Convert horsepower to amps, amps to HP, HP to KW and watts, and torque & RPM to horsepower for DC, single-phase AC, and 3-phase AC motors — 5 modes with efficiency and power factor, NEMA MG1 verified.
✓Verified: NEMA MG1-2021 & NEC 2023 Article 430 Motor Standards
Motor shaft output in HPEnter valid HP.
Motor supply voltageEnter valid voltage.
Select motor circuit typeSelect type.
Motor efficiency (typical 0.80–0.96)Enter efficiency 0.01–1.0.
PF for AC motors (DC: leave blank)Enter PF 0.01–1.0.
Motor full-load currentEnter valid current.
Motor supply voltageEnter valid voltage.
Select motor circuit typeSelect type.
Motor efficiencyEnter efficiency 0.01–1.0.
PF for AC motorsEnter PF 0.01–1.0.
Select conversion directionSelect direction.
Enter horsepower valueEnter valid value.
1 mech HP = 745.7W | 1 metric HP = 735.5WSelect HP type.
Shaft torqueEnter valid torque.
Select torque unitSelect unit.
Shaft rotational speedEnter valid RPM.
Motor shaft output in HPEnter valid HP.
Shaft rotational speedEnter valid RPM.
Result
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⚠️ Disclaimer: Results use NEMA MG1 and NEC Article 430 motor formulas. Always use motor nameplate full-load current for breaker and wire sizing — nameplate values account for actual motor design and are more accurate than calculated values. Consult a licensed electrician for motor installations.
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📚 Sources & Methodology
All horsepower and motor formulas verified against:
NEMA MG1-2021 — Motors and Generators: HP definitions, efficiency classes, full-load current, and motor performance standards nema.org
NEC 2023 (NFPA 70) Article 430 — Motors, Motor Circuits, and Controllers: Tables 430.248 (single-phase FLC), 430.249 (2-phase), 430.250 (3-phase FLC) nfpa.org
IEEE Std 112-2017 — Test Procedure for Polyphase Induction Motors and Generators, efficiency measurement standards.ieee.org
Complete Motor Power Guide — HP, Amps, KW & Torque
Horsepower to Amps: The Complete Formula
Converting horsepower to amps requires knowing the motor type (DC or AC), supply voltage, motor efficiency, and for AC motors, the power factor. The fundamental relationship is: input watts = HP × 745.7, and then amps = watts / (V × PF × efficiency). For 3-phase motors, the line voltage is multiplied by √3 (1.732) to account for the three-phase power relationship.
HP to Amps Formulas (NEMA MG1 Standard)
DC Motor: I = HP x 746 / (V x Eff)Single-phase AC: I = HP x 746 / (V x Eff x PF)3-phase AC: I = HP x 746 / (V x 1.732 x Eff x PF)Amps to HP (DC): HP = I x V x Eff / 746HP to KW: KW = HP x 0.7457 (1 HP = 745.7 W)Torque to HP: HP = Torque(ft-lb) x RPM / 5252Torque to HP: HP = Torque(Nm) x RPM / 9549HP to Torque: Torque(ft-lb) = HP x 5252 / RPM
NEC Full-Load Current Reference Table
NEC Article 430 Tables 430.248 and 430.250 provide standard full-load current (FLC) values for wire and breaker sizing. Always use these for circuit protection — do not size from calculated HP values alone.
HP
115V 1-phase
230V 1-phase
208V 3-phase
230V 3-phase
460V 3-phase
1/4 HP
5.8 A
2.9 A
2.4 A
2.2 A
1.1 A
1/2 HP
9.8 A
4.9 A
3.5 A
3.2 A
1.6 A
3/4 HP
13.8 A
6.9 A
4.7 A
4.3 A
2.1 A
1 HP
16 A
8 A
6.1 A
5.5 A
2.8 A
2 HP
24 A
12 A
11.0 A
10.0 A
5.0 A
3 HP
34 A
17 A
14.0 A
12.6 A
6.3 A
5 HP
56 A
28 A
21.0 A
19.0 A
9.6 A
10 HP
—
50 A
37.0 A
34.0 A
17.0 A
15 HP
—
—
54.0 A
49.0 A
25.0 A
25 HP
—
—
89.0 A
80.0 A
40.0 A
50 HP
—
—
167 A
151 A
76.0 A
100 HP
—
—
318 A
287 A
143 A
Torque, RPM, and Horsepower Relationship
The relationship HP = Torque × RPM / 5252 (in ft-lb) comes from the definition of one horsepower as 33,000 ft-lb per minute. Since RPM × 2π radians gives angular velocity, and power = torque × angular velocity: Power (ft-lb/min) = Torque (ft-lb) × RPM × 2π, converted to HP by dividing by 33,000, simplifying to HP = T × RPM / 5252. In SI: HP = T(Nm) × RPM / 9549.3, or KW = T(Nm) × RPM / 9549.3 × 0.7457.
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Motor breaker sizing rule (NEC 430.52): Motor branch circuit breaker = NEC Table FLC × 250% for standard inverse-time breakers. Wire = FLC × 125% minimum. Example: 5 HP 230V single-phase: NEC FLC = 28A. Breaker = 28 × 2.5 = 70A. Wire = 28 × 1.25 = 35A minimum = 8 AWG copper. Always use the NEC table values, not calculated amps, for circuit protection sizing.
