Transformer Calc.

[maddenm]

We have 3 single phase 7.5 KVA transformers wired together 3 phase Delta/Y. 480v to 240/120v What is the combined KVA and what is the proper overcurrent protection?

 

[charlie b]

A delta - wye will not give you 240/120 on the secondary. How about giving us your answer, or at least tell us what you have tried.

 

[maddenm]

My mistake, 120/208 volts. Our disagreement is between 12.9 KVA and 22.5 KVA. The question would be what is the proper size main breaker at the Distribution panel.

 

[LarryFine]

Mad, welcome to the forum!

Three 7.5 kva units can supply (3 x 7.5) 22.5 kva.

Do you know how to convert that to amps at 208v?

 

[charlie b]

Our disagreement is between 12.9 KVA and 22.5 KVA.

I notice that 12.9 kva (someone's notion of the total) divided by 7.5 kva (the rating of the individual components) equals a number that is very close to the square root of three. That number will always turn up somewhere, when you work with three phase systems. But it does not turn up, when all you are doing is adding up values of kva. The total is 22.5, as Larry has already said.

 

[Lady Engineer]

This happens for residential, because they have single phast tranformers, but will add a third, in order to give a store or business a three phase service. They often used three-240/120V single phase transformers then give you the 240/120V wild leg (or high leg), but you can't used the high led for 120V loads, which will be rated 208V on that phase leg.

Anyway, I agree with Charlie, 22.5KVA, you can't add amps of different voltages, but you can add KVA.

 

[david luchini]

what is the proper overcurrent protection?

The proper overcurrent protection would be:

35A max on the primary for primary only protection,

or

60A max on the primary/80A max on the secondary for primary/secondary protection.