Motors & generators: Texas Journeyman Electrician Practice Questions

Branch-circuit conductors supplying a single continuous-duty motor must have an ampacity of at least what percentage of the motor's full-load current?

Single-motor branch-circuit conductors are sized at 125% of the motor's full-load current.

When sizing conductors for a general-purpose motor, the full-load current is taken from:

Conductors, short-circuit and ground-fault protection use the full-load current from the Code tables; the nameplate current is used for overload protection.

For a motor with a marked service factor of 1.15 or more, separate overload protection is generally sized at no more than what percentage of the nameplate full-load current?

With a service factor of 1.15 or more (or a temperature rise of 40 °C or less), overloads are sized at 125% of nameplate current.

Which NEC article covers generators?

Generators are in Article 445. 430 is motors, 440 is air-conditioning and refrigerating equipment, 450 is transformers.

A single continuous-duty motor has a full-load current of 28 A (from the Code table, given). What minimum ampacity must its branch-circuit conductors have?

Multiply the table full-load current by 125%.

A 10-hp, 460-V, three-phase motor has a nameplate current of 13 A. Which current is used to size its branch-circuit conductors?

Section 430.6(A)(1) requires the table full-load current (not the nameplate) for sizing conductors, switches and short-circuit and ground-fault protection of general motors.

Which current is used to size the separate overload protection of a general-purpose motor?

Section 430.6(A)(2) bases separate motor overload protection on the motor nameplate current rating.

Which NEC table gives full-load currents for three-phase alternating-current motors?

Table 430.250 covers three-phase ac motors; 430.248 is single-phase, 430.52 gives protective device percentages and 310.16 gives conductor ampacities.

Which NEC table gives full-load currents for single-phase alternating-current motors?

Table 430.248 covers single-phase ac motors; 430.247 covers dc motors and 430.7(B) gives locked-rotor code letters.

Full-load currents for direct-current motors are found in which NEC table?

Table 430.247 lists full-load currents for dc motors; 430.251(B) converts horsepower to locked-rotor current.

A single continuous-duty motor has a table full-load current of 52 A. What minimum ampacity must its branch-circuit conductors have?

Section 430.22 requires 125% of the full-load current: 52 A × 1.25 = 65 A.

Conductors supplying several motors must have an ampacity of at least:

Section 430.24 requires 125% of the highest-rated motor's full-load current plus the sum of the other motors' full-load currents (and other loads as required).

When sizing conductors for several motors, how does the NEC decide which motor is the highest rated?

Section 430.17 defines the highest-rated motor as the one with the highest rated full-load current, not the highest horsepower.

A motor has a service factor of 1.0 and no temperature-rise marking of 40°C or less. Its separate overload device may be rated no more than what percentage of the nameplate full-load current?

Section 430.32(A)(1) allows 125% for a service factor of 1.15 or more or a temperature rise of 40°C or less, and 115% for all other motors.

A motor is marked with a temperature rise of 40°C. Its separate overload device may be rated no more than what percentage of the nameplate full-load current?

Under 430.32(A)(1), a marked temperature rise of 40°C or less allows overloads up to 125% of nameplate current.