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HVAC Design Criteria and Guidelines
ELECTRICAL DATA FOR HVAC DESIGNERS
For most HVAC engineers, electrical engineering in general and Article 430 of the National Electrical Code
(NEC), "Motors, Motor Circuits, and Controllers" in particular is a real mystery. But, this appendix defines
basic electrical information valuable to HVAC designers, including the rules governing the sizing of
conductors and overcurrent protective devices (OPD) for HVAC pump and fan motors, normally singlespeed, non-reversing, single- and three-phase induction motors in fractional and integral horsepower sizes.
Motor full load amps (FLA): From the beginning, the HVAC engineer must determine the FLA of
the fan or pump motor(s) being circuited. The table in Section D of this appendix lists the rated FLA
of typical "premium efficiency" HVAC motors. For other types of motors, the FLA can be
determined by using NEC tables 430.248 through 430.250.
For refrigeration equipment with hermetically sealed motors, manufacturers’ catalog cut sheets
must be used to determine full load requirements because refrigeration motors do not conform to
standard horsepower ratings. Typically, two values are provided by the supplier: "minimum circuit
amps" (MCA), which defines the circuit loading for wire sizing purposes, and "maximum over
current protection" (MOCP) which defines the required overload protection rating.
Overload protection: If the motor starter uses thermal overload elements, select the thermal
overload for that starter with an ampacity range within which the full load ampacity falls. For the
newer, electronic overload elements, the motor full-load amps can be programmed into the unit.
Short-circuit protection: For one single-phase motor, multiply the full-load current by the factor in
Table 430.52, based on the type of motor and the type of overcurrent protection device (OPD). For
the case of inverse time breakers and single-phase motors, that factor is 250%. Then, select the
next smaller standard breaker size below the derived OPD ampacity. It is important to note that any
OPD ampacity that is smaller than19.9 Amps will always use a 15 Amp breaker.
For a single three-phase motor, multiply the full load ampacity by 250% and select a thermal
magnetic breaker with a standard trip rating just smaller than this OPD.
When there are multiple motors on a single feeder, such as from a distribution panel, short-circuit
protection takes a different methodology. For example, you have a 480 V motor control center with
one 50 hp fan, two 30 hp pumps, one 20 hp pump, and three 10 hp fans. The FLA for each of these
motors are 65, 40, 27, and 14 Amps, respectively, at 460 V. The largest FLA is multiplied by 250%
and the sum of the rest of the motors is taken as their FLA. This results in an OPD requirement of
311 Amps, so taking the next smaller size will result in a 300 Amp thermal magnetic breaker.
The conductor rating should match the short-circuit protection rating, in this case 300 Amps.
Motor branch circuit conductor sizing: Data for sizing of fan and pump motor conductors is
provided in Section G of this appendix.
For wiring up to the primary motor disconnecting means, conductors and ground wires are sized on
the basis of the fuse or breaker size protecting that circuit.
Downstream of the motor disconnecting means, select the required conductor size by rounding UP
to a conductor rating greater than the imposed motor load computed as follows:
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HVAC Design Criteria and Guidelines
For a single motor on an individual overload protection device, multiply the FLA of the
motor by 1.25 and then size the conductors the next size larger than the resultant value.
NEC article 430.22 states in part, “…shall have an ampacity not less than 125%.”
For multiple pump or fan motors served by a single circuit, for circuit conductor sizing,
NEC 430.24 states that the largest motor ampacity is multiplied by 1.25 and the full load
ampacity of each additional smaller motors are added to this value.
Chiller motor electrical design: Designing the branch circuit and overcurrent protection for a
refrigeration machine, whether a hermetic centrifugal, screw, scroll, reciprocating, or any other
compressor, is similar to sizing for fan and pump motors.
A 200 ton screw compressor, for example, may require two circuits, each with a full load ampacity
of 168 Amps. From the manufacturer's data, the MCA is listed as 414 Amps and the MOCP as 500
Amp.
Size the ampacity of the fuse or breaker protecting the chiller circuit on the basis of the nameplate
MOCP. In this example, the MOCP would be a 600 Amp fused switch with 500 Amp fuses,
preferably dual element fuses. It is very important to understand that the notation on the nameplate,
which requires that the MOCP be 500 Amp fuses, means that only fuses may be used for
protecting this chiller. According to the NEC article 110.3(B): “Listed or labeled equipment shall be
installed and used in accordance with any instructions included in the listing or labeling.” Because
the label (nameplate) designates that there is a maximum fuse size, this article requires that only a
fuse may be used. If there had been no mention of a fuse, then a 500 Amp thermal magnetic
breaker could be used instead.
