding electrode such as the Intersystem Bonding Termination is not within 20 feet, it is necessary to drive a ground rod for that purpose. However, even after this dedicated grounding means is established, in order to be NEC-compliant, the installation must have a bonding jumper not smaller than 6 AWG or equivalent, which is connected between the CATV system's grounding electrode and the power grounding electrode system for the building. Omitting this jumper is a serious Code violation, second only to no grounding at all. You must bond all system grounds, antenna, power, CATV, telephone, and so on with a heavy bonding jumper. 3. Non-installation of GFCIs where required. Recent Code editions have mandated increased use of GFCIs. In dwelling units, GFCIs are required on all 125V, single-phase, 15A and 20A receptacles in: bathrooms; garages; accessory buildings with a floor at or below grade level not intended as a habitable room, limited to storage, work and similar areas; outdoors; kitchens along countertops; within 6 feet of outside edge of laundry, utility, and wet bar sinks; and boathouses. In other than dwelling units, GFCIs are required on all 125V, single-phase, 15A and 20A receptacles in bathrooms, kitchens, rooftops, outdoors, and within 6 feet of the outside edge of sinks. 11 Other areas requiring the use of GFCIs include: boat hoists, aircraft hangars, drinking fountains, cord- and plug-connected vending machines, high-pressure spray washers, hydromassage bathtubs, carnivals, circuses, fairs (and the like), electrically operated pool covers, portable or mobile electric signs, electrified truck parking space supply equipment, elevators, dumbwaiters, escalators, moving walks, platform lifts/stairway chairlifts, fixed electric space heating cables, fountains, commercial garages, electrical equipment for naturally and artificially made bodies of water, pipeline heating, therapeutic pools and tubs, boathouses, construction sites, health-care facilities, marinas/boatyards, pools, recreational vehicles, sensitive electronic equipment, spas, and hot tubs. 4. Improperly connecting the equipment-grounding conductor to the system neutral. You must connect a grounded neutral conductor to normally noncurrent-carrying metal parts of equipment, raceways, and enclosures only through the main bonding jumper (or, in the case of a separately derived system, through a system bonding jumper). Make this connection at the service disconnecting means, not downstream. When you buy a new entrance panel, a screw or other main bonding jumper is usually included in the packaging. 12 Attached to it are instructions stipulating that it is to be installed only when the panel is to be used as service equipment. It's a major error to install a main bonding jumper in a box used as a subpanel fed by a 4-wire feeder. It's also wrong not to install it when the panel is used as service equipment. Improper redundant connection of grounded neutral to equipment-grounding conductors can result in objectionable circulating current and presence of voltage on metal tool or appliance casings. You should connect grounded neutral and equipment-grounding conductors at the service disconnect. Then separate them — never to rejoin again. Additional optional ground rods may be connected anywhere along the equipment-grounding conductor but never to the grounded neutral. 5. Improperly grounding frames of electric ranges and clothes dryers. Prior to the 1996 version of the NEC, it was common practice to use the neutral as an equipment ground. Now, however, all frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlet or junction boxes that are part of these circuits must be grounded by a fourth wire: the equipment-grounding conductor. An exception permits retention of the pre-1996 arrangement for existing branch-circuit installations only where an equipment-grounding conductor is not present. Several other conditions must be met. If possible, the best course of action is to run a new 4-wire branch circuit from the panel. If you must keep an old appliance, be sure to remove the neutral to frame bonding jumper if an equipment-grounding conductor is to be connected. 13 6. Failure to ground submersible well pumps. At one time, submersible well pumps were not required to be grounded because they were not considered accessible. However, it was noted that workers would pull the pump, lay it on the ground, and energize it to see if it would spin. If, due to a wiring fault, the case became live, the overcurrent device would not function, causing a shock hazard. The 2008 NEC requires a fourth equipment-grounding conductor that you must now lug to the top of the well casing. Many people assume that in a 3-wire submersible pump system one wire is a “ground.” In actuality, submersible pump cable consists of three wires (plus equipment-grounding conductor) twisted together and unjacketed. Yellow is a common 240V leg, black is run, and red is start, which the control box energizes for a short period of time. Prior to the new grounding requirement, everything was hot. 7. Failure to properly attach the ground wire to electrical devices. Wiring daisy-chained devices in such a way that removing one of them breaks the equipment grounding continuity is a common problem. The preferred way to ground a wiring device is to connect incoming and outgoing equipmentgrounding conductors to a short bare or green jumper. The bare or green insulated jumper is then connected to the grounding terminal of the device. 14 8. Failure to install a second ground rod where required. A single ground rod that does not have a resistance to ground of 25 ohms or less must be augmented by a second ground rod. Once the second ground rod is installed, it's not necessary for the two to meet the resistance requirement. As a practical matter, few electricians do the resistance measurement. Figure. Non-overlapping effective resistance areas reduce net resistance. You cannot use a simple ohmmeter because that would require a known perfect ground. Special equipment and procedures are needed, so it's common practice to simply drive a second ground rod. You must locate them at least 6 feet apart. Greater distance is even better (Figure). If both rods and the bare ground electrode conductor connecting them are directly under the drip line of the roof, ground resistance will be further diminished. This is because the soil along this line is more moist. Ground resistance 15 greatly increases when soil becomes dry. 9. Failure to properly reattach metal raceway that is used as an equipmentgrounding conductor. When equipment is relocated, replaced, or removed for repair, many times equipment ground paths are broken. If these connections are not fixed, there's an accident waiting to happen (Photo 3). Setscrews, locknuts, and threads should be fully engaged and continuity tests performed before equipment is put back into service. Dirt and corrosion can also compromise ground continuity. Photo 3. Standard locknuts or bushings shall not be the sole connection for grounding purposes. NEC Article 250.4 requires that electrical equipment, wiring, and other electrically conductive material likely to become energized shall be installed in a manner that creates a low-impedance circuit from any point on the wiring system to the electrical supply source to facilitate the operation of overcurrent devices. 