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Motor Full Load Amp Calculator

Calculate motor FLA from HP, voltage, phase, power factor, and efficiency. Get NEC 430.22 wire ampacity, standard breaker size, and minimum wire gauge.

Motor Nameplate Data
Motor Electrical Results
Full Load Amps (FLA)
—
A
NEC Wire Ampacity
—
A (FLA × 1.25)
NEC Breaker Size
—
A (FLA × 2.5)
Input kW
—
kW
NEC 310 Wire Sizing (THHN/THWN 75°C, Conduit)
AWG/MCMAmpacityAdequate for FLA × 1.25?
NEC 430.22: Branch-circuit conductors must be rated at least 125% of motor FLA. NEC 430.52: Inverse-time breaker sized at 250% of FLA (round up to next standard size). Always verify with AHJ and local code amendments. Breaker standard sizes: 15,20,25,30,35,40,45,50,60,70,80,90,100,110,125,150,175,200,225,250,300,350,400,450,500A.
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About This Calculator

This motor full load amp calculator estimates the running current of a single- or three-phase motor from its horsepower, voltage, phase, power factor, and efficiency. It also derives the NEC branch-circuit wire ampacity at 125% of full-load current, a standard inverse-time breaker size, the input kW, and a minimum THHN/THWN copper conductor from a 75°C ampacity table.

It is a quick design aid for motor branch circuits. For permit and construction documents, conductor and protection sizing must use the table full-load current per the National Electrical Code, and the design must be verified with the authority having jurisdiction and any local amendments.

Formula & Method
3-phase: FLA = (HP × 746) ÷ (V × PF × η × √3)
1-phase: FLA = (HP × 746) ÷ (V × PF × η)
Wire ampacity = FLA × 1.25  ·  Breaker = next std ≥ FLA × 2.5

The constant 746 converts horsepower to watts; η is efficiency and PF is power factor; √3 ≈ 1.732 accounts for three-phase line-to-line voltage. The 1.25 conductor factor is NEC 430.22, and the 2.5 breaker factor is the inverse-time-breaker limit of NEC 430.52, rounded up to a standard size. For final design the code requires using the motor full-load current from NEC Table 430.250 (three-phase) rather than this calculated value.

Frequently Asked Questions
How is motor full load amps (FLA) calculated?
For a three-phase motor, FLA = (HP times 746) divided by (volts times power factor times efficiency times the square root of 3). For single phase, drop the square root of 3. The 746 converts horsepower to watts. This computed value is a design estimate; the National Electrical Code requires conductor and overload sizing to be based on the table values in NEC Table 430.250, not the nameplate amps.
Why does the NEC use table FLC instead of nameplate amps?
NEC 430.6 directs that branch-circuit conductor and protection sizing use the full-load current values from Table 430.247 through 430.250 rather than the motor nameplate amps. The table values are standardized and conservative, so wiring and protection remain adequate across motors of the same horsepower and voltage. Nameplate amps are still used for setting the running overload device.
How are wire size and breaker size determined?
NEC 430.22 requires branch-circuit conductors rated at least 125 percent of the motor full-load current, so this tool multiplies FLA by 1.25 and finds the smallest conductor with adequate ampacity. NEC 430.52 allows an inverse-time breaker up to 250 percent of full-load current, rounded up to the next standard size, which is why the tool shows FLA times 2.5.
What power factor and efficiency should I use?
Use the motor nameplate values when you have them. As defaults, a power factor near 0.85 and an efficiency in the low 90 percent range are reasonable for typical NEMA Premium induction motors. Lower power factor or efficiency raises the calculated current. For final code sizing, remember that the NEC table FLC governs, not this calculated FLA.
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