Motor amps calculator
This computes a current from the horsepower, efficiency and power factor you supply — a nameplate calculation, not the tabulated full-load current your code may require for conductor and overload sizing. Use it to sanity-check, and size the circuit from your code book and your authority having jurisdiction, never from this figure alone.
Enter the motor's horsepower and voltage, read the efficiency and power factor off the nameplate, and the tool works out the running current. Because it uses the motor's own numbers rather than a lookup, it stays honest across motor types and code editions.
Worked example
A 5 hp single-phase motor at 240 V, 90% efficient with a 0.85 power factor, takes (5 × 746 ÷ 0.9) input watts = 4,144 W, then 4,144 ÷ (240 × 0.85) ≈ 20.3 A. Wire the same motor three-phase at 240 V and the √3 drops each line to about 11.7 A.
The formula
amps = (hp × 746 ÷ efficiency) ÷ (phase × V × PF)
Horsepower becomes watts at 746 W each; dividing by efficiency gives the input power the supply must deliver. Dividing that by the voltage and power factor — and by √3 for three phase — turns power into current. Every term is a value you enter, so no code table is involved.
Questions
- How do I calculate motor amps from horsepower?
- Convert the horsepower to watts (1 hp is 746 watts), divide by the motor efficiency to get the input power, then divide that by the voltage — and by √3 as well for three phase — and by the power factor. The result is the running current the motor draws from the supply.
- Is this the same as the code full-load current?
- No, and that is the point. This is a calculated value from the motor’s own nameplate figures — its efficiency and power factor — not a figure read from a code table. The two are usually close, but where your rules require the tabulated full-load current for sizing, use that table, not this arithmetic.
- Where do I find efficiency and power factor?
- They are on the motor nameplate, often written as a percentage and a decimal respectively. Enter them as you read them — efficiency as a fraction like 0.9 and power factor as a decimal like 0.85. They vary by motor, so this tool asks for them rather than assuming a value.
- Why is my three-phase current lower than single phase?
- The √3 in the three-phase formula spreads the same power across three lines, so each line carries less current for the same horsepower. That is one of the practical reasons larger motors are wired three-phase — thinner conductors carry the load.
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