Download PROBLEM B A commercial building has a 120/240 volt, 3 phase

ELECTRICAL SYSTEMS & FORMULAS
Previously, the discussion covered the introduction of
electrical design, from the generation of the power through
its distribution to a residential consumer.
The basics of service of 240 maximum volts provided
the necessary requirements of electrical service to operate
such things as lights, convenience outlets, small motors
such as the furnace fan, and larger devices found in a
residence such as water heaters, automatic clothes dryers,
cooking ranges, and air conditioning compressors.
Rarely do residences have such equipment that have
power requirements that exceed the basic single phase 240
volt system. So much for residences, but they constitute a
small part of the service requirements for our present day
life style.
Commercial applications require the big stuff. Such as a
motor that will propel a loaded elevator car upward to 100
stories at a speed of 1000 feet per minute. Or consider the
electrical requirements of a mechanical system that
provides constant environmental comfort to a building the
size of Sears tower in Chicago.
But then consider the grocery store where you buy your
food. While the system is not large enough to supply a city
the size of Lubbock, the facility does require sufficient
energy to store frozen food for several days at a time, or to
maintain a sufficient level of lighting over 20,000 square
feet of merchandise. All the while providing environmental
comfort to the facility for its customers.
The same system that works for a residence is not
sufficient for general commercial applications – or even
meet the power needs to sustain this school classroom. So a
system of electrical power is available to service the bigger
stuff.
Actually two types of service are available, and they
come in two sizes;
And considering size, realize that in electrical service
for buildings there are single phase systems and three
phase systems. Two phase electrical service is not utilized
in buildings.
It is easy to think of three basic types of outlets, and
three formulas associated with them - - - and to think that
well,
formula 1, outlet type one is single phase.
formula 2, outlet type two is two phase.
formula 3, outlet type three is three phase.
THAT IS ABSOLUTELY NOT THE CASE !
Consider the two types of electrical service. The system
spoken of previously is called a “DELTA” system.
You may remember the very elementary example of the
electrical generator where three coils of wire are arranged
to form a tube, with a magnetized shaft that turns inside
the tube to create electrical flux.
You also may remember the cylindrical container
spoken of on the electrical pole in your alley, that is used
to reduce the 20,000 or so volts down to 240 volts for use
in a simple residence.
In a delta system, if you remove the lid of the
transformer and see what is inside, you will find three coil
windings illustrated by the following diagram.
There are 3 wire coil windings inside, connected in the
form the form of a triangle, as the Greek letter DELTA,
A wire extends from each connection point to the place
that requires service. Since there must be a neutral, or
ground wire, it is simply connected to one of the coils,
which then extends with the other three phase wires.
Voltage between any two phase wires is 240. Voltage
between any phase wire and the neutral is 120. But
connection of all three phase wires and the neutral will
result in 415.69 volts.
The other type is called a “WYE” system, also called
because of the arrangement of the coils in the transformer.
Remove the lid and find three wire coils connected as
in the following diagram.
The electrical service in both the wye and the delta
configuration, each which contain 3 phase wires and a
neutral, are THREE PHASE SYSTEMS. ALL THREE PHASE
SYSTEMS HAVE THREE PHASE WIRES AND A NEUTRAL.
Anything less is not three phase.
That is why the 240 volt service to the residence with 3
wires, two phase and a neutral, is a SINGLE PHASE SYSTEM.
There are 3 wire coil windings inside, connected in the
form the form of the letter “Y” hence the name.
A wire extends from each connection point to the place
that requires service. The neutral, or ground wire is
connected to the center, common coil connection, which
then extends with the other three phase wires.
Voltage between any two phase wires is 208. Voltage
between any phase wire and the neutral is 120. But
connection of all three phase wires and the neutral will
result in 360 volts.
So now consider the formulas: the first two are from
the previous discussion.
Formula One: one phase wire (poles) , one neutral wire.
single phase
amperes = watts / volts
here volts is always 120.
Formula Two: two phase wires (poles) , one neutral wire
single phase
amperes = watts / volts here volts is 240 for delta,
and 208 for wye.
Formula Three: three phase wires (poles), one neutral wire
three phase
amperes = watts / √3 x volts here volts is 240 for
delta and 208 for wye.
In considering formula three, the bottom amounts to
the square root of 3, which equals 1.732, multiplied by the
appropriate voltage.
1.732 x 240 = 415.69
1.732 x 208 = 360
So in the case of Delta:
amperes = watts / 415.69
And for wye:
amperes = watts / 360
For the designer, a building will have either a wye system
or a delta system, not both. The designer chooses, based
on the need and what is available.