Motor starting: Common starting methods available for HVAC motors are across-the-line starters,
wye-delta starters, part winding starters, autotransformer starters, and solid state starters, including
variable frequency drives.
The simplest and most common starting device is the across-the-line starter where the equipment
consists of a main contactor and a thermal or electronic overload relay. The disadvantage of the
direct-on-line method is very high starting current (6 to 10 times the rated motor currents) and high
starting torque, causing slipping belts, heavy wear on bearings and gear boxes damaged products
in the process water hammers in piping systems. Across-the-line starting is typically applied to
motors only 40 Hp or less
For larger motors, reduced voltage starters are typically applied, either "wye-delta" or "star-delta",
part-winding, autotransformer, or solid state type.
1. Wye-delta starting device consists normally of three contactors, an overload relay and
a timer for setting the time in the star-position (starting position). The starting current is
about 30 % of the direct-on-line starting device. The starting torque is about 25 % of the
direct-on-line starting torque. The stress on an application is reduced compared to the
direct-on-line starting method.
2. Part-winding starting uses only a portion (usually one-half, but sometimes two-thirds) of
the motor winding, increasing the impedance seen by the power system. It is to used only
for voltage recovery and must not be left on the start connection for more than 2 to 3
seconds. The motor is not expected to accelerate on the start connection.
The advantages of part-winding starting are as follows:
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HVAC Design Criteria and Guidelines
a. Starting current is 60-75% of normal, depending on the specific winding
connection.
b. Starting torque is very low (may not even turn the shaft).
c. Winding heating is very high on start connection.
3. An autotransformer starter is connected so the motor is on the secondary of an
autotransformer during starting. The autotransformer has taps, to limit the voltage, applied
to the motor at 50%, 65%, or 80% of full-voltage. Because the line current varies as the
square of the impressed voltage, these same taps equate to 25%, 42%, and 64% of the
full-voltage value of line current.
The latter values reveal the advantage of the autotransformer over the primary impedance
starter. Those same taps on a primary-impedance starter equate to 50%, 65%, and 80%
of the full-voltage value of the line current.
Advantages of the autotransformer starter include lower relative cost and simplicity.
Features now found with autotransformer starters include solid-state motor protection
relays and vacuum contactors. While these starters may have complex equipment, they
operate on a concept much simpler than solid-state starters.
Disadvantages of the autotransformer starter include its non-continuous acceleration and
inflexibility. Acceleration is non-continuous because the torque developed by the motor is
practically constant during the initial starting period and then changes to another value
after the transition period. With the typical three taps, the autotransformer starter was
historically the most flexible of reduced voltage starters until the advent of the solid-state
starter. However, its flexibility pales in comparison to the solid-state starter.
4. Solid-state type starters use back-to-back thyristors for each line to the motor. These
six thyristors control power to the motor. The power adjusts by not completely turning on
the thyristors during starting. In other words, only a portion of the 3-phase sinusoidal wave
is supplied to the motor during start.
Because of these control features, the big advantage of the solid-state starter is the large
number of starting characteristics. The standard soft-start mode simply ramps the voltage
from a preset initial torque value to 100% during a user-selected time of 0 sec to 30 sec.
Another available control mode is a start based on current limitation. In this mode, you
select how much you want to limit the current (between 50% and 600%), and the duration
(between 0 sec and 30 sec). If you try to limit the current to a level lower than required to
start the motor, the motor won't start. But there is much more flexibility using a solid-state
starter than an auto-transformer starter with its three taps.
Other available operating modes include kickstart, soft stop, and pump control options.
The last option starts a pump motor on a curve rather than a straight line ramp. This
causes the hydraulic system to react as if there were a closed discharge valve behind the
pump, opening during starting.
The major disadvantage of the solid-state starter is its higher relative cost.
5. Variable frequency drives (VFDs) that incorporate integral solid state starters are
commonly applied to HVAC fans and pumps. In these cases, our specifications require
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HVAC Design Criteria and Guidelines
that the motor is rated for drive applications (inverter duty motor). [Standard motors may
do OK when connected to a VFD, or they may not. Due to the abnormal stresses applied
by the VFD to the motor windings and rotor laminations, any small weakness of a motor
will be amplified when the motor is operated at a lower speed/frequency. The result can
range from just a very noisy motor to a shorted winding or complete motor failure.]
Typically, modern drives produce either little or no harmful harmonics ("electrical noise")
because they have an integral filter or an internal design that reduces the reflected wave
harmonics.
