3Phase on 1phase

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arnettda

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I am wiring a 3ph 10hp mtr fed buy a 1ph service/ They are using a 25 hp controller compared to a 10hp controller due to losses when they convert it. Do I size everything to the 10hp or 25hp? I will check with controller supplier today but wanted an opinion here. Thanks
 
You size for whatever the amperage is. I'm assuming your 'controller' here is a variable speed drive since they are the only controllers I know of that can convert single phase to 3 phase. Anyway, the input to the drive will be a known value you can get from the drive specs. The output from the drive to the motor will be a known value you can look up in the NEC using the hp rating on the motor.

I guess that would mean you would use the 25 hp rating for the drive and the 10 hp rating for the motor.
 
arnettda said:
I am wiring a 3ph 10hp mtr fed buy a 1ph service/ They are using a 25 hp controller compared to a 10hp controller due to losses when they convert it. Do I size everything to the 10hp or 25hp? I will check with controller supplier today but wanted an opinion here. Thanks
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You must size the input based on the maximum HP size of the controller, not what is actually connected.
 
I am pretty sure that you must size the circuit to the VSD based upon the _rated input current_ of the VSD, not its horsepower. Of course the two numbers are related; but in the OP's case I would not want to size the feeder for a single phase 25 HP load.

-Jon
 
I sized everything according to the amperage ratting of the VFD. I may have oversized it though? the value was 78.4 amps the same as a 3ph 25hp mtr. So I used 125% of that for conductor sizing and so on as you would for any motor according to the code. So my new question is could I have just used the 78.4 or did I do the right thing buy treating it like a motor?
 
arnettda said:
I sized everything according to the amperage ratting of the VFD. I may have oversized it though? the value was 78.4 amps the same as a 3ph 25hp mtr. So I used 125% of that for conductor sizing and so on as you would for any motor according to the code. So my new question is could I have just used the 78.4 or did I do the right thing buy treating it like a motor?
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I believe you could have just used the 78.4, but sizing wire too large isn't a problem (other than the cost) so I wouldn't worry about it.
 
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