16 10. Failure to bond equipment ground to water pipe. Improper connections are often seen in the field. Screw clamps and other improvised connections do not provide permanent low impedance bonding. The worst method would be to just wrap the wire around the pipe or to omit this bonding altogether. Photo 4. Someone used a water pipe clamp to improperly connect a ground wire to this ground rod. In a dwelling, a conductor must be run to metallic water pipe, if present, and connected with a UL-listed pipe grounding clamp (Photo 4). This bonding conductor is to be sized according to Table 250.66, based on the size of the largest ungrounded service entrance conductor or equivalent area for parallel conductors. Herres is a licensed master electrician in Stewartstown, N.H. 17 Motor Calculations: Motors and Branch-Circuit Conductors Overcurrent and short-circuit protection aren’t the same for motors 18 The best method for providing overcurrent protection for most circuits is to use a circuit breaker that combines overcurrent protection with short-circuit and ground-fault protection. However, this isn't usually the best choice for motors. With rare exceptions, the best method for providing overcurrent protection in these cases is to separate the overload protection devices from the short-circuit The best method for providing overcurrent protection for most circuits is to use a circuit breaker that combines overcurrent protection with short-circuit and ground-fault protection. However, this isn't usually the best choice for motors. With rare exceptions, the best method for providing overcurrent protection in these cases is to separate the overload protection devices from the short-circuit and ground-fault protection devices (Fig. 1). Motor overload protection devices like heaters protect the motor, the motor control equipment, and the branch-circuit conductors from motor overload and the resultant excessive heating (430.31). They don't provide protection against short-circuits or ground-fault currents. That's the job of the branch and feeder breakers, which don't provide motor overload protection. This arrangement makes motor calculations different from those used for other types of loads. Let's look at how to apply Art. 430, starting at the motor. 19 Overload protection. Motor overload devices are often integrated into the motor starter. But you can use a separate overload device like a dual-element fuse, which is usually located near the motor starter, not the supply breaker. Fig. 1. Overcurrent protection is generally accomplished by separating the overload protection from the short-circuit and ground-fault protection device. If you use fuses, you must provide one for each ungrounded conductor (430.36 and 430.55). Thus, a 3-phase motor requires three fuses. Keep in mind that these devices are at the load end of the branch circuit and that they don't 20 provide short-circuit or ground-fault protection. Motors rated more than 1 hp without integral thermal protection and motors rated 1 hp or less that are automatically started [430.32(C)] must have an overload device sized per the motor nameplate current rating [430.6(A)]. You must size the overload devices no larger than the requirements of 430.32. Motors with a nameplate service factor (SF) rating of 1.15 or more must have an overload protection device sized no more than 125% of the motor nameplate current rating. Fig. 2. When working with motors that have a service factor rating of 1.15 or higher, size overload protection devices no more than 125% of the motor nameplate rating. 21 Let's look at Fig. 2 and work through a sample calculation. Example No. 1: Suppose you use a dual-element fuse for overload protection. What size fuse do you need for a 5-hp, 230V, single-phase motor with a service factor of 1.16 if the motor nameplate current rating is 28A? (a) 25A (c) 35A (b) 30A (d) 40A The overload protection shall be sized according to the motor nameplate current rating [430.6(A), 430.32(A)(1), and 430.55]. You also have to consider another factor: nameplate temperature rise. For motors with a nameplate temperature rise rating not over 40°C, size the overload protection device no more than 125% of the motor nameplate current rating. Thus, 28A×1.25=35A [240.6(A)] 22 Fig. 3. Size the overload protection device of a motor with a nameplate temperature rise rating of 40°C or less at no more than 125% of the motor nameplate current rating. 23 Let's look at Fig. 3 and work through another example problem. Example No. 2: Again, suppose you're using a dual-element fuse for the overload protection. What size fuse do you need for a 50-hp, 460V, 3-phase motor that has a temperature rise of 39°C and motor nameplate current rating of 60A (FLA)? (a) 40A (c) 60A (b) 50A (d) 70A The overload protection is sized per the motor nameplate current rating, not the motor full load current (FLC) rating. Thus, 60A×1.25=75A. Overload protection shall not exceed 75A, so you need to use a 70A dual-element fuse [240.6(A) and 430.32(A)(1)]. Motors that don't have a service factor rating of 1.15 or higher or a temperature rise rating of 40°C and less must have an overload protection device sized at not more than 115% of the motor nameplate ampere rating (430.37). 24 Fig. 4. Refer to Table 310.16 when selecting the proper size conductor to serve a single motor. 25 Sizing branch-circuit conductors. Branch-circuit conductors that serve a single motor must have an ampacity of not less than 125% of the motor's FLC as listed in Tables 430.147 through 430.150 [430.6(A)]. You must select the conductor size from Table 310.16 according to the terminal temperature rating (60°C or 75°C) of the equipment [110.14(C)]. Let's reinforce this concept by working through a sample calculation. Refer to Fig. 4. Example No. 3: What size THHN conductor do you need for a 2-hp, 230V, singlephase motor? (a) 14 AWG (c) 10 AWG (b) 12 AWG (d) 8 AWG Let's walk through the solution: Step 1: Conductor sized no less than 125% of motor FLC Step 2: Table 430.148 shows the FLC of 2-hp, 230V, single-phase as 12A Step 3: 12A × 1.25 = 15A Step 4: Per Table 310.16, you need to use 14 AWG THHN rated 20A at 60°C 26 The minimum size conductor the NEC permits for building wiring is 14 AWG [310.5]. However, local codes and many industrial facilities have requirements that 12 AWG be used as the smallest branch-circuit wire. So in this example you might need to use 12 AWG instead of 14 AWG. Fig. 5. Short-circuit and ground-fault protection devices are designed for fast current rise, short-duration events. On the other hand, overload protection devices 27 are designed for slow current rate, long-duration situations. Branch-circuit protection for short-circuits and ground-faults. Branch-circuit short-circuit and ground-fault protection devices protect the motor, motor control apparatus, and conductors against short circuits or ground faults. They don't protect against an overload (430.51) (Fig. 5). The short-circuit and ground-fault protection device required for motor circuits isn't the type required for personnel (210.8), feeders (215.9 and 240.13), services (230.95), or temporary wiring for receptacles (527.6). Per 430.52(C), you must size the short-circuit and ground-fault protection for the motor branch circuit — except those that serve torque motors — so they're no greater than the percentages listed in Table 430.52. When the short-circuit and ground-fault protection device value that you find in Table 430.52 doesn't correspond to the standard rating or setting of overcurrent protection devices as listed in 240.6(A), use the next higher protection device size [430.52(C)(1) Ex. 1]. 28 Did that statement stop you? Does it strike you as incorrect? That's a common response, but remember, motors are different than other system components. Motor overload protection devices, such as heaters and fuses, protect the motor and other items from overload. The short-circuit and ground-fault protection doesn't need to perform this function. Therefore, oversizing won't compromise protection. Undersizing will prevent the motor from starting. Use the following two-step process to determine what percentage from Table 430.52 you should use to size the motor branch-circuit short-circuit groundfault protection device. Step 1: Locate the motor type on Table 430.52. Step 2: Select the percentage from Table 430.52 according to the type of protection device, such as non-time delay (one-time), dual-element fuse, or inverse-time circuit breaker. Don't forget to use the next higher protection device size when necessary. Let's see if you have this concept down with a short quiz. Of the following statements, which one is true? Use Table 430.52 to look up the numbers. 