Motors and Motor Circuit Calculations - Dan Dudley

Copyright 1999-2019 Dan Dudley

Motors and MotorCircuit Calculations

2017 NEC Requirements


2017 NEC Motor CircuitRequirements and

Calculations


NEC Motor CalculationsBasic 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.


NEC Motor CalculationsBasic 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 allother motors supplied by the same feeder

• No provision for going UP to next standard size must go down to nextstandard size


Hz

Hp Phases FLC Voltage

TempRise

ServiceFactor

LockedRotorLetter

InsulationClass


Line side

Nameplate Data

Actuating Coil

Overload Relays

Load Side


For all othercalculations use

Table values

Copyright 1999-2019 Dan Dudley25-208 50 sync – 440 200 - 480

50 HP

Synchronous

440 Volts200 HP

480 Volts

25 HP

208 Volts


What minimum size THHN conductors are required for thismotor ?

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 FLI44


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 FLI32.5


Circuits with more than one Motor• Step 1.

• Step 2.

• Step 3.

• Decide which motor has largestFLI rating

(This may not be the largest HP)

• Largest FLI of one motor X 125%

(only increase one)

• Add FLI rating of all othermotors

(straight from tables)


What is the minimum ampacity of the branch circuit conductors for thetwo 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 208V5HP 3Φ 208V


What is the minimum ampacity of the branch circuit conductors supplying the twomotors 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


What is the minimum ampacity of the branch circuitconductor serving the three motors as shownabove?

10HP 3Φ 480VSquirrel Cage


30HP 3Φ 480VSynchronous


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





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

DesignA





What is the minimum size branch circuit conductors supplying the motor andincandescent 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 wattsIncandescent Lighting

1HP 1Φ 120VFLI 12.4


81.25

26.25

123.25


Time Delay Fuses 20HP 3Φ 480VFLI 29


Time DelayFuses 20HP 3Φ 480V

FLI 29

T.430.250: 20hp = 27 ampsT.430.52: Time Delay Fuse = 175%Max. O.C. device = 27 X 175% = 47.25 amps430.52(c)(1) exception #1: Next Larger size permitted240.6(a) Next Larger Std size = 50 amps

Answer: 50 amp Time Delay Fuse


Non-Time DelayFuses

20HP 3Φ 480VFLI 29

T.430.250: 20hp = 27 ampsT.430.52: Non-Time Delay Fuse = 300%Max. O.C. device = 27 X 300% = 81 amps430.52(c)(1) exception #1: Next Larger size permitted240.6(a) Next Larger Std size = 90 amps

Answer: 90-amp Non-Time Delay Fuse


Inverse TimeBreaker

20HP 3Φ 480VFLI 29

T.430.250: 20hp = 27 ampsT.430.52: Inverse Time Breaker = 250%Max. O.C. device = 27 X 250% = 67.5 amps430.52(c)(1) exception 1: Next Larger size permitted240.6(a) Next Larger Std size = 70 amps

Answer: 70-amp Inverse Time Circuit Breaker


Selecting Minimum Size Overloads 430.32(A)(1)n Service Factor

n Less than 1.15 use 115%

n 1.15 or greater use125%

n No Service Factor Listeduse 115%

n Temperature Rise

n 40°C or less use 125%

n Over 40°C use 115%

n No Temp Listed use115%


Selecting Maximum Size Overloads 430.32(C)

n Service Factor

n Less than 1.15 use 130%

n 1.15 or greater use140%

n No Service Factor Listeduse 130%

n Temperature Rise

n 40°C or less use 140%

n Over 40°C use 130%

n No Temp Listed use130%


Solution:

Welder No.1630.11(a) Nonmotor GeneratorDuty cycle 30 Multiplier = 0.55I=RPCXMI = 28 amps x 0.55 = 15.4 amps

Welder No.2630.11(a) Nonmotor GeneratorDuty cycle 80 Multiplier = 0.89I=RPC xMI = 18 amps x 0.89 = 16.02 amps630.11(b)Total = No.1+ No.2

Total = 15.4 + 16.02 = 31.42 ampsAns. 31.42 amps

RPC 28ADuty Cycle

30%RPC 18A

Duty Cycle80%

Motors with a Duty Cycle - Welders


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 1ΦFLI 24.5

Periodic Duty15 Min


Solution:

Nameplate FLI is used430.22(e)Continuous use, varying duty= 200%Ampacity = nameplate FLI x 200 %Ampacity= 16.5 x 200 = 33 amps

Answer: 33 amperes

15HP 230V 3ΦFLI 16.5

Varying Duty


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


Motor Duty Ratingn Intermittent Duty = T.430.22(e)n Nameplate F.L.C. usedn 50HP,480V,3Æ, 15-minute rated elevator motor, Name Plate FLC

=61.2An 430.22(e) = 85%n Name Plate FLC = 61.2 X 85% = 52.02A Branch Circuit

Ampacityn Conductor Required = #6 (60°C column)

240.6 310.15(b)(16) 110.14430.52


Motor CalculationsPractice Questions

30 Minutes


Question 1 Keywords

What size short circuit/ground fault protection isrequired for the branch circuit serving motor #1 inmotor diagram #1?

What size short circuit/ground fault protection isrequired for the branch circuit serving motor #1 inmotor diagram #1?

We are looking for the smallest fuse or circuit breakerrequired to protect motor #1 on Motor Diagram #1.


Question 2 Keywords

What is the minimum size overloads required for motor #2 onmotor diagram #1 ?

What is the minimum size overloads required for motor #2 onmotor diagram #1 ?

We are looking for the smallest heater/overload size allowed formotor #2 on motor diagram #1.

Overloads = name plate full load current times either 115% or125% depending on S.F. and/or Temp Rise.


Question 3 Keywords

What is the minimum size THHN feeder required for the six motors asshown on Motor Diagram #1 ?

What is the minimum size THHN feeder required for the six motors asshown on Motor Diagram #1 ?

We are looking for the smallest legally allowed THHN copper conductorto 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

Motors and Motor Circuit Calculations - Dan Dudley

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