So, what is all this good for?
Remember the design of an electrical system requires
the connection of all the necessary lighting, convenience
outlets, and special devices required for the building
function, and in a manner that is in compliance with all
applicable building codes, specifically the National Electric
Code.
So what is involved? All the things that are between
the electrical service wires and the outlets . . . Namely the
proper size of circuit
the size of wires required to carry the amperage
the size of conduit (pipe) in which the wires are placed
the size of the circuit breaker required for safety.
Since electrical current is transferred from service to
devices by wires, and in commercial applications, wire
must be placed inside a conduit or pipe, then data must be
available to determine the requirements.
In your packet is a chart that shows electrical wire,
electrical conduit, and requirements for ground wires;
ELECTRICAL WIRE SIZE
Allowable Ampacities of insulated conductors from 1993 edition of the National
Electrical Code. (NEC table 3 10-16 using copper wire) Not more than 3 current
carrying conductors in raceway.
Size of
Conductor
14
12
10
8
6
4
3
2
1
Allowable
Amperage
20
25
35
50
65
85
100
115
130
Area
KCMIL
Dia.
inches
4.11
6.53
10.38
16.51
.064
.081
.102
.128
26.24
41.74
52.62
66.36
83.69
.184
.232
.260
.292
.332
1/0
2/0
3/0
4/0
150
175
200
230
105.6
133.1
167.8
211.6
.373
.419
.470
.528
250
300
350
400
500
256
285
310
336
380
250
300
350
400
500
.575
.630
.681
.728
.813
600
700
750
800
900
1000
1250
1500
1750
2000
420
460
475
490
520
545
590
625
650
665
600
700
750
800
900
1000
1250
1500
1750
2000
.893
.964
.998
1.03
1.09
1.15
1.29
1.41
1.52
1.63
STANDARD AMPERE RATING FOR BREAKERS & FUSES
NEC 240-6 Standard Ampere Ratings For Overcurrent Protection:
Fuses and Fixed Trip Circuit Breakers shall be considered 15, 20, 25,
30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250,
300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000,
4000, 5000, and 6000 amperes.
CONDUIT SIZE
1/2" 3/4"
WIRE
SIZE
AWG &
KCMIL
14
12
10
8
6
4
3
2
1
1/0
2/0
3/0
4/0
NEC table 3A
DIAMETER OF CONDUIT
2"
3"
1"
4"
5"
6"
6
4
4
1
10
8
6
3
16
13
11
5
29
24
19
10
40
32
26
13
65
53
43
22
93
76
61
32
143
117
95
49
192
157
127 163
66
85
1
1
1
1
2
1
1
1
1
1
1
1
4
3
2
2
1
1
1
1
1
7
5
4
4
3
2
1
1
1
10
7
6
5
4
3
3
2
1
16
12
10
9
6
5
5
4
3
23
17
15
13
9
8
7
6
5
36
27
23
20
14
12
10
9
7
48
36
31
27
19
16
14
12
10
62
47
40
34
25
21
18
15
13
97 141
73 106
63 91
54 78
39 57
33 49
29 41
24 35
20 29
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
1
1
1
1
1
1
4
3
3
2
1
1
1
1
6
5
4
4
3
3
2
2
8
7
6
5
4
4
3
3
10
9
8
7
6
5
4
4
16
14
12
11
9
7
7
6
23
20
18
16
14
11
10
9
4"
5"
6"
250
300
350
400
500
600
700
750
1/2" 3/4"
1"
2"
3"
DIAMETER OF CONDUIT
133
GROUND WIRE SIZE
NEC table 250-94 copper wire
SIZE OF LARGEST SERVICE ENTRANCE
CONDUCTOR OR EQUIVALENT AREA
FOR PARALLEL CONDUCTORS
SIZE OF
GROUNDING ELECTRODE
CONDUCTOR
2 or smaller
1 or 1/0
2/0 or 3/0
over 3/0 thru 350 kcmil
over 350 kcmil thru 600 kcmil
over 600 kcmil thru 1100 kcmil
over 1100 kcmil
8
6
4
2
1/0
2/0
3/0
Also in your packet is a page with two charts:
SINGLE PHASE MOTORS
THREE PHASE MOTORS
Realize that standard voltage has not been the same
designation back through electrical history. Many charts
that have been carried over time after time did not change
with the modernization of generated voltage – especially
the delta system, which is the one Thomas Edison used in
the 1800’s.
Often you will see 110v, or 115v, which is now 120v.
You will also see 210v, 215v, or 230v, which is really
240v.
The motor charts are two that show discrepancy from what
you have been previously told, but are still useful in
determining the amperage of motors.
So why can’t you simply use the formulas to determine
the amperage of motors, especially since all of you know
that one horsepower is rated at 746 watts ? ?
You remember you were told in the first introduction to
ignore p.f. which is the abbreviation for “power factor.”
Power factor deals with the inefficiency of electrical
gadgets. Motors fit into this category. In the windings of
the wires is a great deal of static trash – the armature of a
motor is heavy, and takes a certain amount of power to get
it started turning, and a certain amount of power to keep it
turning.
Power factor is taken into account in the motor charts.
If it were not, look at a single phase, 1 horsepower motor
and see the amperage is 8. So amps = watts/volts x p.f.
p.f. = 746 / 230 x 8 = .40
which means a 1 hp motor is about 60% efficient.
CHARTS TO DETERMINE THE AMPACITY OF MOTORS
The motors chart is useful when items of specification
rate motors in horsepower and phase, rather than in
running amps.
Amperes for all outlets must be known, because wire
size and breaker size is based on their ampere carrying
capacity.
Designing circuitry for lighting and convenience outlets
remains simple, since the NEC specifically states that these
circuits be limited to 20 amperes.
There are, however, lighting circuits that are exempt
from that limit because of their separation from common
public access. These include systems for parking lot and
street lighting, lighting in sports arenas, and specialty type
lighting for display and theatrics. And in most cases, these
types of lighting circuits are not limited to 120 volts.
EXAMPLE PROBLEMS involving electrical connections.




