Overload sizing (NEC)
Where to set a motor overload relay: at the nameplate full-load amps, and no higher than the code maximum — 125% of FLA for a 1.15 service factor, 115% otherwise.
Motor overload relay setting · NEC 430.32
Set to FLA; max = 125% (SF ≥ 1.15) or 115% (SF < 1.15) — NEC 430.32
Trip class — 10 = tight/submersible, 20 = general purpose, 30 = high-inertia loads
What this gives you
A motor overload relay protects the winding from a slow, running overcurrent — a jammed conveyor, a failing bearing, a plugged impeller — not from a bolted short circuit, which is the breaker's job. You dial the relay to the motor's nameplate full-load amps (FLA), and the trip point must not be set above the code ceiling. Under NEC 430.32(A)(1) that ceiling is 125 percent of FLA for a motor marked with a service factor of 1.15 or greater, or a temperature rise of 40 °C or less, and 115 percent for every other motor. This tool takes the nameplate FLA and the service-factor class and returns the maximum amps you're allowed to set, plus the FLA starting point.
Set at FLA, capped at the code maximum
The dial setting and the code maximum are two different numbers, and confusing them is where people get into trouble. You start at the nameplate FLA — that is what the relay's overload element is sized around. The maximum is 125 or 115 percent of that FLA, and it is a hard ceiling, not a target. A motor with a 1.15 service factor runs a little hotter than its base rating and tolerates the higher 125 percent band; a 1.0 service-factor motor gets the tighter 115 percent so it isn't cooked at sustained overload. Always read the FLA off the motor itself, not from NEC Table 430.250 — the table value is deliberately high for sizing wire and breakers and will let the overload drift too loose.
Field note — nuisance trips and the 430.32(C) bump
If the overload trips on a legitimately hard start or a heavy but normal load, NEC 430.32(C) lets you raise the maximum to 140 percent (for the 1.15 SF / 40 °C class) or 130 percent (all others) — and no higher. That is a documented exception for a motor that won't start or carry its load at the base setting, not a free pass to keep bumping the dial every time it trips. If you're reaching for 140 percent, first rule out a mechanical problem, a low-voltage condition, or a wrong trip class on the relay.
Overload is not short-circuit — the 430.52 branch device
The overload relay handles the slow running overcurrent. The branch-circuit short-circuit and ground-fault device — the breaker or fuse ahead of the starter — handles the fast, high-magnitude fault, and NEC 430.52 sizes it off the same FLA but with much larger multipliers. The tool reports all four common device types as code maximums: an inverse-time (thermal-magnetic) breaker at 250 percent of FLA, a non-time-delay fuse at 300 percent, a dual-element (time-delay) fuse at 175 percent, and an instantaneous-trip breaker at 800 percent. These are ceilings, not target settings: in practice you round down to the next standard rating from NEC 240.6, and 430.52(C)(1) Exception 1 lets you step up to the next standard size only when the calculated value doesn't correspond to one and the motor won't start otherwise.
Trip class — how long the relay tolerates locked-rotor current
The trip class is the maximum time, in seconds, a relay will carry roughly 600 percent of its current rating before tripping: Class 10 trips within 10 seconds and suits tight, low-thermal-mass loads like submersible pumps; Class 20 is the general-purpose default; Class 30 tolerates up to 30 seconds for high-inertia loads — large fans, centrifuges, loaded conveyors — that take a long time to come up to speed. Pick the class that lets the motor start without nuisance-tripping, but no looser: a Class 30 relay on a submersible pump can let the winding cook on a locked rotor.
Worked example
A motor with a nameplate FLA of 28 A and a 1.20 service factor falls in the 1.15-or-greater class, so the overload multiplier is 125 percent: 28 × 1.25 = 35.0 A maximum, with the relay dialed to the 28 A FLA to start. The same motor built at 1.0 service factor would use 115 percent: 28 × 1.15 = 32.2 A. For the branch device on that 28 A motor, 430.52 caps an inverse-time breaker at 28 × 2.5 = 70 A, a non-time-delay fuse at 84 A, a dual-element fuse at 49 A, and an instantaneous-trip breaker at 224 A — then round down to a standard size. Set the overload at FLA, verify the running current with a clamp meter, and only move toward the ceiling if the motor genuinely needs it.