What is locked rotor amps?

The current a motor draws at zero speed on start, typically six to eight times full-load amps for a NEMA Design B motor.

What does service factor mean?

A 1.15 service factor means the motor can carry 15% above rated horsepower continuously under normal conditions, but running there shortens insulation life.

Why is nameplate FLA different from the NEC table?

NEC table values are conservative generic figures used for wire and breaker sizing; the nameplate reflects that specific motor's design and is what overloads are set from.

Air Compressor Motor Data — NEC FLA, Wire & Breaker Sizing

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Formula

FLC × 1.25 = wire ampacity · FLC × 2.5 = breaker · FLC × 1.75 = TD fuse · FLC × 1.15–1.25 = overload

Phase

Voltage

Motor power (HP)

Unit

NEC table range @ 230 V: 0.5–200 HP

Full-Load Current (FLC)

28.0A

NEC 430.250 · 10 HP @ 230 V

Conductor (Cu, 75°C THWN)

10 AWGrated 35 A

Branch-circuit min: 35.0 A (125% FLC, NEC 430.22)

Branch-Circuit Protection (NEC 430.52)

Inverse-Time Breaker

70 A

250% FLC

Dual-Element Fuse

50 A

175% FLC

Non-Time-Delay Fuse

90 A

300% FLC

Overload Relay (NEC 430.32)

32.2 – 35.0 A

115% FLC (SF < 1.15) · 125% FLC (SF ≥ 1.15 or 40°C rise)

NEMA Starter Size

Size 2

Magnetic across-the-line starter (typical compressor duty)

Reference values from NEC 430.248 / 430.250 (FLC), 310.16 (75°C Cu ampacity), 430.22 (branch ampacity), 430.52 (max OCPD), 430.32 (overload), 240.6 (standard OCPD sizes). FLC values between table HP entries are linearly interpolated — for sizing decisions, round up to the next standard motor HP per NEC 430.6(A)(1). Always verify with the AHJ and motor nameplate. Look up these NEC articles →

How to use this calculator

Find typical full-load current, service factor and starting characteristics by motor size and voltage.

Select motor horsepower and supply voltage.
Select phase and enclosure type.
Read typical full-load amps and locked-rotor current.
Verify against the actual nameplate before sizing conductors or protection.

Frequently asked questions

What is locked rotor amps?: The current a motor draws at zero speed on start, typically six to eight times full-load amps for a NEMA Design B motor.
What does service factor mean?: A 1.15 service factor means the motor can carry 15% above rated horsepower continuously under normal conditions, but running there shortens insulation life.
Why is nameplate FLA different from the NEC table?: NEC table values are conservative generic figures used for wire and breaker sizing; the nameplate reflects that specific motor's design and is what overloads are set from.