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Motor Overload, Fuse & Contactor Selection
Get indicative thermal overload setting, fuse and AC-3 contactor sizing for a three-phase electric motor (DOL start) from its power.
Full-load current Iₙ: — A
Thermal overload setting: — A
Fuse (gG, DOL): — A
Contactor (AC-3) ≥: — A
Iₙ = P / (√3 · U · cosφ · η)
⚠️ Indicative guidance for standard DOL start only. Final selection must follow the manufacturer data and IEC/local standards — confirm with an electrical engineer. Star-delta / soft starter / VFD change these values.
How sizing works
The thermal overload relay is set to the motor full-load current (Iₙ). The fuse must withstand the starting inrush (~6–8×Iₙ for a few seconds), so a gG fuse around ~1.6–2×Iₙ is typical for DOL. The contactor is chosen by its AC-3 rating ≥ Iₙ. Related: full-load current & cable size calculators.
Frequently asked questions
How do I set the thermal overload relay?
Set it to the motor rated full-load current (Iₙ) shown on the nameplate; the calculator estimates Iₙ from power. Fine-tune within the relay range to the actual running current.
What fuse size for a motor?
A motor fuse must ride through the starting inrush. For a direct-on-line start a gG fuse around 1.6–2×Iₙ is typical; aM (motor) fuses can be closer to Iₙ. Always check the device tables.
How is the contactor selected?
By its AC-3 (motor) rating, which must be ≥ the motor full-load current at the operating voltage, considering the motor kW too.
Does star-delta change these?
Yes. Star-delta, soft starters and VFDs reduce starting current, so overload/fuse/contactor selection differs from direct-on-line.