Multiple transformer secondary taps

[wirenut1980]

Hello, I am reviewing a set of drawings that show a 12,470 V - 480/277 V (delta - wye) transformer, 1500 kVA pad-mount oil-filled. It shows 2 connections directly to the transformer secondary. The first is (4) sets of 600 Kcmil Cu in parallel, (4) 4" conduits feeding a 1600 Amp main distribution panel with main breaker. The other is (2) sets of 4/0 Cu in parallel, (2) 2.5" conduits feeding a 400 Amp main distribution panel with main breaker. Is this a code compliant plan? At first look I thought so, but something just doesn't seem right.

 

[charlie b]

Re: Multiple transformer secondary taps

You can have up to six breakers constitute, as a set, the transformer's secondary protection. You have two. Their total ratings can't exceed the limits for secondary protection. In your case, the rated secondary current is 1805 amps, and the limit for over current protection is 125% of that (i.e., 2256 amps). The total of your two breakers is 2000 amps. So that is OK.

But the two breakers have to be "grouped together in one location." I am not certain, but I believe that having them in two panels that are close to each other would be fine. But if the two panels are in different rooms, then I think that would violate Note (2) of Table 450.3(B).

 

[steve66]

Re: Multiple transformer secondary taps

But the secondary conductors also have to be protected. That means the secondary conductors have to be installed per the tap rules of article 240. Unless this is an outdoor installation, that would limit the secondary lengths to 25' (or maybe even 10').

Steve