Distance between the VFD and the motor can become an opportunity for motor winding
failure due to reflected wave high voltages caused by locating the motor distant from the
VFD. Several drive manufacturers have Web-based calculators that will tell you if the
distance between the motor and the drive is too far and give you mitigation means to help
improve the installation. The common-sense approach would be to keep the VFD within
sight of the motor (per NEC, that is less than 50 ft and within line of sight). If this is not
possible, we can specify that the VFD should have an output dv/dt filter to mitigate the
effects of reflections.
ELECTRICAL FORMULAS:
AC SINGLE PHASE ~ 1ø
AMPS=
WATTS=
WATTS÷(VOLTS x PF)
I=P÷(V x PF)
VOLTS x AMPS x PF
P=V x I x PF
VOLTS=
WATTS÷AMPS
V=P ÷ I
VOLT-AMPS=
VOLTS x AMPS
VA = V x I
HORSEPOWER=
(V x A x EFF x PF)÷746
POWER FACTOR=
INPUT WATTS÷(V x A)
EFFICIENCY=
AMPS=
(746 x HP)÷(V x A x PF)
AC THREE PHASE ~ 3ø
WATTS÷(1.732 x VOLTS x PF)
I = W ÷ (1.732 x V x PF)
WATTS=
1.732 x VOLTS x AMPS x PF
W = 1.732 x V x I x PF
VOLTS=
WATTS ÷ AMPS
V=W÷I
1.732 x VOLTS x AMPS
VA = 1.732 x V x I
VOLT-AMPS=
HORSEPOWER=
(1.732 x V x A x EFF x PF)÷746
POWER FACTOR= INPUT WATTS ÷ (1.732 x V x A)
EFFICIENCY=
(746 x HP) ÷ (1.732 x V x A x PF)
Where
V
W
R
I or A
HP
= VOLTS
= WATTS
= OHMS
= AMPERES
= HORSEPOWER
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HVAC Design Criteria and Guidelines
PF
kW
kWh
VA
kVA
C
EFF
= POWER FACTOR [For resistive loads (electric heating coils, etc.), PF =1.0.
For HVAC motors, PF can be estimated from the table below.]
= KILOWATTS
= KILOWATT HOUR
= VOLT-AMPERES
= KILOVOLT-AMPERES
= CAPACITANCE
= EFFICIENCY (decimal)
Typical Motor Power Factor
Motor
Nameplate
(hp)
Speed (rpm)
Power Factor
50% load
75% load
100% load
0-5
1800
0.72
0.82
0.84
5 - 20
1800
0.74
0.84
0.86
20 - 100
1800
0.79
0.86
0.89
100 - 250
1800
0.81
0.88
0.91
HVAC MOTOR CHARACTERISTICS
(NEMA "Premium" Design B, Normal Torque, Continuous Duty, 1800 rpm, ODP or TEFC motors)
Full Load
Full Load
Full Load
Full Load
Nameplate
Amps
Efficiency
Nameplate
Amps
Efficiency
Horsepower
(@460V)*
(%)
Horsepower
(@460V)*
(%)
1
30
1.43
85.5
35.1
93.6
1.5
40
2.00
96.5
48.3
94.1
2
50
2.63
86.5
60.6
94.5
3
60
3.90
89.5
68.4
95.0
5
75
6.49
89.5
84.3
95.4
7.5
100
9.29
91.7
112
95.4
10
125
12.6
91.7
138
95.4
15
150
18.0
92.0
170
95.8
20
200
24.4
93.0
430
96.2
25
250
29.5
93.6
284
96.2
115 V / 1 Ph = 8
FLA Multiplier
230 V / 1 Ph = 4
(Change 460 V to different voltage)
230 V / 3 Ph = 2
460 V / 3 Ph = 1
Motor Characteristics
RPM
Synchronous rpm
Actual (load) rpm
1800
1750
3600
3550
Torque (lb.ft.)
