Motor overload sizing
The overload protects the motor; the breaker protects the wire — size one like the other and motors either nuisance-trip all day or quietly burn up. Six motors, four reads — nameplate FLA, setpoint, running current, trip class — including the two faults that look identical until you clamp the running current: a wrong setpoint versus a real overload.
Before you start — job brief
An overload relay protects the motor from a sustained overcurrent — it does not protect against a short circuit; that is the breaker or fuses ahead of it. Sizing is a code exercise: set the overload to the motor's full-load amps (FLA), never above the maximum the code allows (125% of FLA for a service factor of 1.15 or more). Read the nameplate, not your memory.
- Overload = running-overcurrent protection · Breaker/fuses = short-circuit & ground-fault
- Set the overload to the nameplate FLA; the code max is 125% of FLA (115% if SF < 1.15)
- Trip class is the ride-through: Class 10 trips fast, Class 20/30 let a high-inertia load accelerate
- Read four things: nameplate FLA/SF, the setpoint, the running current, and the class vs. the start
Motor starter · check nameplate, setpoint, running current and class
Read the nameplate, the setpoint, the running current, and the class.
Bench check
Readings taken
- No readings yet.
Your diagnosis
The overload has one job, and it isn't the breaker's
The most common mistake in motor protection is confusing the two devices in the starter. The breaker or fuses ahead of the contactor protect the wiring from a short circuit or ground fault — a fast, huge fault current. The overload relay protects the motor from a sustained running overcurrent — a slow cook, ten or twenty percent over rating for minutes at a time. They trip on completely different things, and sizing one like the other is how motors either nuisance-trip all day or quietly burn up. This trainer is only about the overload.
Sizing is a window, not a number
Set the overload to the motor's full-load amps (FLA) off the nameplate — not a table, not the last motor, the nameplate in front of you. That is the target. The code then gives you a ceiling: for a motor with a service factor of 1.15 or more, the overload may be set no higher than 125% of FLA; for a service factor under 1.15 (or a marked temperature rise over 40 °C), the ceiling is 115%. So a 28-amp motor is set at 28, and may be trimmed up to 35 if — and only if — it nuisance-trips for a real reason like a hot day. Above 35 the motor is no longer protected. Set to FLA first; move toward the ceiling only with a reason; never go past it.
Field note — the two faults that look identical
A motor trips its overload warm, a few minutes after load. There are two completely different causes, and the fix for one is the disaster for the other. If the running current is below the service-factor rating, the motor is healthy and the setpoint is too low — raise it to FLA. If the running current is above the service-factor rating, the motor is genuinely overloaded by something mechanical — and raising the relay would only let it cook. Same complaint, opposite fix. Always clamp the running current and compare it to the rating before you touch the dial. The relay setting tells you if the protection is wrong; the running current tells you if the motor is wrong.
Trip class — the ride-through
Sizing the current is only half of it. The trip class is how long the overload will tolerate locked-rotor current before it trips: a Class 10 trips within 10 seconds, Class 20 within 20, Class 30 within 30. A motor pulls six-or-more times FLA while it accelerates, and the relay has to ride through that inrush without tripping. A small pump up to speed in a couple of seconds is happy on a Class 10. A big centrifugal fan or a loaded flywheel that takes fifteen or twenty seconds to accelerate will trip a Class 10 on every start — not because the current setting is wrong, but because the class is too fast. The cure is a Class 20 or 30, never a higher current setting. If a motor trips only on starting and runs fine once up to speed, look at the class before anything else.
Single-phasing — the quiet killer
Lose one of the three supply legs — a blown fuse, an open contactor pole, a lost utility phase — and the motor tries to carry its whole load on the two remaining legs. The current on them jumps to roughly 1.7 times normal while the dead leg reads zero, and the motor overheats fast. A clamp on all three legs shows it instantly: two high, one at zero. A plain three-heater thermal overload can be slow to catch this, which is why modern electronic overloads include phase-loss protection. On any motor that matters, that protection is worth having — single-phasing has killed more windings than almost anything else.
The discipline
Four reads name every fault in this trainer. The nameplate sets the window (FLA to 125% of FLA). The setpoint has to sit inside it. The running current, against the service-factor rating, separates a wrong setting from a real overload. And the class has to cover the acceleration. Read all four, compare each to its spec, and the motor tells you exactly what it needs — instead of getting a dial cranked up until the tripping stops and the protection is gone.