The document provides detailed guidelines and calculations based on the NEC requirements for motor circuit calculations, including overload sizing, short circuit protection, and conductor sizing for different motors. It emphasizes specific formulas and tables to determine appropriate sizes for electrical components depending on the motor specifications and operating conditions. Additionally, it includes example calculations for various motor types, ensuring compliance with safety and efficiency standards.

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Motors and Motor
Circuit Calculations
2017 NEC Requirements

2. Copyright 1999-2019 Dan Dudley
2017 NEC Motor Circuit
Requirements and
Calculations

3. Copyright 1999-2019 Dan Dudley
NEC Motor Calculations
Basic Information
• 430.6(a)(1) Basic Calc information
• Name Plate Values (430.6(a)(2) are only used to size overloads
• Otherwise use T.430.247, 248, 249, & 250
• 430.22(a) Single motor 125% cont. duty (3 hrs. or more)
• T.430.22(e) Note Cont. Duty unless nature of apparatus is otherwise.

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NEC Motor Calculations
Basic Information
• 430.52(c)(1) exception
• Next higher standard size overcurrent protective device.
• 430.62 Feeder O.C. Protection
• Largest S.C. – G.F. Protective device plus the sum of the F.L.C. of all
other motors supplied by the same feeder
• No provision for going UP to next standard size must go down to next
standard size

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Hz
Hp Phases FLC Voltage
Temp
Rise
Service
Factor
Locked
Rotor
Letter
Insulation
Class

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Line side
Nameplate Data
Actuating Coil
Overload Relays
Load Side

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For all other
calculations use
Table values

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¾ @120 5 @208

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25-208 50 sync – 440 200 - 480
50 HP
Synchronous
440 Volts
200 HP
480 Volts
25 HP
208 Volts

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What minimum size THHN conductors are required for this
motor ?
Name Plate Not used
430.248: 10hp @ 240v = 50 amps
430.22(a): 50 X 125% = 62.5 amps
310.15(b)(16): 62.5 amps = #4 THHN
10HP 240V 1Φ FLI
44

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What is the minimum ampacity of the branch circuit conductors for a 25 HP ,
480-volt, 3 phase, squirrel cage motor with nameplate FLI of 32.5 amps ?
Name Plate value not used
430.250: 25hp @480v = 34 amps
430.22(a): 34 X 125% = 42.5 amps
310.15(b)(16) : # 6 THHN @60ºC and 55 amps
25HP 480V 3Φ FLI
32.5

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Circuits with more than one Motor
• Step 1.
• Step 2.
• Step 3.
• Decide which motor has largest
FLI rating
(This may not be the largest HP)
• Largest FLI of one motor X 125%
(only increase one)
• Add FLI rating of all other
motors
(straight from tables)

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What is the minimum ampacity of the branch circuit conductors for the
two motors as shown ? 3Æ, 208V
430.250: 5 hp @ 208v = 16.7 amps
430.24: Ampacity = FLI 1st motor X 125% + FLI 2nd motor
Ampacity = 16.7 X 125% + 16.7 = 37.575 amps
5HP 3Φ 208V
5HP 3Φ 208V

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What is the minimum ampacity of the branch circuit conductors supplying the two
motors as shown ? 1Æ, 230V
430.248: 2hp @ 230v = 12 amps
430.248: 5hp @ 230v = 28 amps
Ampacity = Largest FLI X 125% + Second FLI
28 X 125% + 12 = 47 amps
5HP 1Φ 230V 2HP 1Φ 230V

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What is the minimum ampacity of the branch circuit
conductor serving the three motors as shown
above?
10HP 3Φ 480V
Squirrel Cage
25HP 3Φ 480V
Squirrel Cage
30HP 3Φ 480V
Synchronous

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430.250: FLI’s, 3-phase, 480v
10hp Squirrel cage = 14 FLI
25hp Squirrel cage = 34 FLI
30hp Synchronous = 32 FLI
34 X 125% + 14 + 32 = 88.5 amps
10HP 3Φ 480V
Squirrel Cage
25HP 3Φ 480V
Squirrel Cage
30HP 3Φ 480V
Synchronous

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What Size THWN conductors are required for:
1: Ungrounded Conductors
2: Grounded Conductor
3: Grounding Conductor
88.5A
T.310.15(b)(16) & 110.14(c) = #2 THWN Ungrounded 60°C
No Grounded Conductor Required
T.250.122 Grounding Conductor (EGC) = #8 Cu or #6 Al
Design
A
10HP 3Φ 480V
Squirrel Cage
25HP 3Φ 480V
Squirrel Cage
30HP 3Φ 480V
Synchronous

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What is the minimum size branch circuit conductors supplying the motor and
incandescent lighting above ?
430.248: 1hp @ 120v = 16 amps
Lighting: I=W / E = 6000 / 120 = 50 amps
430.24(a): Ampacity = motor FLI X 125% + Lighting I
16 X 125% + 50 = 70 amps
#4 THHN Cu (60°C)
6000 watts
Incandescent Lighting
1HP 1Φ 120V
FLI 12.4

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81.25
26.25
123.25

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Time Delay Fuses 20HP 3Φ 480V
FLI 29