❓ Frequently Asked Questions
HP to amps depends on voltage and motor type. DC: I = HP x 746/(V x Eff). Single-phase AC: I = HP x 746/(V x Eff x PF). 3-phase: I = HP x 746/(V x 1.732 x Eff x PF). Example: 1 HP at 120V, Eff=0.88, PF=0.85: I = 746/(120 x 0.88 x 0.85) = 746/89.4 = 8.34 amps. Use the HP to Amps tab above for any combination.
1 HP at 120V single-phase (Eff=0.85, PF=0.85): I = 746/(120x0.85x0.85) = 8.60A. At 240V: 4.30A. At 120V DC (Eff=90%): 6.90A. NEC Table 430.248 rates a 1 HP 115V motor at 16 amps FLC (includes motor design margin for safe circuit sizing). For a 1 HP 460V 3-phase motor, NEC FLC = 2.8 amps.
KW = HP x 0.7457. 1 HP = 745.7 watts = 0.7457 KW. Examples: 5 HP = 3.73 KW. 10 HP = 7.46 KW. 25 HP = 18.6 KW. 100 HP = 74.6 KW. This is shaft output power. Input power = output / efficiency. A 10 HP motor at 91% efficiency: input = 7.46/0.91 = 8.20 KW electrical consumed.
HP = Torque(ft-lb) x RPM / 5252. Or HP = Torque(Nm) x RPM / 9549. Example: 150 ft-lb at 1800 RPM: HP = 150 x 1800/5252 = 51.4 HP. In Nm: 200 Nm at 1500 RPM: HP = 200 x 1500/9549 = 31.4 HP = 23.4 KW. The constant 5252 = 33000/(2 x pi) from the HP definition (1 HP = 33,000 ft-lb/min).
3-phase: I = HP x 746/(V x 1.732 x Eff x PF). For 10 HP at 460V, Eff=0.91, PF=0.88: I = 7460/(460 x 1.732 x 0.91 x 0.88) = 7460/641.8 = 11.6A. For circuit sizing, use NEC Table 430.250: 10 HP 460V 3-phase = 14A FLC. Breaker = 14 x 2.5 = 35A. Wire = 14 x 1.25 = 17.5A minimum = 12 AWG copper.
Mechanical HP (US): 1 HP = 745.7 W = 550 ft-lb/sec. Standard for US motors and engines. Metric HP (PS): 1 PS = 735.5 W. Used for European cars (different from mechanical HP by 1.4%). Electrical HP: 1 HP = 746 W exactly (slightly more than mechanical). Boiler HP: 9,810 W (completely different, for steam boilers). When in doubt, use mechanical HP for all US electrical and mechanical calculations.
Per NEC 430.52: breaker = NEC Table FLC x 250% (inverse-time breaker). Wire = FLC x 125%. Use NEC Tables 430.248/430.250 — not calculated values. Example: 5 HP 230V 1-phase: NEC FLC = 28A. Breaker = 28 x 2.5 = 70A. Wire = 28 x 1.25 = 35A = 8 AWG. The NEC table accounts for motor starting current (6-8x FLC) and ensures breaker clears on overcurrent without tripping on normal starting.
Torque(ft-lb) = HP x 5252 / RPM. Torque(Nm) = HP x 7121 / RPM. Example: 5 HP at 1750 RPM: Torque = 5 x 5252/1750 = 15.0 ft-lb = 20.3 Nm. At 875 RPM (half speed): Torque = 5 x 5252/875 = 30.0 ft-lb — double the torque at half the speed, same power. This inverse relationship explains why gearboxes increase torque while reducing speed.
Efficiency = output shaft power / input electrical power. Typical: 1 HP motor 75-82%, 5 HP 84-89%, 50 HP 91-94%, NEMA Premium 1 HP 85.5%, NEMA Premium 10 HP 91.7%. Power factor (PF) = real power / apparent power. Induction motors PF 0.80-0.92 at full load, drops at partial load. Low PF increases current draw without increasing output power — use PF correction capacitors for large motors.
1 mechanical HP = 745.7 watts (exact: 745.69987... W). 1 HP = 0.7457 KW. Electric motor input power is higher: 1 HP at 88% efficiency = 745.7/0.88 = 847W electrical input. Use 746W per HP for quick calculations; use 745.7W for precision work. The HP to KW/Watts tab above handles all three HP standards (mechanical, metric PS, electrical).
NEMA Premium efficiency (MG1 Table 12-12) minimums: 1 HP TEFC = 85.5%, 5 HP = 89.5%, 10 HP = 91.7%, 25 HP = 93.6%, 50 HP = 94.5%, 100 HP = 95.4%. These meet IEC IE3 standard. NEMA Premium motors cost 10-20% more but save energy continuously. A 10 HP motor running 8 hr/day: standard 89% vs premium 91.7% saves (1/0.89 - 1/0.917) x 7460W x 2920hr/yr = 742 KWh/year = $96/year at $0.13/KWh.
DC: HP = I x V x Eff / 746. Single-phase AC: HP = I x V x Eff x PF / 746. 3-phase AC: HP = I x V x 1.732 x Eff x PF / 746. Example: 20A at 230V, 3-phase, Eff=0.90, PF=0.87: HP = 20 x 230 x 1.732 x 0.90 x 0.87 / 746 = 6224/746 = 8.34 HP. Use the Amps to HP tab above with your specific motor values.