Motor circuit breaker & fuse sizing calculator
Size the branch-circuit protection for a three-phase motor: enter FLA and starting current, and read the recommended inverse-time breaker, non-time-delay fuse and overload settings.
Branch-circuit protection
Locked-rotor current (% of FLA)
Supply voltage
Motor branch-circuit protection is allowed higher than the 125% used for feeders and continuous loads because starting current is transient. The breaker here covers short-circuit and ground-fault protection; the overload relay handles running overloads. NEC 430.52 caps an inverse-time breaker at 250% of FLC (400% where the starting current makes the 250% value inadequate) — the tool picks the next standard size at or above 250% and reports none if no standard size fits within the 400% ceiling. Where the calculated value exceeds the largest standard rating, NEC 430.52 permits the next size up as long as it does not exceed the locked-rotor current.
Inverse-time breakers and non-time-delay fuses are sized at 250% and 300% of FLA; where a time-delay fuse is used, NEC 430.52 table values drop to 175% of FLA. AO Ctrl carries the starters, breakers and fuses for branch-circuit protection:
Frequently asked questions
How do I size a motor circuit breaker under NEC 430?
For an inverse-time breaker, size at 250% of FLA and round up to the next standard size; the calculator uses standard ratings from 6 A to 250 A. With the default 27 A FLA, 27 × 2.5 = 67.5 A rounds up to an 80 A breaker. A non-time-delay fuse uses 300% of FLA and a time-delay fuse uses 175% of FLA.
What is locked-rotor current and how is it calculated?
Locked-rotor current is the inrush a motor draws while stalled at start-up, expressed as a percentage of FLA. The calculator defaults to 600% (the typical NEMA Design B value), so a 27 A motor draws about 162 A locked-rotor. NEC 430.52 permits the next standard size up where the calculated value exceeds the largest standard rating, as long as it does not exceed the locked-rotor current.
What is the difference between a breaker and an overload relay for a motor?
The branch-circuit breaker or fuse provides short-circuit and ground-fault protection, and is allowed higher than the 125% used for feeders and continuous loads because starting current is transient. The overload relay handles running overloads separately. This calculator sizes the breaker and fuse only — you still need a separate overload element set to the motor FLA.
AO Ctrl provides this tool for reference only.