NEC Code Questions Test-2: Motor Circuit Calculations
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Free Online NEC Code Questions : Motor Circuit Calculations (NEC 430)
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Question 1 of 10
1. Question
1 pointsA 5 HP single phase motor is connected to a 240 volts power supply. What is the minimum size branch circuit conductors required for this motor?
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Refer: NEC Table 430.248
FLI of 5 HP motor = 28 A
Refer : NEC 430.22(a)
Branch Conductor Ampacity = 28 * 125% = 35 amperes
Minimum Conductor size : No 8 THWN
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Question 2 of 10
2. Question
1 pointsA 4 hp 230 V single phase motor has a service factor of 1.15 and name plate current rating of 25 A. What size dual element fuse can be used to protect the motor from overloads, short circuit and ground faults?
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The motor name plate rating = 25 A
Refer NEC 430.32(A)(1) The overload protection of motor with service factor 1.15 and above shall have protective device with 125% of rated current.
Overload protection device rating = 25 * 1.25 = 31.25A
Refer NEC 240.6(A), the standard fuse ratings that can be used. In this case 30A fuse can be used to protect the motor
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Question 3 of 10
3. Question
1 pointsWhat size branch circuit conductor that can be used for a 5 HP, 230 volts three phase motor with terminals rated at 750 C?
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Refer: NEC Table 430.250
For alternating current motor of 5HP , 230 volts the permitted FLC of motor is 15.2 A.
Branch circuit rating (continuous load) = 15.2 * 1.25 = 19 A
Refer: NEC Table 310.15(B)(16)
14 AWG with 20 A at 75 0C is suitable branch conductor for the three phase motor.
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Question 4 of 10
4. Question
1 pointsWhat size inverse time circuit breaker is required for a 10HP, 230 volts three phase motor operated at 750C?
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The branch circuit protections (inverse time circuit breaker) should be 250 % of FLC of motor.
Refer: NEC Table 430.250
For a 10 HP, 230 Volts three phase motor the FLC is 28 A
Branch circuit protection = 28 * 2.5 = 70 A
Refer NEC 240.6(A) ( Standard Trip Circuit breaker ratings)
70 A circuit breaker can be used to protect the motor.
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Question 5 of 10
5. Question
1 pointsThe following motors are fed from a single phase motor. What size feeder protection is required for the motors?
- 10 HP, 230 volts three phase motor
- 15 HP, 230 volts three phase motor.
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Refer: NEC 430.250 ( FLC of three phase motors)
10 HP , 230 volts FLC = 28 A
15 HP, 230 volts FLC = 42 A
Branch circuit protection ( 10 HP Motor) = 28 * 2.5 = 70 A
Branch circuit protection (15 HP Motor) = 42 * 2.5 = 105
Refer NEC 430.62(A)
The feeder protection should not be greater than the largest branch circuit OCPD plus FLC of other motors in the group.
The feeder protection not more than = 105 + 70 = 175 A
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Question 6 of 10
6. Question
1 pointsWhat size feeder conductor can be used for the following motors operating at 750 C?
- 15 HP , 460 V Three phase motor
- 10 HP, 460 V Three phase motor
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Refer: NEC 430.250 ( FLC of three phase motors)
10 HP , 460 volts FLC = 14 A
15 HP, 460 volts FLC = 21 A
Refer: NEC 430.24
The feeder conductor size is sum of 125% of highest rated motor FLC plus all other motors FLC.
So feeder conductor size = 21 * 1.25 + 14 = 40.25 A
8 AWG with 50 A is suitable as feeder conductor.
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Question 7 of 10
7. Question
1 pointsWhich of the following motor is highest rated motor?
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3 HP, 208 volts Three phase motor FLC = 10.6 A [NEC Table 430.250]
10 HP, 230 volts Three Phase motor FLC = 28 A
10 HP, 460 volts Three Phase motor FLC= 14 A
3 HP , 115 volts Single phase motor FLC = 34 A [ NEC Table 430.248]
Highest rated motor is one with highest FLC. So 3 HP , 115 volts Single phase with 34 A FLC is highest rated motor.
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Question 8 of 10
8. Question
1 pointsWhat is the size of the branch circuit conductor that can supply 3 HP 115 volts single phase motor with terminals operating at 600 C?
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Incorrect
Refer: NEC Table 430.248
3 HP, 115 volts single phase FLC = 34 A
Refer NEC Table 310.15(B)(16) at 600C column
8 AWG with 40 A rating at 600C is suitable size.
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Question 9 of 10
9. Question
1 pointsA 5 HP Three phase motor with a name plate FLA of 25 A is used for intermittent duty and rated for 5 minute service. What size branch circuit conductor can be used for the motor when the terminals are rated at 75 0C?
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For intermittent duty applications FLA should be used in branch circuit calculations
Refer: NEC Table 430.22(E)
For 5 min rated service the branch circuit should be rated to 85% of FLA
25 * 0.85 = 21.25 A
Refer: NEC Table 310.15(B)(16)
12 AWG with 25 A at 75 0C is suitable as branch circuit conductor.
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Question 10 of 10
10. Question
1 pointsMotor branch circuit protection devices can also protect the motor against overloads.
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Incorrect
Motor branch circuit protection devices can protect the motor from short circuit and ground fault but cannot protect from overload. [NEC 430.51]
Thank you very much.. For this quiz I learned and developed my idea and knowledge about electrical.. Very helpful to me..