Horsepower to Amps Calculator

Why estimate motor current from horsepower

Before a motor is on site — while you’re planning a circuit, checking whether an existing feeder has room, or comparing a spec against a breaker size — the only number you have is the horsepower. This calculation turns that into an expected running current, given the voltage, phase, power factor, and efficiency.

It’s a planning figure, not a wiring figure. Motor circuits are sized from the full-load amps on the nameplate and the code tables, which fold in margins for starting current and continuous operation that a plain power calculation doesn’t carry.

The horsepower to amps formula

Single-phase:

Three-phase, line-to-line voltage:

is the rating, 746 is watts per horsepower, is voltage, is the power factor, and is efficiency as a decimal. Multiplying gives mechanical output watts; dividing by efficiency scales up to the electrical input.

Worked example

The calculator’s defaults are a 5 hp motor at 240 V, single-phase, power factor 0.8, efficiency 90%.

Mechanical output in watts:

Current:

So the motor draws about 21.6 A running. For comparison, the NEC table figure for a 5 hp single-phase 230 V motor is 15.2 A — the table assumes a better power factor and efficiency for that size — and it’s the table value, not this estimate, that sizes the conductors.

Horsepower to Amps Calculator worked example diagram — A visual read of the worked example above — the same numbers the calculator returns for its default inputs.

Approximate running current

Single-phase, PF 0.8, efficiency 88%:

Motor	120 V	240 V
1 hp	~8.8 A	~4.4 A
2 hp	~17.7 A	~8.8 A
3 hp	~26.5 A	~13.2 A
5 hp	~44 A	~22 A
7.5 hp	~66 A	~33 A

Three-phase current per line is these figures divided by about 1.73.

Starting current is a separate problem

The result is steady running current. A motor started direct-on-line briefly draws locked-rotor current — typically 5 to 8 times the running figure — for a second or two while it comes up to speed. Breakers and overload relays for motor circuits are chosen with a time delay so they ride through that surge without tripping.

Common mistakes

Sizing wire to this estimate instead of the nameplate FLA and code table.
Ignoring the phase setting — a 73% error between single- and three-phase.
Using efficiency and power factor of 1, which no real motor achieves.

For the reverse, use the amps to horsepower calculator. For sizing a supply, see horsepower to kVA.

About the formula: Use the stated electrical relationship and enter values in the displayed units.

Frequently Asked Questions

How does this horsepower to amps calculation work?+

Horsepower is multiplied by 746 to get mechanical watts, then divided by efficiency to find the electrical input, and by voltage, phase factor, and power factor to solve for the current the motor draws.

How many amps is a 5 hp motor at 240V?+

About 21.6 A single-phase at a power factor of 0.8 and 90% efficiency. Three-phase at 240 V it's around 12.5 A per line. The NEC full-load-current tables list 15.2 A for a 5 hp single-phase 230 V motor — use the table value for wiring.

Should I size a breaker or wire from this?+

No — size from the motor's nameplate full-load amps and the code table, which build in the margins for motor starting and continuous duty that this calculation leaves out.

Why does a bigger motor draw less current per horsepower?+

Larger motors have better efficiency and power factor, so the amps per horsepower fall as size rises. A 50 hp motor is proportionally more current-efficient than a 5 hp one at the same voltage.

What about starting current?+

This gives running current only. A motor started across the line pulls locked-rotor current of roughly 5 to 8 times the running figure for a second or two, which is why motor breakers and overloads have time delays.

Single-phase or three-phase — how much difference?+

Big. Three-phase divides the current by √3 (about 1.73), so the same horsepower draws roughly 42% less current per line on three-phase.