NEC 2023 · checked by a second reviewer · November 7, 2026
NEC Article 430 Practice Questions (10 Free, With Answers)
Motors: 10 original questions on motor full-load currents, conductor sizing, overloads and short-circuit protection. Click an answer to see the explanation and the NEC section. No sign-up.
NEC Article 430 · 10 questions0 / 10 answered
For a general-purpose 3-phase AC motor, which current is used to size the branch-circuit conductors and the short-circuit and ground-fault protection?
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Correct answer: A. Conductor ampacity and short-circuit/ground-fault protection are based on the table full-load current (such as Table 430.250 for 3-phase motors), not the nameplate. The nameplate current is tempting because it is the real motor, but it is used for sizing separate overload protection.Reference: NEC 430.6(A)
A motor has a nameplate full-load current of 20 A, a service factor of 1.0, and no temperature-rise marking. What is the maximum rating of a separate overload device under the basic rule?
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Correct answer: B. Motors without a service factor of 1.15 or more and without a temperature rise of 40°C or less are limited to 115% of nameplate current: 20 A x 1.15 = 23 A. 25 A is tempting because 125% applies to motors with a 1.15 service factor or a low temperature rise.Reference: NEC 430.32(A)(1)
A motor has a nameplate full-load current of 30 A and a service factor of 1.15. The overload selected under the basic rule trips while starting the motor. What is the maximum trip current of a larger overload relay?
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Correct answer: C. When the basic overload is not enough to start or carry the load, a larger one is allowed up to 140% of nameplate current for motors with a 1.15 or higher service factor: 30 A x 1.40 = 42 A. 39 A uses the 130% value that applies to all other motors, and 37.5 A is the basic 125% value that already failed.Reference: NEC 430.32(C)
A 10-hp, 230-volt, 3-phase motor has a table full-load current of 28 A. It is protected by dual-element time-delay fuses. Using the basic percentage and the next-higher-standard-size allowance, what is the largest fuse permitted?
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Correct answer: D. For dual-element time-delay fuses the basic limit is 175% of table FLC: 28 A x 1.75 = 49 A. 49 A is not a standard size, so the next higher standard size, 50 A, is allowed. 70 A is tempting but comes from the 250% value for inverse time circuit breakers.Reference: NEC 430.52(C)(1) and Table 430.52(C)(1); NEC 240.6(A)
A 10-hp, 460-volt, 3-phase motor has a table full-load current of 14 A. It is protected by nontime-delay fuses. Using the basic percentage and the next-higher-standard-size allowance, what is the largest fuse permitted?
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Correct answer: A. Nontime-delay fuses are allowed up to 300% of table FLC: 14 A x 3 = 42 A. 42 A is not a standard size, so the next higher standard size, 45 A, may be used. 42 A is tempting, but no 42-A fuse exists.Reference: NEC 430.52(C)(1) and Table 430.52(C)(1); NEC 240.6(A)
A motor disconnecting means must be 'in sight from' the motor. What does 'in sight from' mean in the NEC?
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Correct answer: B. 'In sight from' means one piece of equipment is visible from the other and no more than 50 ft away. Being in the same room is not enough if the distance is greater, and 25 ft has no role in this definition.Reference: NEC Article 100, In Sight From; NEC 430.102
A 3-phase motor has a table full-load current of 40 A. What is the minimum ampere rating of its disconnecting means (in addition to a suitable horsepower rating)?
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Correct answer: C. For motor circuits of 1,000 V or less, the disconnecting means must have an ampere rating of at least 115% of the motor's full-load current: 40 A x 1.15 = 46 A. 50 A is the 125% figure used for conductor sizing, so it is the most tempting wrong answer.Reference: NEC 430.110(A)
A feeder supplies three 230-volt, 3-phase motors. The largest motor (FLC 28 A) has a 70-A inverse time breaker as its branch-circuit protection. The other two motors have FLCs of 15.2 A and 9.6 A. What is the largest standard feeder breaker permitted?
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Correct answer: D. The feeder device may not exceed the largest branch-circuit protective device plus the FLCs of the other motors: 70 + 15.2 + 9.6 = 94.8 A. There is no round-up allowance here, so the next lower standard size, 90 A, is the maximum. 100 A is tempting because the branch-circuit rule allows rounding up, but the feeder rule does not.Reference: NEC 430.62(A); NEC 240.6(A)
Unless protected by other approved means, how many overload units are required for a 3-phase AC motor?
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Correct answer: B. A 3-phase AC motor needs three overload units, one in each phase conductor. Two units is tempting because some older installations have only two, but with two units a loss of one phase (single-phasing) can overload the motor without tripping.Reference: NEC 430.37 and Table 430.37
A 15-hp motor is controlled by a magnetic motor controller (not an inverse time breaker or molded case switch). What horsepower rating must the controller have at the application voltage?
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Correct answer: D. Controllers other than inverse time circuit breakers and molded case switches must have a horsepower rating at the application voltage at least equal to the motor horsepower. An ampere rating alone is tempting, but horsepower ratings account for the high inrush current a controller must make and break.Reference: NEC 430.83(A)(1)
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