Sizing transformer secondary

[Sparky3141]

This is something different for me. I have a 3 phase Delta-Wye, 30 kVA, 480 ? 208/120 transformer with #10?s ran to the primary and fused at 30 amps.

The way I figure it that works out to technically (potentially) be a 50 amp primary: 30,000 / (480*1.732) * 1.25 = 45.1. But for some reason, someone put in #10?s. So now that?s what I have to work with.


But with the 10?s at the primary, I?m limited to 30 amps; so my question is ; do I use that as my basis for sizing my secondary?


Thus: Determine primary kVA at 30 amps: 480 * 1.732 * 30 = 24.940 kVA


Then size secondary so as not to overload the 30 amp primary: 24.940 / (208 * 1.732) = 69.2
Which would put me at 70 amps.

It makes sense to me to do it this way so as not to overload the primary but I?ve not had the experience before to not size primary and secondary together based on transformer max rating.

 

[jim dungar]

It makes sense to me to do it this way so as not to overload the primary but I?ve not had the experience before to not size primary and secondary together based on transformer max rating.

Except for the rare occasion where the primary is protecting the secondary, you can do what ever you want.

I would size the secondary based on full load. If it turns out the 30A primary is not enough for the load, then that is all that would need upgrading.

 

[iwire]

I agree with Jim's approach.

I think it is worthwhile to consider that typical transformer overcurrent protection of up to 125% secondary and up to 250% primary does not protect the transformer from overload. The transformer is protected from overload by using good sense what you connect to it.

30,000/208/1.732*1.25=104amps.

I would feel fine using a 100 amp breaker with 3 AWG copper, but the code allows up to a 110 amp breaker.

 

[Sparky3141]

Yeah, I didn't find anything in 240 or 450 that addressed that but thinking of it that way then the worst that could happen would be to blow the fuse(s). Thanks for the input.



Sincerely,
Jerry