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Norm
Norm, Master Electrician
Category: Electrical
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Experience:  Registered professional electrical engineer and licensed master electrician.
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I am installing a 75 KVA 208 3P to 440 3P transformer. What

Customer Question

I am installing a 75 KVA 208 3P to 440 3P transformer. What gauge wire should I use (in and out of the transformer) and what size breaker into the transformer?
Submitted: 1 year ago.
Category: Electrical
Expert:  Rich replied 1 year ago.

Rich :

Hello .. Rich here .. I can help you with this what is the amperage of the panel you are feeding with this transformer ?

Customer :

200a

Rich :

For the 200 amp service you would need 2/0 copper or 4/0 aluminum .. out of the transformer

Rich :

the primary of this transformer will draw about double the current 450 amps .. that would take 400 kcmil and a 450 amp breaker

Expert:  Rich replied 1 year ago.
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Customer: replied 1 year ago.

Just re-read your question the load on the 440 3p side of the transformer will not exceed 90A...

Expert:  Rich replied 1 year ago.
Yes. And you are wiring it to a. 200amp panel
Expert:  Rich replied 1 year ago.
Using the 75kva xformer you would have 350amps in and 155 amps out. So. Wire the primary with 350 copper to a 350 amp bkr.
Customer: replied 1 year ago.

Each phase? Doesnt seem to add up...what is the math?


 


 


208V 3p in


 


440V 74 KVA load at 89 amps - rating on the equipment


 


75 KVA transformer

Expert:  Norm replied 1 year ago.

Hi, I'm Norm.

The primary (208 V.) current for the 75 kVA transformer is, coincidentally, 208 Amps. A breaker for that side must be at least 1.25 times that value, or 260 Amps. The next standard overcurrent device is 300 Amps, so use that with 300 kcmil copper wire with THHN insulation, no more than 3 current-carrying conductors in conduit. The secondary (480 V.) current is 90 Amps, so you can use #4 copper with THHN insulation, also no more than three current-carrying conductors in conduit. I use 480 Volts rather than 440 Volts as that is the standard utility voltage today, even though your load may have a 440-Volt nameplate.

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