Why is overload protection based on FLA and not the NEC table FLC?

Running overload protection guards the actual motor, so NEC 430.6(A)(2) requires it to be sized from the motor nameplate full-load amperes. Branch-circuit short-circuit and ground-fault protection, by contrast, uses the larger NEC table full-load current value.

When do I use 125% versus 115%?

Use 125% of FLA for motors marked with a service factor of 1.15 or higher, or a temperature rise of 40°C or less, because these motors tolerate more current. All other motors use 115% of FLA, which is the more conservative limit.

What is the 140% or 130% maximum for?

If the overload device set at the nominal value trips during normal starting or running, NEC 430.32(C) permits raising it to a maximum of 140% of FLA for the higher class motors, or 130% for the others. You never exceed these ceilings.

Does this size the fuses or breaker too?

No. This calculates only the running overload, which protects the motor from sustained overcurrent. The branch-circuit fuses or breaker that protect against short circuits and ground faults are sized separately under NEC 430.52 and are much larger.

Does this apply to single-phase and three-phase motors?

Yes. The 430.32 percentage rules apply to both single-phase and three-phase continuous-duty motors over 1 HP. You enter the nameplate FLA for whichever motor you have, and the same factor is applied.

Motor Overload (OL) Heater Sizing Calculator

Running overload protection guards a motor against sustained overcurrent that would overheat its windings, and the NEC sizes it from the motor nameplate full-load amperes. This calculator applies the NEC 430.32 percentages, 125% or 115% depending on the motor’s service factor and temperature rise, and shows both the nominal trip current and the 430.32(C) maximum for single-phase and three-phase motors.

How it works

The overload trip is a percentage of the nameplate FLA, never the NEC table FLC:

service factor ≥ 1.15  OR  temp rise ≤ 40°C   →  nominal = FLA × 125%, max = FLA × 140%
all other motors                              →  nominal = FLA × 115%, max = FLA × 130%

You set the relay or heater so it trips at or just above the nominal value, and you may raise it toward the maximum only if nuisance tripping occurs during normal operation.

Example and tips

A 10 HP three-phase motor with a 28 A nameplate FLA and a 1.15 service factor gets 28 × 1.25 = 35 A nominal, with a ceiling of 28 × 1.40 = 39.2 A. A motor without that service factor would use 28 × 1.15 = 32.2 A nominal. Tips: always read the FLA off the motor nameplate rather than a table, since the two values differ; the overload protects the motor and is distinct from the much larger short-circuit branch protection; and only step up toward the maximum if the motor genuinely cannot start at the nominal setting.