Motor Amperage Calculator
Sized to the surge, not the running current.
Work out Motor Amperage. Sized to the surge, not the running current. Free, with no account and nothing to install.
Full-load current
14.15 A
84.9 A at start · overload set at 16.27 A
The nameplate kilowatt figure is mechanical output at the shaft. Electrical input is that divided by the efficiency, and the current follows from the input — which is why a 90%-efficient motor draws about 11% more than the nameplate alone suggests. Starting current is what sizes the protection. A direct-on-line induction motor pulls six to eight times its running current for the first second or two, which is why motor circuits use time-delay devices and why a breaker sized to the running current trips on every start. The overload relay is a separate device from the breaker and does a different job: it protects the motor windings from sustained overload, while the breaker protects the cable from a short. Both are needed, and neither substitutes for the other.
How the Motor Amperage Calculator works
Full-load current from a motor's kilowatt rating, efficiency and power factor, plus the starting surge that actually sizes the protection. A breaker sized to running current trips every time the motor starts.
Also known as: motor kw to amps · full load current of a motor · why does my motor trip on start · motor starting current calculator
Frequently asked questions
How do I convert motor kW to amps?
Divide the mechanical rating by the efficiency to get electrical input, then divide by voltage times power factor — and by √3 as well for three-phase. The efficiency step is the one most often skipped.
Why is starting current so high?
A stationary induction motor looks almost like a short circuit to the supply until it builds back-EMF. Direct-on-line starting draws six to eight times the running current for the first second or two.
What size breaker does a motor need?
Far larger than the running current — typically around 250% of full-load amps — because it must ride through the start without tripping. It is a different sizing rule from a general-purpose circuit.
What does the overload relay do?
It protects the motor windings from sustained overload, set at 115 to 125% of full-load current. It is a separate device from the breaker, which protects the cable from a short, and neither substitutes for the other.
How do you reduce starting current?
Star-delta starting, a soft starter, or a variable frequency drive. A VFD reduces it most and adds speed control, which is why it has become the default on anything but the smallest motors.
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