12,470/480 Transformer Primary conductor size help


Hi all. I would like some assistance on sizing the primary conductors for a transformer.

The situation is: Inside a chemical storage facility, with controlled access. There is a 12,470V supply coming from a PSE-11 distribution box. This will feed the transformer for a new 2500A Motor Control Center. The transformer is a 2,000kVA 12,470/480 Delta Wye. Prior to terminating in the transformer, the 12,470 will pass through a Square D HVL-CB inside the building.

From my past experience, I would use 93 as the FLA and 117 for the FLA x 125%. From there, using table 310.60(C)(77) I would pull 3-1/C #2 and a #2 ground. However, our engineer (without much experience in the industrial field) is saying I need to pull 3/0 conductors, and is quoting table 240.92(B) and the short circuit rating calculation.

I explained that since this is protected where it gets the power (the 150A fuse in the PSE-11) it doesn't fall under the tap rule, and doesn't have to be sized for short circuit rating.

Can you please help.?

charlie b

Staff member
Assuming you are using MV-90 conductors, I agree with #2. I think a #2 ground is overkill (or was that a typo?). I also agree that this is not a tap situation, so that 240.92(B) does not apply.