ECN Forum Forums General Topics of the Trade General Discussion Area Transformer Feeder Upgrade

Hello all,

Here's my dilema, I'm upgrading an exisiting building to a machine shop.

Currently it has a 400 Amp 3 phase 480 Main Service.It has a 227 KVA Transformer on a(480 volts) 200 Amp breaker feeding a 400 amp 208/120 three phase secondary panel.

I want to put the transformer (227 KVA) on a 400 breaker(Primary),and go to parallel 3/0 feeders for the primary side.

I want to add an additional 225 Amp panel and feed directly off the secondary taps on the transformer.The new 225 Amp panel would have a 225 Amp main breaker.

I'm thinking this is legal.

The 227KVA transfomer at 208 volts according to my calculations can handle 618 Amps.

Thanks for your help in advance

I believe you are correct. Your selection of overcurrent devices meets the primary and secondary protection requirements of Table 450.3(B).

BTW, I came up with 630 amps for a 227 kva transformer.

I'm thinking you could actually bump up the size of the new panel to 350 amps.

Its my opinion that your primary feeder for your 227kva transformer should be calculated as such:

227,000 / ( 1.732 x 480) = 273 amps

273 amps x 125% = 341 amp

341A next size up per 240.6 would be 350A breaker

So parallel sets of (3)#2/0 CU THHN/W @ 75 deg. would be ideal. The 400 amp breaker exceeds the maximum 125% as allowed in 430.3(b) for primary protection only.

The transformer (Secondary) will be feeding two panels one 400 amp(Which is currently the Main 208/120 panel for the building) and the additional 225 Amp panel I want to add.

As far as I know it's legal to feed multiple panels from one transformer assuming you don't overload it.

Please correct me if I'm wrong Thanks

There are two ways of protecting a transformer, primary only and primary and secondary. If you use primary and secondary protection, your primary OCPD can be 250% of the primary current, in this case, 273 x 2,5 = 682 amps, so your 400 amp breaker is fine. The secondary full load current is 630 amp x 125% = 787 amps max ocpd. Your proposed total of ocpds for the secondary side is only 625 amps, well within the limits. you are allowed multiple ocpds, see note 2 to table 450.3(B)

Conductor protection is a separate issue from transformer protection, so parallel 3/0 conductors on a 400 amp breaker is a good pick.