29 1. The branch-circuit short-circuit protection (non-time delay fuse) for a 3-hp, 115V, single-phase motor shall not exceed 110A. 2. The branch-circuit short-circuit protection (dual-element fuse) for a 5-hp, 230V, single-phase motor shall not exceed 50A. 3. The branch-circuit short-circuit protection (inverse-time breaker) for a 25hp, 460V, 3-phase synchronous motor shall not exceed 70A. Let's address each question individually. We'll be referring to 430.53(C)(1) Ex. 1 and Table 430.52. 1. Per Table 430.148, 34A×3.00=102A. The next size up is 110A. So this is true. 2. Per Table 430.148, 28A×1.75=49A. The next size up is 50A. So, this is also true. 3. Per Table 430.150, 26A×2.50=65A. The next size up is 70A. This is also true. 30 Remember the following important principles: You must size the conductors at 125% of the motor FLC [430.22(A)]. You must size the overloads no more than 115% to 125% of the motor nameplate current rating, depending on the conditions [430.32(A)(1)]. You must size the short-circuit ground-fault protection device from 150% to 300% of the motor FLC [Table 430.52]. If you put all three of these together, you can see the branch-circuit conductor ampacity (125%) and the short-circuit ground-fault protection device (150% to 300%) aren't related. This final example should help you see if you've been paying attention. 31 Fig. 6. Although this example may bother some people, the 14 AWG THHN conductors and motor are protected against overcurrent by the 16A overload protection device and the 40A short-circuit protection device. 32 Example No. 4: Are any of the following statements true for a 1-hp, 120V motor, nameplate current rating of 14A? Refer to Fig. 6. (a) The branch-circuit conductors can be 14 AWG THHN. (b) Overload protection is from 16.1A. (c) Short-circuit and ground-fault protection is permitted to be a 40A circuit breaker. (d) All of these are true. Walking through each of these, you can see: (a) The conductors are sized per 430.22(A): 16A×1.25=20A; Table 310.16 requires 14 AWG at 60°C. (b) Per 430.32(A)(1), overload protection is sized as follows: 14A (nameplate)×1.15=16.1A. (c) Short-circuit and ground-fault protection is determined based on 430.52(C)(1): 16A×2.50=40A circuit breaker. Therefore all three statements are true. 33 The 16A overload protection device protects the 14 AWG conductors from overcurrent, while the 40A short-circuit protection device protects them from short circuits. This example illustrates the sometimes confusing fact that when you're doing motor calculations, you're actually calculating overcurrent and short-circuit protection separately. Motor calculations have long been a source of confusion and errors for many people. Understanding what makes these calculations different should help you do your motor calculations correctly every time. Next month we'll look at sizing motor feeders in Part 2 34 Motor Calculations Part 2: Feeders Part 1 of this two-part series explained how to size overload protection devices and short-circuit and ground-fault protection for motor branch circuits. Understanding the key point of that article, which was that motor overload protection requires separate calculations from short-circuit and ground-fault protection, clears up a common source of confusion and a point of error. But another source of confusion arises when it comes to sizing short-circuit and ground-fault protection for a feeder that supplies more than one motor. Let's look again at branch-circuit calculations and then resolve the feeder issues so your calculations will always be correct. Branch-circuit conductors and protection devices Per 430.6(A), branch-circuit conductors to a single motor must have an ampacity of not less than 125% of the motor full load current (FLC) as listed in Tables 430.147 through 430.150. To illustrate this, let's size the branch-circuit conductors (THHN) and short-circuit ground-fault protection device for a 3-hp, 115V, single-phase motor. The motor FLA is 31A, and dual-element fuses for short-circuit and ground-fault protection are in use (Fig. 1). 35 Per Table 430.148, the FLC current is 34A. 34A×125%=43A. Per Table 310.16 (60°C terminals [110.14(C)(1)(a)]), the conductor must be a 6 AWG THHN rated 55A. Fig. 1. Don’t make the mistake of using a motor’s FLA nameplate rating when using the short-circuit and ground-fault protection devices. You must use the FLC 36 rating given in Table 430.148. Per the motor FLC listed in Table 430.52, size the branch-circuit short-circuit and ground-fault protection devices by using multiplication factors based on the type of motor and protection device. When the protection device values determined from Table 430.52 don't correspond with the standard rating of overcurrent protection devices listed in 240.6(A), you must use the next higher overcurrent protection device. To illustrate this, let's use the same motor as in the previous example. Per 240.6(A), multiply 34A×175% You need a 60A dual-element fuse. To explore this example further, see Example No. D8 in Annex D of the 2002 NEC. Once you've sized the motor overloads, branch-circuit conductors, and branchcircuit protective devices, you're ready to move on to the next step. Motor feeder conductor calculations From 430.24, you can see that conductors that supply several motors must have an ampacity not less than: 125% of the highest-rated motor FLC [430.17], plus The sum of the FLCs of the other motors (on the same phase), as determined by 430.6(A), plus The ampacity required to supply the other loads on that feeder. 37 Fig. 2. Motor feeder conductors shall be sized not less than 125% of the largest motor FLC plus the sum of the FLCs of the other motors on the same phase. 38 Use Fig. 2 and solve the following problem. Example No. 1. For what ampacity must you size the feeder conductor if it supplies the following two motors? The terminals are rated for 75°C. One 7.5-hp, 230V (40A), single-phase motor One 5-hp, 230V (28A), single-phase motor (a) 50A (b) 60A (c) 70A (d) 80A Let's walk through the solution. The largest motor is 40A. 40A×1.25+28A=78A. 80A is the closest selection that's at least 78A. 39 What size conductor would give us this ampacity (a) 2 AWG (b) 4 AWG (c) 6 AWG (d) 8 AWG Per Table 310.16, a 6 AWG conductor rated at 75°C provides 65A of ampacity, so it's too small. However, a 4 AWG conductor provides 85A of ampacity, which will accommodate the necessary 78A. Therefore, you need to size this feeder conductor at 4 AWG. Next, we have to determine what size overcurrent protection device (OCPD) we must provide for a given feeder. 40 Fig. 3. To size overcurrent protection devices for each feeder, start by determining the ampacities required for each motor and move on from there. 41 Example No. 2. Using a slightly more complex example, try sizing the feeder conductor (THHN) and protection device (inverse-time breakers, 75°C terminal rating) for the following motors (Fig. 3): Three 1-hp, 120V, single-phase motors Three 5-hp, 208V, single-phase motors One wound-rotor, 15-hp, 208V, 3-phase motor Refer to 240.6(A), 430.52(C)(1), Table 430.148, and Table 430.52. Start by determining the ampacities required for each size of motor, then walk through each step until you arrive at the correct OCPD size. 1-hp motor: FLC is 16A. 16A×250%=40A 5-hp motor: FLC is 30.8A. 30.8A×250%=77A (Next size up is 80A.) 15-hp motor: FLC is 46.2A. 42 Fig. 4. Each motor’s FLC will come into play when sizing the conductor. 