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Old 01-23-2011, 08:20 AM   #1
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In-rush Current Measurement


I recently got a new clamp ammeter that reads inrush current. The measurements I got on the 240 volt deep well pump were 60 amps on startup (more or less the same on both legs), and 11 amps steady state.

How does this relate to sizing a portable generator to power this load? I've already run the pump in the past with a generator rated 7550/13500 watts (steady/surge). 60 amps would imply 14400 watts.

Almost 6 times the draw at steady state blows my mind.

Thanks.

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Old 01-23-2011, 10:23 AM   #2
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In-rush Current Measurement


Portable generators employ breakers just like you have inside in your house panel. Inverse time breakers. Breakers are designed to allow for inrush currents. The NEC also addresses motors and breaker sizes in article 430.52. Maximum breaker size for the motor is as follows.

(I) Motor Full Load Current x 2.50 = Breaker size maximum.

Is the motor 1.5 HP or 2 HP?

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Old 01-23-2011, 10:31 AM   #3
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In-rush Current Measurement


Thanks. I have no idea what the HP of the well pump is-no paperwork and it's somewhere down a 450' hole.
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Old 01-23-2011, 11:30 AM   #4
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In-rush Current Measurement


There is no need to size a breaker for inrush amperage, if fact, I would have second thoughts on the accuracy of the reading you have. Rule of thumb for inrush is 7 x plated FLA, but this lasts less than 1 cycle, any longer and the breaker sized for the plated amps would indeed trip.

There are ways around inrush amps, but these are expensive and usually reserved for horse powers in excess of 60.

The generator will experience no ill effects by inrush, as you have seen already.
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Old 01-23-2011, 11:40 AM   #5
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In-rush Current Measurement


Thanks. I am glad the generator started the pump, but I got the generator a while ago and the meter, a couple of days ago. If I was shopping around for one now, I'd probably shop for a larger one, based on the readings.

The meter is an Extech 830: http://www.extech.com/instruments/pr...d=26&prodid=80
The readings were repeatable.

I guess there's a large inertia the motor has to overcome and the motor isn't a fancy soft start model.

I originally tried starting it with a 5kw generator, but I imagine the pump tripped, because the lights dimmed in the basement and then went bright again. I don't want to experiment too much with the pump!
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