How to determine AC motor startup/inrush current?

Thread Starter


Joined Jan 6, 2010
Hello, I am having problem on how to determining the statup/inrush current of a 7.5kW AC motor. The rated current is 15.1A. General guidelines with calculations provided will be very helpful. Thank you. :D


Joined Sep 30, 2009
If you can find a data sheet for the motor(Google is your friend) look for some thing called 'locked rotor current' on the data sheet. Starting current is the same thing, rating.


Joined Jun 7, 2009
you've not told us why you need this info, which always helps in responses.

For wire sizing/overcurrent/overload, the code dictates.

Inrush is typically based on the class of rotor within the motor, your supply internal impedence, and your overall interconnection impedance.

Duration is based on the class of rotor and the characteristics of the load.

Soft starting, wye/delta, etc has a considerable affect on starting currents.

Peak currents are important when spec'ing a supply transformer, but duration is often the larger concern when selecting a starting strategy.

If your starting a blower for instance, across the line with a Nema B rotor, your looking at approx 300%. If it was a Nema A you'd be struggling with around 600%, and the abrupt spool up may be injurious to your equipment.

If your starting a punch press with a Nema D rotor, I'd figure about 500%. Using a Nema B on this same application would still pull about 500%, but the duration would be too long and your overloads would trip.
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Joined Nov 30, 2010
You could always attach an amp meter and measure the amps while the rotor is stalled. It works for me!


Joined Jul 7, 2009
I'm with #12 -- the best way is to measure it. And the best way to measure it is probably to use a scope to measure the startup current across a shunt or use a current transformer or adequately fast scope current probe. Ask questions if you're unsure about using a shunt, as it can be a dangerous method if you don't know what you're doing. The nice thing about a measurement is that you KNOW what the startup current is; you don't have to rely on what some table somewhere tells you.


Joined Jun 7, 2009
that would be an unconventional approach, unless your the manufacturer. Engineers don't even go there. Remember that start current and it's duration equal heat, the real problem. As well, start current is not just instantaneous peak, it's the slope from peak to FLA, in other words, it has a time value. Hermatic equipment must have a locked rotor rating for the simple fact that they often fail in a stalled state.