43 46.2A×150% (wound-rotor) 569A (Next size up is 70A.) Now, let's look at the feeder conductor. Conductors that supply several motors must have an ampacity of not less than 125% of the highest-rated motor FLC (430.17), plus the sum of the other motor FLCs [430.6(A)] on the same phase (Fig. 4). Continuing with this example, add up all the ampacities, multiplying the highest rated motor by 125%. Thus: (46.2A×1.25)+30.8A+30.8A+16A=136A. Table 310.16 shows you need 1/0 AWG THHN because at 150A it's the smallest conductor that accommodates the 136A of ampacity we're working with. When sizing the feeder conductor, be sure to include only the motors that are on the same phase. For that reason, these calculations only involve four motors. You must provide the feeder with a protective device with a rating or setting not greater than the largest rating or setting of the branch-circuit short-circuit and ground-fault protective device (plus the sum of the full-load currents of the other motors of the group) [430.62(A)]. Remember, motor feeder conductors must be protected against the overcurrent that results from short circuits and ground faults but not those that result from motor overload. When sizing the feeder protection, be sure to include only the motors that are on the same phase. 44 Fig. 5. In this example, the largest branch-circuit fuse or circuit breaker allowed for Motor 1 is 70A. 45 Refer to Fig. 5 for this sample motor feeder protection calculation. Example No. 3. What size feeder protection (inverse-time breaker) do you need for the following two motors? 5-hp, 230V, single-phase motor 3-hp, 230V, single-phase motor (a) 30A breaker (b) 40A breaker (c) 50A breaker (d) 80A breaker Let's walk through the solution. Step 1: Get the motor FLC from Table 430.148. A 5-hp motor FLC is 28A. A 3-hp motor FLC is 17A. 46 Step 2: Size the branch-circuit protection per the requirements of 430.52(C)(1), Table 430.52, and 240.6(A) 5-hp: 28A×2.5=70A 3-hp: 17A×2.5=42.5A (Next size up is 45A.) Step 3: Size the feeder conductor per 430.24(A). The largest motor is 28A. (28A×1.25)+17A=52A Table 310.16 shows 6 AWG rated 55A at 60°C as the smallest conductor with sufficient ampacity. Step 4: Size the feeder protection per 430.62. It must not be greater than the 70A protection of the branch circuit plus the 17A of the other motor, which is the total of all loads on that feeder. 70A+17A=87A Choose the next size down, which is 80A. 47 How can you be safe if you're selecting the next size down instead of the next size up? Remember, you've already accounted for all the loads, and the NEC requires that you not exceed the protection of the branch circuit. Again, keep in mind that you aren't calculating for motor overload protection. Motor calculations are different from other calculations. With motor feeders, you're calculating for protection from short circuits and ground faults, only — not overload. Putting it all together Motor calculations get confusing if you forget there's a division of responsibility in the protective devices. To get your calculations right, you must separately calculate the motor overload protection (typically near the motor), branch-circuit protection (from short circuits and ground faults), and feeder-circuit protection (from short circuits and ground faults). Remember that overload protection is only at the motor. Any time you find yourself confused, just refer to NEC Figure 430.1. It shows the division of responsibility between different forms of protection in motor circuits. Example D8 in Annex D of the 2002 NEC illustrates this with actual numbers. Keeping this division of responsibility in mind will allow you to make correct motor calculations every time. 48 Sizing Circuit Protection and Conductors — Part 1 Suppose you have a 60A breaker supplying a branch circuit. What size conductors do you need for that circuit? One table in the NEC says No. 8; another says No. 4. Which one is right? What does the note about ambient temperature correction factors mean? And how do you know you need that size breaker in the first place? Let’s start with looking at the order of the calculations. The overcurrent protection device (OCPD) — whether a breaker or a fuse — defines the circuit. So sizing the OCPD logically comes before sizing the conductors. But before you can size the OCPD, you have to determine what load it will supply. 49 As part of collecting data for sizing branch circuits and feeders for an addition, Seth White, a field technician with Industrial Tests, Inc., Rocklin, Calif., is using the power monitor on a similar installation to determine which loads really are continuous and whether the majority are non-linear. 50 Load calculations Loads may be continuous (operating 3 hr or longer) or noncontinuous. Your first step in branch circuit load calculations is to characterize each load as either continuous or noncontinuous. This distinction is critical. If you mischaracterize just one continuous load as noncontinuous, you could undersize the OCPD and the conductors. Make sure you avoid the trap of assuming lighting is noncontinuous if controlled by occupancy sensors. There’s no guarantee they won’t run less than 3 hr. Even if the controls shut the lights off after 2 hr, sensor trips can keep extending the runtime. Similar logic applies to other types of loads that are on automatic control. A good practice when performing branch circuit calculations is to make a table with two columns — one for continuous and one for noncontinuous. For each load you can determine that will run less than 3 hr at a time, list it in the noncontinuous column. List all other loads in the continuous column. You also need to determine if the load you’re supplying is a specific-purpose load covered by another Article in the NEC. To do that, look for your load type in Table 210.2. If it’s a motor, it’s always covered by Art. 430. If it’s a hermetic motor (used in air-conditioners, refrigeration units, and chillers), it’s covered by Art. 430 as well as by Art. 440 [440.3]. 51 At this point, it seems we’re ready to multiply the total of the continuous loads by 125% [210.19(A)] and then add that number to the total of the noncontinuous loads. For branch circuits, it’s usually just that simple — but not always. Some loads operate in a mutually exclusive fashion. That’s why when you’re sizing feeders, for example, you use the larger of the heating or airconditioning load when determining the total load. Rarely does a branch circuit have loads that are mutually exclusive. However, it can happen. For example, consider an industrial shop that has one portable arc welder and 10 welding outlets. Only one outlet will be in use at any given time (the others are then excluded by dint of not having an arc welder), so all of these are on a single branch circuit. 52 OCPD sizing Once you’ve correctly calculated the load, you’re ready to size the OCPD. Breakers and fuses come in standard sizes [240.6]. To size the OCPD, follow what’s known as the “next-size-up” rule. Look at the standard sizes that are larger than your load, and pick the one that’s closest. Do not apply this to motor overload protection. For motor OCPDs, refer to Art. 430. But wait. You calculated your load in VA, and the OCPDs are rated in amps. You’ll have to convert VA to amps. In a DC circuit, you’d just divide VA by the nominal voltage to derive amps. For AC loads, you must also divide by the appropriate phase factor. For 3-phase loads, you divide the VA by the nominal voltage and by the square root of three (approximately 1.732). If your total 3-phase load in a 480V system is 50,000VA, what size breaker do you need? 50,000VA ÷ (480V × 1.732) = 60.2A. The next size up is 70A. Note that some types of single-phase loads are routinely supplied by a 3phase panel. Lights, for example, are single-phase loads. Just make sure you wire the lighting system to achieve balance across the phases. 