Hp x 5250 / rpm
Hp
Torque x rpm / 5250
Number of Poles
7200 / rpm
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HVAC Design Criteria and Guidelines
MOTOR STARTING AND STOPPING:
Frequent starting and stopping of Design B motors will result in early failure. The following table defines the
allowable number of starts and minimum time between starts, where:
A = Maximum number of starts per hour
B = Maximum product of starts per hour times load wk2
C = Minimum rest (off) time in seconds
Allowable starts per hour for any HVAC motor is equal to the lesser of A or B, as tabulated below:
MOTOR STARTER AND DISCONNECT SIZING:
Starters are sized in accordance with the latest edition of the National Equipment Manufacturers Association
(NEMA) standard ratings for magnetic starters and the NEC, as follows:
NEMA
Starter Size
00
0
1
2
NEMA Starter Sizes for Motors
Maximum HP for System Voltage (V)/ Phase (PH)
120V/1PH
240V/1PH
208V/3PH
240V/3PH
480V/3PH
1/3
1
2
3
1
2
3
7-1/2
1-1/2
3
7-1/2
10
1-1/2
3
7-1/2
15
2
5
10
25
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HVAC Design Criteria and Guidelines
NEMA
Starter Size
3
4
NEMA Starter Sizes for Motors
Maximum HP for System Voltage (V)/ Phase (PH)
120V/1PH
240V/1PH
208V/3PH
240V/3PH
480V/3PH
7-1/2
-
15
-
25
40
30
50
50
100
Provide disconnect switches for all equipment. Disconnect switches are sized in accordance with the latest
edition of the NEC for single motor applications as follows:
Switch
Rating Amps
(A)
30A
60A
100A
200A
400A
600A
120V/1PH
1-1/2
3
5
-
Disconnect Switch Sizes for Motors
Maximum HP at System Voltage (V))/ Phase (PH)
208V/1PH
240V/1PH
208V/3PH
240V/3PH
3
7-1/2
10
-
3
10
10
-
5
15
25
50
100
150
7-1/2
15
25
60
125
200
480V/3PH
15
30
60
100
250
400
Disconnect switches shall be sized for all other applications based on total kW rating of the equipment as
follows:
Disconnect Switch Sizes for Equipment
Switch
Rating
Amps (A)
30A
60A
100A
200A
400A
600A
Maximum kW at System Voltage (V)/ Phase (PH)
120V/
1PH
2.8
5.8
9.6
19.2
38.4
57.6
208V/
1PH
240V/
1PH
277V/
1PH
208V/
3PH
240V/
3PH
480V/
3PH
5.0
10.0
16.6
33.3
66.6
99.8
5.8
11.5
19.2
38.4
76.8
115.2
6.6
13.3
22.2
44.3
88.6
133.0
8.6
17.3
28.8
57.6
115.1
172.7
10.0
19.9
33.2
66.4
132.9
199.3
19.9
39.9
66.4
132.9
265.7
398.6
Dual element fuses are required with disconnect switches. Fuses are sized based on the nameplate rating
for the equipment.
Equipment enclosures for disconnect switches, starters, variable frequency drives, control panels and any
other panel enclosures housing electrical equipment are rated based on NEMA standard ratings. Panel
enclosures must be suitable for the environment in which they will be installed. Unless noted otherwise,
provide NEMA rated enclosures based on the following environment conditions:
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HVAC Design Criteria and Guidelines
NEMA Enclosure Ratings for Electrical Equipment
Environment Condition
NEMA
Type
1
3R
4
4X
7
9
12
Indoors only, dry, low dust, and non-corrosive environment
Outdoors, weatherproof and rainproof
Outdoors, watertight and raintight
Same as 4 plus corrosion resistant
Hazardous locations Class I, Groups A, B, C, or D
Hazardous locations Class II, Groups E, F, or G
Indoors subject to circulating non-hazardous dust, or dripping non-corrosive liquids
MOTOR BRANCH CIRCUIT CONDUCTORS AND RACEWAY:
The following table summarizes the NEMA-rated starter size, thermal and HMCP overload protection
ratings, conductor wire size, ground wire size, and conduit size typically required for 460/3/60 branch circuit
service to an HVAC motor. Note that the feeder serving the breaker is sized on the basis of the breaker
rating.
Motor HP
NEMA
Starter Size
Overload
Motor
Circuit
Protector
Size
HMCP
1
1
W36
3
3/4
12
12
1.5
1
W40
7
3/4
12
12
2
1
W42
7
3/4
12
12
3
1
W45
7
3/4
12
12
Conduit
Size
(in.)
Conductor
Ground
Size (AWG) Size (AWG)
5
1
W50
15
3/4
12
12
7.5
1
W54
15
3/4
12
12
10
1
W56
30
3/4
10
10
15
2
W61
50
3/4
10
10
20
2
W64
50
1
8
8
25
2
W67
70
1 1/4
6
6
30
3
W67
100
1 1/4
6
6
40
3
W69
100
1 1/4
4
4
50
3
W72
100
1 1/4
4
4
60
4
W74
150
1 1/2
2
2
75
4
W77
150
1 1/2
2
2
100
4
W36
200
2
2/0
2
125
5
W38
250
2 1/2
4/0
1/0
150
5
W40
400
2 1/2
4 /0
1/0
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HVAC Design Criteria and Guidelines
Motor HP
NEMA
Starter Size
Overload
Motor
Circuit
Protector
Size
HMCP
200
5
W43
400
3
350 kcml
2/0
250
6
W38
400
4
2 - 250 kcml
2/0
Conduit
Size
(in.)
Conductor
Ground
Size (AWG) Size (AWG)
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