Motor & switching
Motor Overload Relay Setting Calculator
Turn the motor nameplate current, service factor, and starting duty into a preliminary thermal-overload dial setting and trip class for a contactor-mounted overload relay. The nameplate current governs — these are starting points to be trimmed on commissioning.
Setting: dial to the motor, not the cable
A thermal overload relay protects the motor windings, so it is set from the motor’s actual full-load current, taken from the nameplate. The service factor sets the allowable margin above it:
Start at the FLC on the dial and trim upward within the range only if the motor nuisance-trips at a genuinely acceptable load. Setting above the range removes winding protection; setting far below it causes nuisance trips.
Trip class: match the start time
| Class | Trip time at 7.2 × setting | Use |
|---|---|---|
| Class 10 | ≤ 10 s | Normal starting — pumps, fans, standard loads |
| Class 20 | ≤ 20 s | Heavy starting — loaded conveyors, mixers |
| Class 30 | ≤ 30 s | Very heavy / high-inertia — crushers, centrifuges |
The class must let the motor accelerate without tripping, yet still protect the windings during a stalled or locked-rotor condition. A long-starting motor on a Class 10 relay trips on every start; too high a class on a quick-starting motor slows protection.
Frequently asked questions
Do I use the nameplate current or the cable rating?
Always the motor nameplate FLC. The overload protects the motor; the cable and short-circuit protection are handled separately by the breaker or fuse.
My motor trips during start — raise the setting?
Usually no. First check the trip class matches the start time — a heavy or high-inertia load often needs Class 20 or 30. Only trim the current setting within its range, and never above the service-factor limit.
Electronic overload or bimetallic?
Electronic overloads offer wider setting ranges, selectable classes, phase-loss and phase-imbalance protection, and better temperature stability. Bimetallic relays are simple and economical for standard duties.
Build the starter
Pair the overload relay with a correctly rated AC contactor and short-circuit protection for a complete motor starter.
Preliminary setting for guidance only. The final overload setting and trip class must be confirmed by a qualified person against the motor nameplate, the driven load, and the relay manufacturer’s data, then verified on commissioning.