53 Conductor sizing Because sizing conductors isn’t a one-size-fits-all process, the NEC has several ampacity tables. Making matters a bit more complicated, the tables have multiple temperature-rating columns. You can avoid confusion here if you understand the logic of the tables. After you select the right table and column in that table, you must apply the: Temperature correction factors [310.15(B)(2)], and Adjustment factors [310.15(B)(3)]. Depending upon your application, you might also need to apply the correction factors in 310.15(B)(4) through 310.15(B)(7). The temperature correction factors are based on the expected ambient temperature where you’re installing the conductor. Use the peak expected temperature, not an average. 54 Dick Reese, an electrical power engineer with Industrial Tests, Inc., Rocklin, Calif., analyzes his coordination study to determine whether the system’s new conductor size/type will cause any unnecessary faults or nuisance trips. 55 Adjustment factors vary, depending upon the wiring method. At this point, you should already know the type of raceway and conductor you’re using. You should also know if a raceway or cable will carry more than three currentcarrying conductors. Those design decisions logically come ahead of sizing the raceway and conductors, though sometimes it’s necessary to change them late in this process. Once you’ve applied these factors, you have your conductor ampacity number. Now you just need to look it up in the table you selected, using the appropriate temperature rating column. Follow it to the left to see the minimum conductor size. Note: You might increase the conductor size due to voltage drop or other considerations. 56 Typical lighting circuit Let’s apply the process we’ve learned to sizing the OCPD and conductors for a 480V branch circuit consisting of six 400W lights. Here’s the 7-step process: Step 1: Characterize the loads. Regardless of any controls, characterize these lights as continuous loads; there’s no guarantee they will be on for less than 3 hr. Step 2: Calculate total load. Look up the input wattage (manufacturer’s data). For example, assume it’s 452W. Calculate kVA. Since lights are actually single-phase loads, you can skip this step. You do not divide the watts by 1.732 for this type of load, even if supplied by 480V. Step 3: Calculate amps. (452W ÷ 480V) × 6 = 5.65A. 57 Step 4: Size the OCPD. Use the next size up [240.6], which is 15A. Step 5. Identify the table. You’re running 3 THHN conductors in EMT. After reading the table headings, it’s obvious that Table 310.15(B)(16) is the correct ampacity table. Step 6. Apply temperature correction factor. The architect gave you 125°F as the highest expected ambient temperature in the ceiling space. From Table 310.15(B)(2)(b), we see this requires us to multiply the allowable ampacity by 0.5. Alternatively, we can multiply our load amps by 2. Step 7. Size the conductor per the required ampacity. THHN is in the 90° column, but your connectors are rated for 60°C. Using the 60°C column, we find this circuit requires a conductor at least 14 AWG. The process for calculating a feeder is different from that of calculating a branch circuit. In Part 2, we’ll dive into that and see what to do. 58 Sizing Circuit Protection and Conductors — Part 2 In Part 1, we looked at how to size a branch circuit and walked through a simple example. So if you’re sizing a feeder, why don’t you do it the same way? Except in cases where a feeder is also essentially a branch circuit by dint of supplying a single load (e.g., a large motor supplied by a feeder), you’ve already done the branch circuit load calculations. In normal power distribution, the purpose of a feeder is to supply power to multiple branch circuits. The typical arrangement in an industrial facility is a 480V feeder supplies a transformer that supplies a panel (or multiple transformers, each supplying a panel). That transformer might supply the panel with 120V for office loads, 277V for lighting loads, or maybe 480V for production equipment. Each overcurrent protective device (OCPD) in that panel is supplied by the transformer that’s supplied by that feeder. You can follow the flow of power from the service through the feeder through the branch circuit, but the flow of demand is in the opposite direction. The individual loads add up on the branch circuit, and the branch circuits add up on the feeder. This means that as part of the process of determining the load on the feeder, you add up the branch circuit loads that feeder supplies. Because you can’t know your feeder load until you’ve calculated your branch circuit loads, you do branch circuit calculations first. 59 Load analysis You need to do a bit of load analysis before sizing your feeder OCPD and conductors. You don’t need to look at whether loads are continuous or noncontinuous, as you’ve already accounted for this in determining your total VA for each branch circuit. Typically, but not always, you can simply add up the VA of the branch circuits to determine the VA on the feeder. For example, a feeder supplies 10 branch circuits for lighting at 452VA each. That totals up to 4,520VA. But what if those are HID lights, commonly used in high-bay applications (Photo)? 60 Make sure you know how to account for non-linear lighting loads in your calculations. 61 HID lights are an excellent choice for many reasons, but the downside is their ballasts make them nonlinear loads. This has some ramifications. First, you’ll probably want to derate the conductors. This is especially true if you have long runs, and voltage drop comes into play. The good news is that manufacturers of highly nonlinear loads like HID lighting typically provide derating factors. And because high-bay lighting is a ceiling application, you also must account for the higher ambient temperature using the correction factors in Tables 310.15(B)(2)(a) and (b). Second, this type of load means you have to treat the neutral as a currentcarrying conductor [310.15(B)(5)(c)]. Third, not all loads run at a given time. Lighting, which we’ve been talking about, isn’t a diverse load — that is, if you turn the lights on, they all operate. That is not true of all loads, however. Some loads operate in a mutually exclusive fashion. That’s why, for example, you use the larger of the heating or air-conditioning load when determining the total load. That’s part of accounting for load diversity. Load diversity is really an engineering decision, and you base it on the expected operation of the equipment. Consider an industrial shop that has one portable arc welder and 10 welding outlets. Only one outlet will be in use at any given time. Suppose this is a 250A 220V/380V arc welder. If you didn’t factor in load 62 diversity, you’d need to add a service instead of just a single breaker. You do this analysis to determine the actual maximum VA that will be on the feeder. Once this is done, you can turn your attention to sizing the OCPDs and then the conductors. To illustrate the basics of feeder sizing, we’ll expand on the example we used in Part 1. Now we’re sizing the feeder for six of those branch circuits. We’ll start with the OCPD. OCPD sizing In this example, we can simply add up the branch circuit loads to determine the feeder load and size the OCPD. Calculate the feeder load per Art. 220, Parts III, IV, and V. For our example, we multiply our 452VA load by 10 branch circuits to arrive at 4,520VA. Because lighting is a single-phase load, we divide by the voltage to determine the current. In Part 1, these were 480V lights, but the corporate office has just sent out a memo stating that all lights must be 277V because the company has contracted for special discount pricing for 277V lamps. Previously with 480V lights, each branch circuit drew 5.65A. Now with 277V lights, it’s going to draw 9.8A. 63 If you divide your total of 4,520VA by 277, you see the feeder carries 16.31A. Using the next-size-up rule, the OCPD is 20A. But suppose it’s a warehouse supplied by just that one feeder, and it has four additional loads: • Electric heaters totaling 250,000VA • Office air-conditioner: 15,000VA • Convenience receptacles: 10,000VA • Temperature-controlled exhaust fans: 50,000VA You don’t heat and cool at the same time, so applying load diversity gives us an additional load equal to the heater load plus the convenience receptacles. Odds are that the air conditioner and receptacles are powered by the same 120/240 transformer. Who’s to say that someone won’t add more circuits in the future? It’s a good engineering decision to allow for the maximum transformer VA minus the air-conditioner VA and add that number to the 250,000VA. This way, the feeder will have to be pulled only once rather than needing to be replaced due to a simple upgrade or small expansion project. 