Why use the NEC table instead of the calculation?

NEC 430.6(A)(1) requires you to use Tables 430.247 through 430.250 for branch circuit conductor sizing, OCPD sizing, and ampacity rating - not the nameplate or calculated current. The tables are conservative and account for the variability across motor designs.

When do I use the nameplate amps?

Per NEC 430.6(A)(2), you use nameplate FLA for sizing motor overload protection (the protection that trips on prolonged overcurrent), but not for conductors or branch circuit OCPD.

What is a typical motor efficiency?

85-90% for general-purpose induction motors under 10 HP. 90-95% for premium efficiency motors and motors above 10 HP. Older motors (pre-EPACT) can be as low as 75%. The motor nameplate lists the actual efficiency at full load.

HP to Amps Calculator

Convert horsepower to amps for electric motors. Free calculator with efficiency and power factor for single-phase and three-phase.

Inputs

Horsepower

Voltage

Efficiency

Power Factor

Phase

Single-PhaseThree-Phase

Result

Current

8.604A

I = (HP × 746) / (V × Eff × PF) = (1 × 746) / (120 × 0.85 × 0.85)

About this calculator

Motor sizing starts from horsepower because that is what motor nameplates and equipment specs use. To wire the motor you need amps. Converting horsepower to full-load amps requires voltage, motor efficiency, and power factor - which is why NEC Tables 430.247 through 430.250 just publish typical values rather than asking you to do the math each time.

Formula

One horsepower is 746 watts of mechanical output. Motor electrical input is the mechanical output divided by efficiency, then divided by voltage and (for AC) power factor to get amps. Three-phase adds √3.

Single-phase: I = (HP × 746) ÷ (V × η × PF)
Three-phase: I = (HP × 746) ÷ (√3 × V × η × PF)
(η = efficiency as a decimal)

Reference: NEC 430.247-430.250 (use the table values for sizing, not calculated values, per 430.6(A))

How to use

Pick single-phase or three-phase.
Enter motor horsepower, voltage, efficiency (typical 0.85-0.95), and power factor (typical 0.80-0.90).
Read the calculated full-load amps.
For NEC sizing, compare to the value in NEC Table 430.248 (single-phase) or 430.250 (three-phase) - per 430.6(A), the table value is what you use for conductor and OCPD sizing, not the nameplate or calculated value.

Worked example

Setup

A 5 HP three-phase 460 V motor with 87% efficiency and 0.85 power factor.

Calculation

I = (5 × 746) ÷ (1.732 × 460 × 0.87 × 0.85) = 3,730 ÷ 588.9 = 6.33 A calculated.

Answer

Calculated: 6.33 A. NEC Table 430.250 lists 7.6 A for a 5 HP, 460 V three-phase motor. Use 7.6 A for branch circuit conductor and OCPD sizing per 430.6(A).

Related study guides

Motor Calculations for the NEC Exam: FLC, Conductors & OCP

Frequently asked questions

Why use the NEC table instead of the calculation?: NEC 430.6(A)(1) requires you to use Tables 430.247 through 430.250 for branch circuit conductor sizing, OCPD sizing, and ampacity rating - not the nameplate or calculated current. The tables are conservative and account for the variability across motor designs.
When do I use the nameplate amps?: Per NEC 430.6(A)(2), you use nameplate FLA for sizing motor overload protection (the protection that trips on prolonged overcurrent), but not for conductors or branch circuit OCPD.
What is a typical motor efficiency?: 85-90% for general-purpose induction motors under 10 HP. 90-95% for premium efficiency motors and motors above 10 HP. Older motors (pre-EPACT) can be as low as 75%. The motor nameplate lists the actual efficiency at full load.