Wire size for transformer

[Fossil]

I need to feed a 35KVA transformer with 208v primary
and a 400v secondary . What size copper wire should i
feed the transformer with my calculations say 1/0 awg
are there any considerations i should take into account
the transformer is feeding a continuious load CNC machine.

 

[George Stolz]

Re: Wire size for transformer

I come up with 4/0 CU or 300 AL. How do you come to your numbers?

(I'm not saying I right, I'm probably not. )

 

[iwire]

Re: Wire size for transformer

Fossil you left to much info out.

Is this single phase or 3 phase?

What size overcurrent devices are you planing on using?

What is the actual load of the CNC machine?

If it is single phase the smallest primary conductor I come up with to get the full capacity of the transformer is 2/0.

35 * 1000 / 208 = 168 amps.

1/0 is rated only 150 amps @ 75 C, you will not be able to use the 90 C rating.

2/0 CU is rated 175 amps @ 75 C.

But we need to know the load and overcurrent protection sizes to really pin down an answer.

George I am figuring that you carried an 'extra' 25% for the continuous load?

With the info given it is tough to say that would be needed.

The CNC machine may run all day but will cycle and for the primaries to be fully loaded the CNC machine would have to draw the full 35 KVA on the secondaries, it is doubtful the machine is rated as high as the selected transformer.

 

[George Stolz]

Re: Wire size for transformer

Originally posted by iwire:
George I am figuring that you carried an 'extra' 25% for the continuous load?

I don't know much about transformers, so I looked at 450 briefly and got under the impression that I should (or could) size the conductors and OCPD at 125% per table 450.3(B).

I didn't think that was entirely correct, but I figured the only way to know was to throw it out there to get shot down.

 

[Fossil]

Re: Wire size for transformer

Ok guys sorry for not giving all the info.
The CNC machine is rated at 47amps at 400volts 3 phase.
The transformer is 208 3 phase in and 400volt 3 phase out.
I figured 35000kva/208x1.732 = 97 amps

 

[George Stolz]

Re: Wire size for transformer

So, yeah, if it's not continuous, looks as though you could use #3 CU for it, bare minimum.