64 Conductor sizing For the current-carrying conductors, use the same approach we outlined in Part 1. Whether sizing conductors for feeders or branch circuits, you work with the ampacity tables the same way. Select a feeder conductor with sufficient ampacity to carry your calculated load [215.2]. The complicating issue here is the neutral, which is normally the grounded conductor. Any time the majority of the load on a feeder is nonlinear, you must treat your neutral as a current-carrying conductor. In any other situation, size the feeder neutral at least as large as the grounding electrode conductor (GEC) [250.122], but also review 220.61 — because you may need to go larger than the size of the GEC. Sizing per 220.61 involves some load analysis and calculations. The only downside to an oversized neutral is a slightly higher cost of materials. If a significant part of, though not a majority of, the load is nonlinear, it often makes sense for a small project to just treat it as a current-carrying conductor. Sometimes, it makes no difference, and we’ll see that play out in our example. 65 Typical lighting circuit feeder Let’s apply what we’ve learned to sizing the OCPD and conductors for a 277V feeder supplying 10 branch circuits, each consisting of six 400W lights. Here’s the 6-step process: Step 1: Calculate total load. The easiest way to do this is to add up your branch circuit VA. Remember that you characterized these as continuous and noncontinuous already, thereby satisfying the last sentence of 215.2(A)(1). Step 2: Calculate amps. In our example, 4,520VA ÷ 277V = 16.4A. Don’t forget to divide by the square root of three for 3-phase loads. If your feeder has a mix of single-phase and 3-phase loads, you have a mix of calculations to perform. Step 3: Size the OCPD. Use the next size up [240.6]. For our example, that’s 20A. Step 4. Identify the table. You’re running 3 THHN conductors in EMT. Use Table 310.15(B)(16). 66 Step 5. Apply temperature correction factor. For the branch circuits, we had to consider the ceiling temperature. But the feeder runs to a transformer sitting on a poured pedestal on the floor. The warehouse will be maintained at a maximum 80°F. From Table 310.15(B)(2)(b), we see this requires us to multiply the allowable ampacity by 1.22. Alternatively, we can multiply our amps by 0.82, and that gives us 13.5A. Step 6. Size the conductor per the required ampacity. Using the 60°C column, we find this circuit requires a conductor at least 14 AWG. It’s interesting that for the feeder and branch circuits, you’re going to run 14 AWG for the whole installation, including the neutral. This is something to keep in mind before delving into complex complications for sizing the neutral. Things get a bit turned on their head when motors are involved. In Part 3, we’ll look at why that is and what to do about it. 67 Sizing Circuit Protection and Conductors — Part 3 Motors differ from other types of loads in one important way: The motor needs much more current to start than to run. This temporary, but significant, inrush current is what complicates motor circuit protection. The overcurrent protection device (OCPD) must accommodate inrush current while still protecting the conductors. The conductors must be able to handle and dissipate the short-term increase in heat from that starting current. Much of Art. 430 is concerned with getting both of these requirements right. For other types of loads, a single device provides overcurrent protection and overload protection. Because of inrush, motor circuits handle those functions separately — that is, the job of protecting the conductors and the load gets split up for motor circuits. Fault protection opens the circuit when there’s a high level of excess current, such as from a fault or short circuit. But an overload is a relatively small amount of excess current. OCPDs protect against current in excess of the rated current of the equipment or ampacity of a conductor, whether it’s a motor circuit or not. Normally, the OCPD also handles overloads. But with motor circuits, separate thermal overloads do that. 68 Though you can’t see the motor, this coal feeder won’t run without it. Among the many calculations Black & Veatch engineers perform when designing a coal power station are the sizing of circuits and protection for the motors that drive the coal feeders. They have to get those right, 69 or the plant won’t have coal to burn and thus won’t produce power. Load analysis As with any circuit, analyze the loads before sizing motor OCPDs and conductors. Here are some highlights: 1. If multiple motors are on the same feeder and/or branch circuits, look at load diversity. Do any of these operate in a mutually exclusive fashion? 2. Which loads are continuous? Noncontinuous? 3. Does the system include a variable-frequency drive (VFD)? Is it power-factor corrected? Does it have harmonics mitigation? 4. Do you need to derate conductors for voltage drop? 5. What type of motor(s) are you installing? For example, part-winding motors have additional requirements. Review Art. 430 Part I when making this assessment. 6. Is this application covered by another article [Table 430.5]? 70 Sizing motor circuit conductors Normally, we size the OCPD and then the conductors. You find this order of calculation in the examples of Appendix D. But with motors, your second step is to size the conductors [Table 430.1]. Not coincidentally, the requirements are in Art. 430 Part II. In Part III, you’ll find the requirements for providing thermal protection to the motor; that’s outside the scope of this discussion. Part II applies to motors operating at under 600V, such as typical 480V industrial motors. If your motor operates at over 600V, use Part XI instead. How you proceed depends on whether these conductors are for: Single motor. Apply 430.22. Then see if any of subsections (A) through (I) apply to your installation. Wound rotor secondary. Apply 430.23. More loads than just that motor. Apply 430.24. Combination load equipment. Apply 430.25. Motors with PF capacitors installed. Apply 430.27. Constant voltage DC motors. Apply 430.29. 71 If you have load diversity, you can apply a feeder demand factor [430.26]. If you’re using feeder taps, apply 430.28. To make things simple, let’s assume you need to size the conductors for a single, continuous-duty 40HP motor. Those conductors must have an ampacity of at least 125% of the motor FLC. Use the FLC from the motor nameplate. If, for some reason, you can’t get this from the nameplate or the motor data sheet, use the applicable NEC table (e.g., Table 430.250). It’s best, however, to obtain the information from the motor manufacturer (and then affix a new nameplate to the motor). Your motor’s nameplate says its FLC is 53A. Coincidentally, this is close to the 52A shown in Table 430.250. You now have a three-step process for sizing the conductors: Step 1. Identify the table. You’re running three THHN conductors in intermediate metal conduit (IMC). Use Table 310.15(B)(16). Step 2. Apply the temperature correction factor. Determine the maximum ambient temperature — not for where the motor is, but for where the conductors are running. Suppose these will run overhead, and you know the maximum temperature will be 110°F (43°C). 