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Time Delay
Fuses
20HP 3Φ 480V
FLI 29
T.430.250: 20hp = 27 amps
T.430.52: Time Delay Fuse = 175%
Max. O.C. device = 27 X 175% = 47.25 amps
430.52(c)(1) exception #1: Next Larger size permitted
240.6(a) Next Larger Std size = 50 amps
Answer: 50 amp Time Delay Fuse

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Non-Time Delay
Fuses
20HP 3Φ 480V
FLI 29
T.430.250: 20hp = 27 amps
T.430.52: Non-Time Delay Fuse = 300%
Max. O.C. device = 27 X 300% = 81 amps
430.52(c)(1) exception #1: Next Larger size permitted
240.6(a) Next Larger Std size = 90 amps
Answer: 90-amp Non-Time Delay Fuse

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Inverse Time
Breaker
20HP 3Φ 480V
FLI 29
T.430.250: 20hp = 27 amps
T.430.52: Inverse Time Breaker = 250%
Max. O.C. device = 27 X 250% = 67.5 amps
430.52(c)(1) exception 1: Next Larger size permitted
240.6(a) Next Larger Std size = 70 amps
Answer: 70-amp Inverse Time Circuit Breaker

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Selecting Minimum Size Overloads 430.32(A)(1)
n Service Factor
n Less than 1.15 use 115%
n 1.15 or greater use
125%
n No Service Factor Listed
use 115%
n Temperature Rise
n 40°C or less use 125%
n Over 40°C use 115%
n No Temp Listed use
115%

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Selecting Maximum Size Overloads 430.32(C)
n Service Factor
n Less than 1.15 use 130%
n 1.15 or greater use
140%
n No Service Factor Listed
use 130%
n Temperature Rise
n 40°C or less use 140%
n Over 40°C use 130%
n No Temp Listed use
130%

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Solution:
Welder No.1
630.11(a) Nonmotor Generator
Duty cycle 30 Multiplier = 0.55
I=RPCXM
I = 28 amps x 0.55 = 15.4 amps
Welder No.2
630.11(a) Nonmotor Generator
Duty cycle 80 Multiplier = 0.89
I=RPC xM
I = 18 amps x 0.89 = 16.02 amps
630.11(b)
Total = No.1+ No.2
Total = 15.4 + 16.02 = 31.42 amps
Ans. 31.42 amps
RPC 28A
Duty Cycle
30%
RPC 18A
Duty Cycle
80%
Motors with a Duty Cycle - Welders

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Solution:
Nameplate current rating is used.
Table 430.22(e)
FLI 15 min. periodic duty = 90%
Ampacity -Nameplate FLI x 90%
= 24.5 x 90% = 22.05 amps.
Answer: 22.05 amperes
5HP 230V 1Φ
FLI 24.5
Periodic Duty
15 Min

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Solution:
Nameplate FLI is used
430.22(e)
Continuous use, varying duty= 200%
Ampacity = nameplate FLI x 200 %
Ampacity= 16.5 x 200 = 33 amps
Answer: 33 amperes
15HP 230V 3Φ
FLI 16.5
Varying Duty

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Motor Efficiency
• Efficiency = Output / Input
• 5HP, 240V, 1Æ = 28A F.L.C.
T.430.248
• W = E X I = 240V X 28A = 6720 VA
• 6720 VA input and 5712 VA output
• 5712/6720 = .85
• 6720 VA X .85 = 5712 VA output
• 85% efficiency
No motor is 100% efficient
Efficiency = Output / Input

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Motor Duty Rating
n Intermittent Duty = T.430.22(e)
n Nameplate F.L.C. used
n 50HP,480V,3Æ, 15-minute rated elevator motor, Name Plate FLC
=61.2A
n 430.22(e) = 85%
n Name Plate FLC = 61.2 X 85% = 52.02A Branch Circuit
Ampacity
n Conductor Required = #6 (60°C column)
240.6 310.15(b)(16) 110.14
430.52

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Motor Calculations
Practice Questions
30 Minutes

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25-208 50 sync – 440 200 - 480

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Question 1 Keywords
What size short circuit/ground fault protection is
required for the branch circuit serving motor #1 in
motor diagram #1?
What size short circuit/ground fault protection is
required for the branch circuit serving motor #1 in
motor diagram #1?
We are looking for the smallest fuse or circuit breaker
required to protect motor #1 on Motor Diagram #1.

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Question 2 Keywords
What is the minimum size overloads required for motor #2 on
motor diagram #1 ?
What is the minimum size overloads required for motor #2 on
motor diagram #1 ?
We are looking for the smallest heater/overload size allowed for
motor #2 on motor diagram #1.
Overloads = name plate full load current times either 115% or
125% depending on S.F. and/or Temp Rise.

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Question 3 Keywords
What is the minimum size THHN feeder required for the six motors as
shown on Motor Diagram #1 ?
What is the minimum size THHN feeder required for the six motors as
shown on Motor Diagram #1 ?
We are looking for the smallest legally allowed THHN copper conductor
to feed these 6 motors. Feeder conductor = three steps:
1 find largest FLA motor and multiply by 1.25 (125%)
2. Add FLA of all other motors on same feeder
3. Can’t go up in size = drop down to standard at 240.6