g/electrical • Posted by u/voltage_vince • 1 days ago

How do you handle motor nameplate FLA vs actual measured current?

Running into this more often with variable speed drives and newer motor designs. Motor nameplate says 12.4A FLA. Actual measured current at full load is 9.8A. This is a 5HP, 460V, 3-phase induction motor driving an exhaust fan.

Overload relay in the MCC is set to 115% of nameplate (14.3A). The motor runs cool, efficiency seems fine, no faults.

For overload sizing, do you go by nameplate FLA per NEC 430, or actual measured current? I've always gone nameplate, but a PM on this job is pushing for measured current, claiming we're "over-protected." Never heard that argument before.

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u/sparky420 • 1 days ago

We set overloads to nameplate FLA always. If the motor runs cooler than expected, that's a good thing — it means it's undersized for the application (not unusual with fans that have variable seasonal load). Nameplate is the contract between you, the NEC, and the insurance company.

u/admin_jake ADMIN • 1 days ago

NEC 430.6(A)(1) is explicit: use nameplate FLA for overload sizing, not measured current. The PM is wrong. Measured current being lower than nameplate doesn't mean the motor won't pull nameplate under different load conditions, temperature, or voltage sag. Overloads protect against locked rotor and overheating — setting them below nameplate invites nuisance trips.

u/nec_nerd • 1 days ago

Also worth noting: 430.52 and Table 430.52 set the maximum sizing for branch circuit protection (breaker/fuse), not overloads. Overloads are covered under 430.32. They're separate calculations. Make sure your PM understands the difference between branch circuit overcurrent protection and overload protection.