Motor current calculator

Knowing how much current a motor draws helps you choose the motor protection (motor circuit breaker or overload relay), the contactor and the cable, and check with a clamp meter whether a motor is overloaded. The right value is always the one on the nameplate; this calculator gives an estimate for when you do not have the nameplate in front of you or want to check whether a reading makes sense.

How it is calculated

The nameplate power is the output power at the shaft. The power taken from the supply is higher, because you have to divide by the efficiency. So:

• Three-phase: I = P / (√3 × V × cos φ × η)
• Single-phase: I = P / (V × cos φ × η)

With P in watts, V in volts (line-to-line on three-phase), cos φ between 0 and 1 and the efficiency η as a fraction (88 % = 0.88).

Rated in horsepower? Multiply by 0.746 to get kW: a 5 HP motor is 3.73 kW.

Example

A 4 kW three-phase motor at 400 V with a power factor of 0.85 and 88 % efficiency: 4000 / (1.732 × 400 × 0.85 × 0.88) ≈ 7.7 A. The nameplate of a real motor of that size usually shows between 7.5 and 8.5 A, depending on the manufacturer and the efficiency class. The same motor on a 480 V supply would draw about 6.4 A.

What to keep in mind

• Direct-on-line starting can draw 5 to 8 times the rated current for a few seconds. That is why motors are protected with motor circuit breakers or breakers with a suitable trip curve.
• A lightly loaded motor draws less than its rated current, with a worse power factor. Measuring less than the nameplate current is normal if the motor is not working at full load.
• The overload relay is set to the nameplate current, not to this calculator’s result.
• In North America, the NEC sizes the conductors and the short-circuit protection from its full-load current tables (Table 430.250 for three-phase motors), and the overload protection from the nameplate.

To understand the topic in depth: Motors and variable frequency drives, with theory and a practice test.

Quick guide: Overload relay or motor protection circuit breaker: which one?.

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