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ONE: 120/240 v delta
1.
6 lighting circuits, each with connected load of 1800
watts, single pole. For each circuit find:
Quantity of wires_________________
Amperage of each circuit__________
Circuit breaker size in amps________
Size of wires (awg)_______________
2. One electric range outlet, 2 pole, with total load of
9,600 watts. Find:

Amperage of circuit_______________

Quantity of wires required__________

Size of wires required_____________

Circuit breaker size in amps________


3. Two pole outlet for water heater with heat element
load of 27 amperes. Find:
Amperage of circuit_______________
 Quantity of wires required__________
 Size of wires required_____________
 Circuit breaker size in amps________






4. A single phase 2 pole attic fan motor rated at 2
horsepower. Find:
Amperage of circuit_______________
Size of wires ____________________
Quantity of wires required__________
Circuit breaker size in amps________
PROBLEM B
 A commercial building has a 120/240 volt, 3 phase, delta
electrical system with the following outlets:

1. A 3 pole A/C compressor rated at 33 amperes. Find:

Amperage for the circuit___________

Quantity of wires required__________

Size of wires required_____________

Circuit breaker size in amps________

Size of conduit required___________







2. A 2 pole electric heater element with a connected load
of 5600 watts. Find:
Amperage for the circuit___________
Quantity of wires required__________
Size of wires required_____________
Circuit breaker size in amps________
Size of conduit required___________
3. If the total load for the electrical panel from all
connected circuits amounts to 86,000 watts, find:

Amperage for the panel____________

Quantity of wires required__________

Size of wires required_____________

Circuit breaker size in amps________

Size of conduit required___________

Size of ground wire_______________

END OF DAY 14