3Phase on 1phase

[arnettda]

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

 

[hillbilly]

Not enough info. to give a answer.

steve

 

[drbond24]

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.

 

[jim dungar]

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 must size the input based on the maximum HP size of the controller, not what is actually connected.

 

[winnie]

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

 

[arnettda]

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?

 

[drbond24]

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?

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.