Skip to main content
Tool Belt Calc

Electrical

Motor Amps Calculator

Motor full load and starting current from horsepower.

Your measurementsEvery assumption editable01

Small motors run 75–85%; larger ones 90% and above.

Direct-on-line starting typically draws 6–8× full load current.

Motor currentCalculated live02

Result

12.66A full load

5 hp draws 12.7 A — 76 A starting

Shaft power5 hp at 746 W per hp
3730 W
Electrical inputat 87% efficiency
4287 W
Full load current
12.661 A
Starting current6× full load
76 A
Phase
Three phase

Horsepower is shaft output, not electrical input. A motor draws more than its rating suggests because efficiency and power factor both take their cut before mechanical work appears at the shaft.

Starting current is the figure that sizes protection and generators. Direct-on-line starting commonly draws six to eight times full load current for a second or two, which is why soft starters and variable frequency drives exist.

This is a calculated estimate. The nameplate full load amps on the motor itself is the authoritative figure, and code sizing for motor circuits uses published tables rather than calculated values.

Horsepower is shaft output, not electrical input — a motor draws more than its rating implies because efficiency and power factor both take their cut first. Starting current is shown separately because it is what sizes protection.

Why use this tool?What it does differently03

Why use this tool?

Input versus output

Shaft power and electrical input shown separately, which is where the efficiency loss appears.

Starting inrush

Direct-on-line starting draws six to eight times full load. That is what trips generators and breakers.

Single or three phase

Both formulas, since the same motor rating draws very different current on each.

Nameplate flagged

The motor’s own plate is authoritative; this is a calculated estimate.

How this worksThe method04

How this motor amps calculator works

Shaft power in watts is horsepower times 746. Dividing by efficiency gives the electrical input, and dividing that by voltage, power factor and √3 on three phase gives full load current.

Both efficiency and power factor reduce the useful work per amp drawn, which is why a motor’s current is always higher than a naive watts-over-volts calculation suggests.

Starting current is full load times the starting multiplier. Direct-on-line starting commonly draws six to eight times running current for a second or two — brief, but more than enough to trip protection sized on running load alone.

How to use itStep by step05

How to use it

  1. Step 1: Enter the rating

    Nameplate horsepower, which is shaft output.

  2. Step 2: Enter voltage and phase

    The same motor draws very different current at 120 V and 480 V.

  3. Step 3: Set efficiency and power factor

    From the nameplate where available; small motors are less efficient.

  4. Step 4: Size protection on starting

    Not on full load current, or it will trip every time the motor starts.

Example usageWorked figures06

Example usage

A 5 hp three-phase motor
At 230 V, 87% efficient and 0.85 power factor: 12.66 A full load, and about 76 A while starting at 6×.
Why the naive figure is wrong
5 hp is 3,730 W, which at 230 V three phase would suggest 9.4 A. Efficiency and power factor take it to 12.66.
Sizing a generator
That 76 A starting surge is what determines the generator, not the 12.66 A running figure.
Frequently asked questionsCommon questions07

Frequently asked questions

How do I convert horsepower to amps?

Multiply horsepower by 746 for watts, divide by efficiency, then divide by voltage times power factor — and by √3 as well on three phase.

How much current does a motor draw at startup?

Typically six to eight times full load current with direct-on-line starting, for a second or two. Soft starters and variable frequency drives reduce it substantially.

Why does a motor draw more than its horsepower suggests?

Because horsepower is shaft output. Efficiency losses and power factor both mean more electrical input than mechanical output.

Should I use this for circuit sizing?

No. Motor circuits are sized from published full load current tables and have their own protection rules. The motor nameplate is the authoritative figure.

What efficiency should I assume?

Small motors run 75–85%; larger ones 90% and above. Premium efficiency motors are higher still, and the nameplate states it.

Related toolsElsewhere on the site08

Browse every tool in Electrical Calculators.