Thermal overload relay calculator
Pick the right thermal overload relay (bimetallic / electronic) for a three-phase motor from its full-load current and starting duty.
Full-load current from nameplate
Formula: I = (kW × 1000) / (√3 × V × PF × η) — fills the relay sizing below whenever all four values are valid.
Overload relay setting
Starting duty
Ambient temperature
Range: normal duty 0.9 × FLA … 1.1 × FLA, frequent starts 1.0 × FLA … 1.2 × FLA. Class 10 trips within ~2 minutes at 1.2 × FLA and in under 10 s at 7.2 × FLA; Class 20 tolerates longer starting times. The ambient factor derates the relay’s thermal rating — at 50°C it can carry only 90% of its nominal current continuously (NEC 430.32 / IEC 60947-4-1).
The overload relay protects against sustained overcurrent, not short-circuits — that is the MCCB or fuse job. Match the relay’s motor-terminal FLC range to the setting range above, and confirm the derated FLA still brackets the motor nameplate current at the installation ambient. AO Ctrl stocks the motor starters, overload relays and contactors for industrial panels:
Frequently asked questions
What setting range should a thermal overload relay use for a motor?
For normal (non-frequent) starting duty the tool sets the range at 0.9 × FLA to 1.1 × FLA, and for frequent starts 1.0 × FLA to 1.2 × FLA. With the default 15 kW / 400 V motor the calculated FLA is 28.3 A, so the normal-duty range is 25.5–31.1 A. Pick the relay whose motor-terminal range brackets the FLA and stays below the locked-rotor figure (6 × FLA here).
How is the motor full-load current worked out for relay sizing?
When power, voltage, PF and efficiency are all entered, the tool computes FLA = (kW × 1000) / (√3 × V × PF × η) and uses it in place of a manual entry. The default 15 kW, 400 V motor at PF 0.85 and η 0.90 computes to about 28 A. A nameplate FLA you enter directly also works.
What do overload relay trip classes 10 and 20 mean?
Class 10 trips within roughly 2 minutes at 1.2 × FLA and in under 10 s at 7.2 × FLA; Class 20 tolerates longer starting times, which is why the tool recommends Class 10 for normal starts and Class 20 for frequent starts. The ambient-temperature factor also derates the relay — at 50 °C it can carry only 90% of its nominal current continuously (NEC 430.32 / IEC 60947-4-1).
AO Ctrl provides this tool for reference only.