The overload, sized from the nameplate
What you'll learn
Set the one device that follows the actual machine: 125% with service-factor headroom, 115% without — and why a tripping overload is a message, not a nuisance.
Every number so far came from Table 430.250. The overload is the exception — the one device sized from the nameplate of the actual motor on the floor (430.6(A)), because it protects that motor, not a statistical one.
The rule: 430.32(A)(1)
For a continuous-duty motor over 1 HP, the overload trips at:
| the motor's nameplate says | OL setting |
|---|---|
| service factor 1.15 or more | 125% of nameplate FLA |
| temperature rise 40°C or less | 125% |
| anything else (SF 1.0, no rating) | 115% |
Service factor is the manufacturer's honesty rating: SF 1.15 means "this motor can run 15% over nameplate continuously without damage," so the code lets its overload sit higher. An SF 1.0 motor has no headroom — its overload hugs tighter at 115%.
The compressor
The air compressor is 25 HP at 480 V — table FLA 34 A — but it's an SF 1.0 machine:
- overload max = 115% × 34 = 39.1 A
- if it were SF 1.15: 125% × 34 = 42.5 A
Tap the starter and overload in the one-line, then flip the service factor checkbox in the inputs and watch only one number move. The breaker, the wire, the disconnect — all table-driven — don't care. The overload does.
Field notes
- At install, you set heaters or dial an electronic OL from the nameplate you can read on the machine — never from memory, never from the drawing.
- An overload that trips repeatedly is a message, not a nuisance: jammed load, low voltage, single-phasing, dying bearings. Bumping the dial up to "fix" it is how motors end up as rewinds.
One device left on the one-line — the disconnect — and one big idea: what voltage does to money.
Check your understanding
Question 1 of 2
The overload is the one device sized from the nameplate instead of Table 430.250. Why?