HP to Amps | Electrical Calcs

How to convert HP to amps (the formula)

Horsepower is mechanical output power — 1 HP = 746 watts. To get the electrical current, the motor's electrical input is the output divided by its efficiency, and AC current also depends on power factor. So:

System	Formula	Example
DC	I = HP × 746 ÷ (V × η)	10 HP, 240 V, 90% = 34.5 A
AC single-phase	I = HP × 746 ÷ (V × η × PF)	5 HP, 230 V, 90%, 0.85 = 21.2 A
AC three-phase	I = HP × 746 ÷ (√3 × V × η × PF)	10 HP, 460 V, 90%, 0.85 = 12.24 A

Here η is efficiency (as a decimal) and PF is power factor. The 746 converts horsepower to watts, and the √3 (about 1.732) handles three-phase — the same HP draws fewer amps per line on three-phase.

Formula vs the NEC table: which amps do I use?

This is the part that trips people up. The formula gives a motor's actual input current from its own efficiency and power factor — useful to know. But for sizing conductors and overcurrent protection, the NEC does not let you use the formula or even the nameplate. It requires the standardized full-load current from Table 430.250 (three-phase) or 430.248 (single-phase), so every motor of a given HP and voltage is sized the same way.

⚠ Table for sizing, nameplate for overload

A 10 HP, 460 V three-phase motor calculates to about 12.2 A, but NEC Table 430.250 lists 14 A — and the code makes you size the wire and breaker from that 14 A. The motor's nameplate FLA is used only for the overload device (NEC 430.6). Use the table below, not the calculator, for the circuit.

Efficiency and power factor

Both push the current the same direction: as efficiency or power factor drops, the input amps rise. Standard motors run around 85–92% efficiency and 0.8–0.9 power factor; premium-efficiency motors pull a little less. When you have the motor's real nameplate values, use those in the calculator — but remember they set the actual current, while the NEC table sets the sizing current.

NEC motor full-load-current chart (HP to amps) — three-phase

Standard three-phase motor full-load currents from NEC Table 430.250 — the values the code requires for sizing:

Motor HP	208 V	230 V	460 V
1 HP	4.6 A	4.2 A	2.1 A
2 HP	7.5 A	6.8 A	3.4 A
3 HP	10.6 A	9.6 A	4.8 A
5 HP	16.7 A	15.2 A	7.6 A
7½ HP	24.2 A	22 A	11 A
10 HP	30.8 A	28 A	14 A
15 HP	46.2 A	42 A	21 A
20 HP	59.4 A	54 A	27 A
25 HP	74.8 A	68 A	34 A
30 HP	88 A	80 A	40 A
40 HP	114 A	104 A	52 A
50 HP	143 A	130 A	65 A

Single-phase motors (Table 430.248)

Single-phase motor full-load currents from NEC Table 430.248. Notice a single-phase motor pulls far more amps than a three-phase motor of the same HP — there's no √3 helping it:

Motor HP	115 V	230 V
½ HP	9.8 A	4.9 A
¾ HP	13.8 A	6.9 A
1 HP	16 A	8 A
1½ HP	20 A	10 A
2 HP	24 A	12 A
3 HP	34 A	17 A
5 HP	56 A	28 A

From HP to the motor circuit (size the wire and breaker)

Motor circuits have their own rules. The conductors must be at least 125% of the table full-load current (NEC 430.22) — so a 10 HP, 460 V motor at 14 A needs wire good for at least 17.5 A. The branch-circuit breaker follows NEC 430.52 and is often sized higher than the conductor, because a motor's starting inrush would trip an ordinary breaker. Run the table FLC through the Wire Size Calculator and the Breaker Size Calculator, and see the AWG ampacity chart for conductor ratings. Motor protection is specialized — for a real install, confirm with the code or an electrician.

Frequently asked questions

ConversionsWhat is the formula to convert HP to amps?

Amps = HP × 746 ÷ (volts × efficiency × power factor) for single-phase, or ÷ (√3 × volts × efficiency × power factor) for three-phase. For DC, drop the power factor.

⚡ Size the circuit →

ConversionsHow many amps is a 10 HP motor?

Per NEC Table 430.250, a 10 HP three-phase motor is 14 amps at 460 volts, 28 amps at 230 volts, and 30.8 amps at 208 volts. Use these table values for sizing, not the formula.

Code & SafetyDo I use the formula or the NEC table for motor amps?

Use NEC Table 430.250 (three-phase) or 430.248 (single-phase) for sizing conductors and overcurrent protection. The formula and the nameplate FLA give the actual current — the nameplate is used only for the overload device.

ConversionsHow many amps is a 5 HP motor?

A 5 HP three-phase motor is 7.6 amps at 460 volts and 15.2 amps at 230 volts (Table 430.250). A 5 HP single-phase motor is 28 amps at 230 volts (Table 430.248).

ConversionsWhy is a single-phase motor more amps than three-phase?

Single-phase has no √3 in the denominator, so the same horsepower draws more current per conductor. That is why larger motors are almost always three-phase.

ConversionsDoes efficiency change the amps?

Yes. Lower efficiency means more electrical input for the same mechanical output, so the amps rise. Power factor works the same way — a lower power factor increases the current.

ConversionsHow do I convert HP to amps for a 3-phase motor?

Use amps = HP × 746 ÷ (√3 × volts × efficiency × power factor), or simply read the full-load current from NEC Table 430.250 for the motor's HP and voltage.

Code & SafetyWhat size wire for a motor?

Motor conductors must be at least 125% of the table full-load current (NEC 430.22). For a 10 HP, 460 V motor at 14 A, that is wire rated for at least 17.5 amps. Use the Wire Size Calculator for the exact gauge.

Dr. Artie Vance

Technical reviewer · Ph.D. in Physics, MIT · 14 years' experience · FLC values from NEC Tables 430.250 & 430.248

This calculator gives the calculated input current from the standard motor formula. For real installations, size motor conductors and protection from the NEC full-load-current tables (430.250 / 430.248) and Article 430, per your local authority having jurisdiction (AHJ). Motor circuits are specialized — when in doubt, hire a licensed electrician.