72 From Table 310.15(B)(2)(b), you see you must multiply the allowable ampacity by 0.87. Alternatively, you can multiply 53A by 1.15, which gives you 61A. But what if your ceiling temperature is, say, 160°F (71°C)? In the 60°C column, the ambient temperatures end at 55°C. In such a case, split the run; see Annex D3(a) for an example of how to do this. Step 3. Size the conductor per the required ampacity. Using the 60°C column, you see this circuit requires a conductor at least 4 AWG. OCPD sizing Size the OCPD per 430 Part III, noting that you use the motor nameplate current rating (FLA), not the FLC [430.6(A)]. In our example, you sized the conductors for a single motor that has 53A FLC. Let’s assume the motor is on its own branch circuit. How do you size the OCPD for that circuit? The answer is in 430.52. How would you know to go there? Earlier, you referred to Table 430.1 to see what your second step is. Go back to Table 430.1 as you continue to work out your motor requirements. For this step, Table 430.1 directs you to Part IV. There, you’ll read that the OCPD must be capable of carrying the starting current of the motor [430.52(B)]. This doesn’t mean you size the OCPD for the starting current, however. The meaning of this emerges in 430.52(C). You need to specify an OCPD per Table 430.52. 73 When you size the overload, you use the FLA. But to size the OCPD, you use the FLC. First, find your motor type and OCPD type in Table 430.52. Then, multiply your FLC by the percentage of FLC required by the chart. Using the 53A FLC of our example with an inverse time breaker, you multiply the FLC by 2.5 for a maximum rating of 132.5A. This isn’t a standard OCPD size [240.6(A)]. Since 430.52(C)(1) says the OCPD can’t exceed this calculated value, do you use the next size down? If you read on just a bit, you’ll see that Exception No. 1 lets you use the next size up. So for this branch circuit, you need a 150A breaker. If it turns out that the breaker trips every time you try to start the motor, then what? Determine if the trip is due to a fault or from overload. If it’s overload, Exception No. 2 lets you use a larger OCPD. But there are limits to how big that OCPD can be. You may have voltage drop or power factor problems that lead to excess starting current. In addition to correcting these, consider a soft-start or VFD so you eliminate across-the-line starting. A big advantage of using a soft-start or VFD is you eliminate a common cause of cable failure. The power anomalies resulting from across the line starting can damage loads on other feeders, not just the motor 74 system. Multiple loads Now let’s change our example slightly. Suppose this motor is on a branch circuit with two other motors and four electric heaters. The loads are 40A, 27A, and 40A, respectively. How do you size the OCPD? Unless you have a compelling reason to put these loads on the same branch circuit, it’s generally best to put each motor on its own branch circuit. First, analyze the loads. If that 40A motor runs an HVAC compressor, you can disregard the 40A heaters; these are mutually exclusive loads. To see why not to disregard the motor instead, review the calculations we just did. Next, turn to 430.53. The first requirement is that you must use fuses or inverse time circuit breakers. So when you use Table 430.52, ignore the two middle columns. Apply Table 430.52 to the sum of your motor loads, then add to the sum of the other loads. 75 Fig. 1. These are the major steps in sizing conductors and OCPDs for branch circuits. 76 Feeders For feeder OCPD requirements, turn to Part V. The key here is the “don’t exceed the rating” requirement of 430.52(C)(1) applies only to the largest load. For the conductor requirements on that feeder, turn to Part II. To determine the minimum conductor ampacity [430.24]: 1. Multiply the FLC of the largest motor by 125%. 2. Add up the FLCs of the other motors. 3. Multiply the continuous non-motor loads by 125%. 4. Add up all of the above to the total of the non-continuous loads. 77 Avoiding confusion Fig. 2. The major steps in sizing conductors and OCPDs for feeders are almost identical to the branch circuit ones, but change as shown here. 78 To size conductors and OCPDs for branch circuits, follow the steps shown in Fig. 1. The steps for feeders modify the steps for branch circuits, as shown in Fig. 2. But if you have motor circuits, remember that the inrush current of motors changes things: The normal functions of the OCPD are split. With motors, you have an additional device that does the overload protection job normally done by the OCPD. You use multipliers for sizing the conductors (125%) and the OCPDs [Table 430.52]. If you understand the branch circuit conductor and OCPD sizing steps, it’s just a matter of modifying them a bit for feeders and/or motors. 79 Dwelling Unit Calculations Apply demand factors for correct load calculations A dwelling unit is a single unit that provides complete and independent living facilities, according to the NEC definition found in Art. 100 (Fig. 1 ). Fig. 1. The definition of dwelling unit, as described above, is found in Art. 100. 80 Dwelling units have special requirements for load calculations. Although most of the actual load calculation requirements are in Art. 220, others are scattered throughout the Code and still come into play when making certain calculations . Keep the following considerations in mind when making dwelling unit calculations: Voltages. Unless other voltages are specified, calculate branch-circuit, feeder, and service loads using the nominal system voltage [220.5(A)]. For a singlefamily dwelling unit, the nominal voltage is typically 120/240V. Motor VA. Use motor table voltage and current values, such as 115V, 230V, or 460V — not 120V, 240V, or 480V [430.248 and 430.250]. A much more accurate VA rating is obtained by using the motor’s rated voltage and current, which were used in developing the Code Tables. Rounding. Where calculations result in a fraction of less than 0.50A, you can drop the fraction [220.5(B)]. Receptacles. You can use 15A or 20A receptacles on 20A circuits as long as there is more than one receptacle on the circuit. For these purposes, a duplex receptacle is considered to be two receptacles [210.21(B)(3)]. Continuous loads. A continuous load is one in which the maximum current is expected to continue for 3 hr or more, according to the Art. 100 definition. Fixed electric heating is one example of a continuous load [424.3(B)]. When sizing branch circuit conductors and overcurrent devices for a continuous load, 81 multiply the load by 125% [210.19(A)(1) and 210.20(A)]. Laundry rooms. A laundry area receptacle is required [210.52(F)], at least one of which must be within 6 ft of a washing machine [210.50(C)]. Any receptacle within 6 ft of the outside edge of a laundry sink must be GFCI protected [210.8(A)(7)]. Required circuits. In addition to the circuits required for dedicated appliances and those needed to serve the general lighting and receptacle load, a dwelling unit must have the following circuits: A minimum of two 20A, 120V small-appliance branch circuits for receptacles in the kitchen, dining room, breakfast room, pantry, or similar dining areas [220.11(C)(1)]. These circuits must not be used to serve other outlets, such as lighting outlets or receptacles from other areas [210.52(B)(2) Ex]. These circuits are included in the feeder/service calculation at 1,500VA for each circuit [220.52(A)]. One 20A, 120V branch circuit for the laundry receptacle(s). It can’t serve any other outlet(s), such as lighting, and can serve only receptacle outlets in the laundry area [210.52(F) and 210.11(C)(2)]. In your feeder/service load calculation, include 1,500VA for the 20A laundry receptacle circuit [220.52(B)], as shown in Fig. 2. 82 Feeder and service calculations. Occupants don’t use all loads simultaneously under normal living conditions, so “demand factors” can be applied to many of the dwelling unit loads in order to size the service. Some demand factors provided in the Code are intended for use in dwellings only; others are allowed only in non-dwellings. Therefore, be careful to apply demand factors only as allowed by the NEC. Fig. 2. Per Sec. 210.11(C)(2), one 20A, 120V branch circuit is required for the laundry area receptacles. 