Hone

Lessons · Electrical · motor full-load current

A motor is sized from its full-load current, and the breaker is allowed to be big

The nameplate gives full-load amps (FLA); the conductors are sized at 125 percent of it, and because a motor draws several times FLA for a moment when it starts, the breaker or fuse may be far larger than the wire's ampacity, with the motor's overload relay guarding the wire instead.

Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.

What it is for

A well pump on a breaker sized like a heater circuit trips every time it starts. A technician who does not know the motor rules puts a bigger wire in to stop the tripping; the rule that actually applies lets the breaker go up while the wire stays, because the overload in the starter is the real protection.

How to think about it

Read FLA off the nameplate (or the code table for the horsepower). Conductors: FLA × 1.25, then the wire whose ampacity covers it. Short-circuit protection: an inverse-time breaker up to 250 percent of FLA, rounded up to the next standard size when needed. Overload: set from the nameplate, 115 to 125 percent of FLA depending on the service factor. Before touching any motor terminal box: de-energize, lock out, verify.

Worked example

Nameplate: 230 V, 1 phase, 3 hp, FLA 17 A, SF 1.15
The numbers you need are all on the plate.
Conductors: 17 × 1.25 = 21.25 A, so 12 AWG (20 A) is too small; 10 AWG copper is rated 30 A
Wire covers 125 percent of FLA.
Breaker: up to 17 × 2.5 = 42.5 A, next standard size 45 A
Inverse-time breaker, 250 percent, so the start-up surge does not trip it.
Overload: 17 × 1.25 = 21.25 A, because the service factor is 1.15
The overload relay in the starter is what protects the wire from a stalled motor.

Your turn

A motor with FLA 28 A. Write the conductor sizing line.

28 × 1.25 =  A

The trap

Sizing the breaker to the wire like an ordinary circuit. A 45 A breaker on a 30 A motor circuit looks wrong to anyone who learned the earlier lesson, and it is right: on a motor circuit the overload relay protects the wire, and the breaker only has to stop a dead short.

Practise motor full-load current on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.