NEC Motor Calculation Practice Questions

One horsepower is defined as 746 watts.

Memorize 1 hp = 746 W, then multiply any horsepower by 746 (e.g., 10 hp = 7,460 W). Write it on your test-day paper.

1. Using 550 — that's ft-lb per second, not electrical watts.
2. Confusing mechanical hp output with the electrical VA the motor actually draws.

The conductor to a single motor = 125% of the table FLC (not the nameplate).

Table FLC (not nameplate) × 1.25, then find the wire that carries it in the terminal's temperature column (75 °C here).

1. Using nameplate amps.
2. Using the single-phase table.
3. Reading the 90 °C column instead of 75 °C.

Convert with 746 W per horsepower: watts = hp × 746, or hp = watts ÷ 746. For actual current draw, single-phase FLA = (hp × 746) ÷ (E × efficiency × power factor); add × 1.732 for three-phase.

1. Using 550 — that is ft-lb per second, not electrical watts.
2. Dropping efficiency or power factor from the FLA formula.
3. Forgetting the 1.732 on a three-phase FLA.

Convert with 746 W per horsepower: watts = hp × 746, or hp = watts ÷ 746. For actual current draw, single-phase FLA = (hp × 746) ÷ (E × efficiency × power factor); add × 1.732 for three-phase.

1. Using 550 — that is ft-lb per second, not electrical watts.
2. Dropping efficiency or power factor from the FLA formula.
3. Forgetting the 1.732 on a three-phase FLA.

Motor FLA from nameplate hp

Convert with 746 W per horsepower: watts = hp × 746, or hp = watts ÷ 746. For actual current draw, single-phase FLA = (hp × 746) ÷ (E × efficiency × power factor); add × 1.732 for three-phase.

1. Using 550 — that is ft-lb per second, not electrical watts.
2. Dropping efficiency or power factor from the FLA formula.
3. Forgetting the 1.732 on a three-phase FLA.

Rule 430.32(A)(1): a continuous-duty motor over 1 hp with a service factor of 1.15 or more gets overload protection at 125 % of the nameplate full-load amps.

430.32(A)(1) — overload sizing

Overload is sized from the NAMEPLATE full-load amps: 125 % for a motor with a service factor of 1.15+ or a 40 °C temperature rise, otherwise 115 %.

1. Using the table FLC — overload uses the nameplate FLA.
2. Using 115 % when the service factor is 1.15 or more (it is 125 %).
3. Confusing the overload with the branch short-circuit / ground-fault device.

#12 copper is rated 20 A, which covers the required 17 A.

430.22(E) — duty-cycle conductors

Size the wire at 125 % of the motor's TABLE full-load current (FLC from 430.247–430.250, not the nameplate), then pick the conductor from Table 310.16 in the terminal's temperature column.

1. Using nameplate amps — the table FLC sizes the conductor.
2. Using the single-phase table for a three-phase motor (or the reverse).
3. Skipping the 125 %.

430.24 — motor feeder conductors

Feeder ampacity = 125 % of the LARGEST motor's FLC + 100 % of every other motor's FLC. Only one motor (the largest) gets the 25 % bump.

1. Applying 125 % to every motor — only the largest.
2. Using nameplate amps instead of table FLC.
3. Confusing 430.24 (conductors) with 430.62 (feeder overcurrent device).