83 The NEC provides two dwelling service load calculation methods: the standard method and the optional method. Standard method for feeder and service load calculations The standard method consists of three calculation steps: General lighting VA load. When calculating branch circuits and feeder/service loads for dwellings, include a minimum 3VA per sq ft for general lighting and general-use receptacles [220.12]. When determining the area, use the outside dimensions of the dwelling. Don’t include open porches, garages, or spaces not adaptable for future use. Small appliance and laundry circuits. The 3VA per sq ft rule includes general lighting and all 15A and 20A, 125V general-use receptacles, but doesn’t include small-appliance or laundry circuit receptacles. Therefore, you must calculate those at 1,500VA per circuit. See 220.14(J) for details. Number of branch circuits. Determine the number of branch circuits required for general lighting and general-use receptacles from the general lighting load and rating of the circuits [210.11(A)]. Although this is explained in Annex D, Example D1(a) of the NEC, let’s look at an another example. 84 Fig. 3. Sample calculation showing how to follow the rules in Sec. 220.12 regarding general lighting and receptacles for a 2,000-sq-ft dwelling unit. 85 Question: What’s the general lighting and receptacle load for a 2,000-sq-ft dwelling unit that has 34 convenience receptacles and 12 luminaires rated 100W each (Fig. 3)? The calculation is pretty simple. 2,000 sq ft x 3VA = 6,000VA. No additional load is required for general-use receptacles and lighting outlets because they are included in the 3VA per sq ft load specified by Table 220.12 for dwelling units. See 220.14(J). Now let’s work through an example to determine the number of circuits required. Question: How many 15A circuits are required for a 2,000-sq-ft dwelling unit? Step 1: General lighting VA = 2,000 sq ft x 3VA = 6,000VA Step 2: General lighting amperes: I = VA ÷ E I = 6,000VA ÷ 120V* I = 50A *Use 120V, single-phase unless specified otherwise. 86 Step 3: Determine the number of circuits: Number of circuits = General lighting amperes ÷ circuit amperes Number of circuits = 50A ÷ 15A Number of circuits = 3.30, or 4 circuits. Any fraction of a circuit must be rounded up. Optional method for feeder and service load calculations You can use the optional method [Art. 220, Part IV] only for dwelling units served by a single 120/240V or 120/208V 3-wire set of service or feeder conductors with an ampacity of 100A or larger [220.82]. The optional method consists of three calculation steps: General loads [220.82(B)] Heating and air-conditioning load [220.82(C)] Feeder/service conductors [310.15(B)(6)] Step 1: General loads [220.82(B)] The general calculated load must be at least 100% for the first 10kVA, plus 40% of the remainder of the following loads: General lighting and receptacles: 3VA per sq ft Small-appliance and laundry branch circuits: 1,500VA for each 20A, 120V small-appliance and laundry branch circuit specified in 220.52. Appliances: The nameplate VA rating of all appliances and motors that are fastened in place (permanently connected) or located on a specific circuit, not 87 including heating or air-conditioning.. Be sure to calculate the range and dryer at their nameplate ratings. Step 2: Heating and air-conditioning load [220.82(C)] Include the larger of (1) through (6): Air-conditioning equipment: 100% Heat-pump compressor without supplemental heating: 100% Heat-pump compressor and supplemental heating: 100% of the nameplate rating of the heat-pump compressor and 65% of the supplemental electric heating for central electric space-heating systems. If the control circuit is designed so that the heat-pump compressor can’t run at the same time as the supplementary heat, omit the compressor from the calculation. Space-heating units (three or fewer separately controlled units): 65%. Space-heating units (four or more separately controlled units): 40%. Thermal storage heating: 100%. Step 3: Feeder/service conductors [310.15(B)(6)] 88 400A and less. For individual dwelling units of one-family, two-family, and multi-family dwellings, use Table 310.15(B)(6) to size 3-wire, single-phase, 120/240V service or feeder conductors (including neutral conductors) that serve as the main power feeder. Feeder conductors aren’t required to have an ampacity rating greater than the service conductors [215.2(A)(3)]. Size the neutral conductor to carry the unbalanced load per Table 310.15(B)(6). Table 310.15(B)(6) can’t be used for sizing the feeder or service conductors that supply more than a single dwelling unit. Over 400A. Size ungrounded conductors and the neutral conductor using Table 310.16 for feeder/services over 400A and those that do not fill all of the requirements for using Table 310.15(B)(6). Let’s try a calculation example. Question: What size service conductor is required for a 1,500-sq-ft dwelling unit containing the following loads? Cooktop: 6,000VA Disposal: 900VA Dishwasher: 1,200VA Dryer: 4,000VA Ovens (two each): 3,000VA Water heater: 4,500VA A/C: 17A, 230V Electric heating (one control unit): 10kVA 89 Step 1: General loads [220.82(B)] General lighting: 1,500 sq ft x 3VA = 4,500VA Small-appliance circuits: 1,500VA x 2 circuits = 3,000VA Laundry circuit: 1,500VA Appliances (nameplate): Cooktop: 6,000VA Disposal: 900VA Dishwasher: 1,200VA Dryer: 4,000VA Ovens (each 3 kW): 6,000VA Water heater: 4,500VA Total connected load: 31,600VA First 10kW at 100%: 10,000VA x 1.00 = 10,000VA Remainder at 40%: 21,600VA x 0.40 = 8,640VA Calculated general load: 10,000VA + 8,640VA Calculated general load: 18,640VA 90 Step 2: Air-Conditioning versus heat [220.82(C)] Air-conditioning at 100% [220.82(C)(1)] vs. electric space heating at 65% [220.82(C)(4)] Air conditioner [Table 430.248]: A/C VA = V x A A/C VA = 230V x 17A A/C VA = 3,910VA (omit) Electric space heat: 10,000VA x 0.65 = 6,500VA Step 3: Feeder/service conductors [310.15(B)(6)] Calculated general load (Step 1): 18,640VA Heat calculated load (Step 2): 6,500VA Total calculated load = 18,640VA + 6,500VA = 25,140VA I = VA ÷ E I = 25,140VA ÷ 240V = 105A 91 Therefore, the feeder/service ungrounded conductor is sized to 110A, 3 AWG [310.15(B)(6)]. The Code doesn’t explain how demand factors were derived, and it’s not essential that you understand this in order to apply them correctly. Be sure to work on some practice calculations so you understand how to apply the various demand factors to a dwelling unit calculation. The standard calculation and the optional calculation methods were both discussed in this article. These are two distinctly different calculation methods, so be careful not to mix them. Remember that the standard method is in Part III of Art. 220, and the optional method is contained in Part IV. When you are evaluating the necessary loads in either type of calculation method, follow the requirements for specific loads covered in other Articles outside of Art. 220. Which method is better to use? On an exam, you’ll likely be told which method to use on a specific question. However, if the question doesn’t specify a method, use the standard calculation. The optional method is usually faster and easier to apply, so it has a natural advantage for daily use on the job. 92 Where to Find Dwelling Unit Code Requirements Outside Art. 220 Branch circuits — Art. 210 Areas supplied by small appliance circuits — 210.52(B)(1) Feeders — Art. 215 Services — Art. 230 Overcurrent protection — Art. 240 Wiring methods — Art. 300 Conductors — Art. 310 Appliances — Art. 422 Electric space-heating equipment — Art. 424 Motors — Art. 430 Air-conditioning equipment — Art. 440 93 94