What is the HP to amps formula for a three-phase motor?

For a three-phase AC motor use: A = (HP × 746) ÷ (√3 × V × PF × η). The √3 factor (≈ 1.7321) accounts for the three-phase system. For a 10 HP motor at 460 V, PF 0.85, efficiency 90 %: A = 7460 ÷ (1.7321 × 460 × 0.85 × 0.90) ≈ 12.24 A.

What is NEC full-load amperage and why does it differ from the calculated value?

NEC Table 430.248 (single-phase) and Table 430.250 (three-phase) list full-load ampere (FLA) values for standard motor ratings. These are empirical values used to size conductors and breakers per NFPA 70. They may differ from the formula because they reflect typical motor efficiencies and power factors at rated load rather than nameplate values.

What power factor and efficiency should I use for a motor?

Standard induction motors typically have a power factor of 0.80–0.90 and efficiency of 85–95 % at rated load. Premium-efficiency (IE3/NEMA Premium) motors can reach 92–96 % efficiency. If the nameplate is unavailable, 0.85 PF and 0.90 efficiency are common conservative defaults.

Horsepower to Amps Calculator

Q: How do you convert HP to amps for a single-phase motor?

For a single-phase AC motor use: A = (HP × 746) ÷ (V × PF × η). For example, a 5 HP motor at 230 V, 0.85 power factor and 90 % efficiency draws (5 × 746) ÷ (230 × 0.85 × 0.90) = 3730 ÷ 175.95 ≈ 21.2 A.

Q: How many amps does a 1 HP motor draw?

A 1 HP single-phase 120 V motor with PF 0.85 and efficiency 85 % draws approximately (1 × 746) ÷ (120 × 0.85 × 0.85) ≈ 8.6 A. At 230 V the same motor draws about 4.5 A. Always check the motor nameplate for the rated full-load current.

Built by an electrical engineering student Last updated July 2026 Estimates only — how we check

Enter motor horsepower, voltage, phase, power factor and efficiency to calculate full-load amps (FLA) instantly. Results follow the NEC motor-sizing formula.

Motor Horsepower (HP)

Voltage (V)

Phase

Power Factor (PF)

Efficiency (η) — 0 to 1

Result — Full-Load Amps

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Enter HP and voltage to see the formula used.

How do you convert HP to amps for a motor?

Multiply horsepower by 746 (watts per HP), then divide by the product of voltage, power factor and efficiency. For a 5 HP single-phase motor at 230 V, PF 0.85, η 0.90: A = (5 × 746) ÷ (230 × 0.85 × 0.90) = 3730 ÷ 175.95 = 21.2 A.

This formula is consistent with NEC Article 430. NEC Tables 430.248 and 430.250 provide standardised full-load amps for common voltage/HP ratings and are the values legally required for conductor and breaker sizing; use this calculator for quick estimates and always cross-check with the applicable NEC table and motor nameplate.

HP to Amps Reference Table (Typical Motor FLA)

Motor HP	Single-phase 120 V	Single-phase 230 V	Three-phase 230 V	Three-phase 460 V
1 HP	9.8 A	5.1 A	3.4 A	1.7 A
2 HP	19.5 A	10.2 A	6.8 A	3.4 A
5 HP	—	25.4 A	16.9 A	8.5 A
10 HP	—	50.9 A	33.9 A	17.0 A
25 HP	—	—	84.7 A	42.4 A
50 HP	—	—	169.5 A	84.7 A

Calculated at PF 0.85, efficiency 0.90. Always verify against the motor nameplate and NEC Tables 430.248/430.250.

Formula

Single-phase: A = (HP × 746) ÷ (V × PF × η)
Three-phase: A = (HP × 746) ÷ (√3 × V × PF × η)
DC: A = (HP × 746) ÷ (V × η)

Where: HP = rated horsepower, V = supply voltage in volts, PF = power factor (0–1), η (eta) = motor efficiency (0–1), √3 ≈ 1.7321 for three-phase systems.

NEC Table 430.250 Full-Load Currents — Three-Phase AC Motors

When sizing conductors and overcurrent protection for motors, the National Electrical Code (NFPA 70, NEC) requires using the full-load ampere (FLA) values from Table 430.250 (three-phase motors) or Table 430.248 (single-phase motors), not the calculated formula result or motor nameplate. The table below reproduces selected values from NEC Table 430.250 for 460 V three-phase induction motors — the most common US industrial voltage. These are the authoritative code values used by electricians and engineers for conductor sizing, breaker selection, and starter sizing.

Motor HP	NEC FLA at 460 V (3φ)	NEC FLA at 230 V (3φ)	Minimum conductor size (NEC 430.22, 75°C Cu)
1 HP	2.1 A	4.2 A	14 AWG
2 HP	3.4 A	6.8 A	14 AWG
5 HP	7.6 A	15.2 A	14 AWG
10 HP	14 A	28 A	12 AWG
15 HP	21 A	42 A	10 AWG
25 HP	34 A	68 A	8 AWG
50 HP	65 A	130 A	4 AWG
100 HP	124 A	248 A	3/0 AWG

Source: NFPA 70, NEC Table 430.250 (2023 edition). Conductor sizes shown are for 125 % of FLA per NEC 430.22(A), using 75°C copper conductors per Table 310.16. Always refer to the current edition of NFPA 70 and consult a licensed electrician for actual installation work. Note that NEC FLA values differ from the formula-calculated values in the table above — for 5 HP at 460 V the code specifies 7.6 A; a formula using typical PF 0.85 / η 0.90 gives approximately 7.9 A. Both are valid in context; use NEC table values for code-required sizing.

Frequently Asked Questions

How do you convert HP to amps for a single-phase motor?

Use the formula A = (HP × 746) ÷ (V × PF × η). For example, a 5 HP motor at 230 V, PF 0.85, efficiency 90 % draws approximately 21.2 A.

What is the HP to amps formula for three-phase?

For three-phase AC use: A = (HP × 746) ÷ (√3 × V × PF × η). The √3 ≈ 1.7321 factor accounts for the three-phase supply. A 10 HP motor at 460 V, PF 0.85, efficiency 90 % draws about 12.2 A.

What is NEC full-load amperage and how does it differ?

NEC Tables 430.248 and 430.250 give standardised FLA values for conductor and breaker sizing per NFPA 70. They may differ from the formula because they represent typical motor characteristics rather than specific nameplate values.

What power factor and efficiency should I use?

Standard induction motors typically have PF 0.80–0.90 and efficiency 85–95 % at rated load. If the nameplate is unavailable, PF 0.85 and efficiency 90 % are conservative defaults commonly used in load-estimating.

How many amps does a 1 HP motor draw?

A 1 HP single-phase 230 V motor at PF 0.85, efficiency 85 % draws about 4.5 A. At 120 V the same motor draws roughly 8.6 A. Always check the motor nameplate for the actual full-load current rating.