HP to Amps Formula

Motor horsepower is a mechanical output rating. Converting it to electrical amps at the terminals means dividing by voltage, efficiency (because no motor is 100% efficient), and for AC motors the power factor. The three-phase version also carries a √3 factor. This page lists all three variants (DC, single-phase AC, three-phase AC) and shows the NEC 430.250 / 430.248 table FLC values you should actually use for branch-circuit sizing.

Formulas

DC: I = (HP × 746) ÷ (V × Eff)

AC single-phase: I = (HP × 746) ÷ (V × Eff × PF)

AC three-phase: I = (HP × 746) ÷ (√3 × V_L-L × Eff × PF)

1 HP = 746 watts

A motor rated at 1 HP delivers 746 watts of mechanical output. Because of efficiency losses (heat, friction, copper and iron losses), it draws more electrical power than 746W at the terminals. The correct formula depends on the supply type. DC divides HP×746 by voltage and efficiency. Single-phase AC adds power factor (typically 0.75-0.85 at full load). Three-phase AC adds power factor and the √3 factor in the denominator because a three-phase load shares current across three conductors. Using the single-phase formula on a three-phase motor overstates the current by a factor of √3 (~73%), which materially oversizes wire and breakers and is the single most common mistake when working from motor nameplates.

Worked Examples

Example 1: 1 HP single-phase at 120V (85% eff, 0.85 PF)

I = (1 × 746) ÷ (120 × 0.85 × 0.85) = 746 ÷ 86.7 = 8.6 amps

Example 2: 5 HP single-phase at 240V (85% eff, 0.85 PF)

I = (5 × 746) ÷ (240 × 0.85 × 0.85) = 3,730 ÷ 173.4 = 21.5 amps

Example 3: 5 HP three-phase at 240V L-L (89% eff, 0.85 PF)

I = (5 × 746) ÷ (√3 × 240 × 0.89 × 0.85) = 3,730 ÷ 314.4 = 11.9 amps per line. Less than half the single-phase draw at the same voltage.

Example 4: 25 HP three-phase at 480V L-L (92% eff, 0.88 PF)

I = (25 × 746) ÷ (√3 × 480 × 0.92 × 0.88) = 18,650 ÷ 673.3 = 27.7 amps per line. Compare to NEC 430.250 Table FLC for sanity-check values.

Example 5: Same 5 HP single-phase at 90% efficiency

I = 3,730 ÷ (240 × 0.90 × 0.85) = 3,730 ÷ 183.6 = 20.3 amps. About 6% less current than 85% efficiency.

Common Mistakes

Using the single-phase formula for a three-phase motor (or vice versa). On a three-phase motor the single-phase formula overstates amps by a factor of √3 and oversizes wire and breakers. Always confirm the nameplate before picking a formula.
Using 750W per HP instead of 746W. The extra 4W is inside the slide rule but adds up on large motors.
Forgetting efficiency. A 1 HP motor does not draw 746W; it draws 746 ÷ efficiency, so roughly 878W at 85% efficiency.
Sizing the branch circuit for running amps only. NEC 430.22 requires conductors rated for 125% of motor full-load current, and NEC 430.52 allows OCP well above FLC (up to 250% for inverse-time breakers) to handle starting inrush.

Try the Calculator

Use the interactive HP to Amps Calculator for instant results with any values. Every result page shows the formula applied with your specific numbers.

All Formulas

functionsWatts to Amps Formula functionsOhm's Law Formula functionsEnergy Cost Formula functionsWire Sizing Formula functionsVoltage Drop Formula functionsThree-Phase Power functionsPower Factor functionsSeries & Parallel functionsAC vs DC functionsElectrical Units functionsNEC Reference

This is a formula reference page for educational use. Always consult a licensed electrician and verify compliance with the National Electrical Code (NEC) and local electrical codes before applying any of these formulas to real installations.

Standards & References

This page cites the following electrical codes and standards. Always consult the current edition of your local adopted standard for authoritative requirements.

NEC 430.6(A)(1). Motor full-load current values for general motor applications shall be taken from Tables 430.247 through 430.250 rather than the motor nameplate for the purposes of sizing conductors, switches, and overcurrent devices.
National Electrical Code (NFPA 70), Article 430, Motors, Motor Circuits, and Controllers. Reference →
NEC 430.22. Motor branch-circuit conductors supplying a single continuous-duty motor shall have an ampacity of not less than 125 percent of the motor full-load current rating. The conductor ampacity is taken from NEC Table 310.16 at the applicable temperature column, and motor branch circuits are exempt from the 240.4(D) small-conductor rule via 240.4(G).
National Electrical Code (NFPA 70), Article 430, Motors, Motor Circuits, and Controllers. Reference →
NEC 430.52 and Table 430.52(C)(1). Motor branch-circuit short-circuit and ground-fault protection. Maximum ratings from Table 430.52(C)(1): non-time-delay fuse 300%, dual-element (time-delay) fuse 175%, instantaneous-trip breaker 800%, and inverse-time breaker 250% of full-load amps. The elevated percentages allow for motor locked-rotor startup current.
National Electrical Code (NFPA 70), Article 430, Motors, Motor Circuits, and Controllers. Reference →
NEC Table 430.250. Full-load currents in amperes for three-phase alternating-current motors (squirrel-cage, wound-rotor, and synchronous), by horsepower and nominal voltage (115, 200, 208, 230, 460, 575, and 2300V). Used with NEC 430.6(A)(1) as the authoritative FLC source for three-phase motor branch-circuit sizing.
National Electrical Code (NFPA 70), Article 430, Motors, Motor Circuits, and Controllers. Reference →
NEC Table 430.248. Full-load currents in amperes for single-phase alternating-current motors, by horsepower and nominal voltage (115, 200, 208, and 230V). Used with NEC 430.6(A)(1) as the authoritative FLC source for single-phase motor branch-circuit sizing.
National Electrical Code (NFPA 70), Article 430, Motors, Motor Circuits, and Controllers. Reference →
NEMA MG 1. National standard for motors and generators. Defines motor efficiency classes, power factor values, insulation temperature ratings, service factor, and locked-rotor currents. The industry reference for motor nameplate interpretation.
National Electrical Manufacturers Association. Reference →
DOE 10 CFR Part 431. US Department of Energy energy conservation standards for electric motors. Sets minimum nominal full-load efficiencies for small, medium, and large electric motors sold in the US, including NEMA Premium and IE3/IE4 equivalents.
US Department of Energy. Reference →

Disclaimer: The information on this page is provided for reference. Always consult a licensed electrician and the current edition of your local adopted electrical code before performing electrical work.