ECN Forum Forums Code Related Discussion NEC & other Code issues Subpanels fed from Transformer-SDS

Got a few design issue questions to post, in order to get some feedback. Has to do with separately derived systems [SDS].

Example: Two [2] 225 amp subpanels for 208y120 3 phase 4 wire power fed from a 150 KVA 480 / 208Y120 3 ph 4 wire step down Transformer. Z% = 2.0%. Subpanels located within 25 linear feet of transformer and all equipment is in one common room. For the sake of argument, we will say the max SCA is 24 KAIC.

Would like your input [code related, personal preference and cost wise] to the Secondary Overcurrent Protection and disconnects, plus preferred Primary Protection.

My example would have the subpanels run with 250 MCM THHN cu, and each to have a main OCPD rated at 225 amps, 42 KAIC. From the Transformer, a fused switch with 3-400 amp RK-5 [200KAIC] fuses and paralleled 250s-300 MCM cu. Time-current curves to allow selectivity between branch C/Bs, Main C/B and finally the 400 amp fuses.
Primary to be protected via 175/3 Circuit Breaker with a AIC rating of, like 65,000 [or next step above what ever is available at the gear] and fed via 3-4/0 THHN cu.
Grounding connected via common grid connections.

Please reply with your opinions, questions, suggestions and such. This is an example situation only. It is only posted for a conversation piece .

Scott. "S.E.T."

P.S. I hope these "design" example ideas are OK with the group. Just trying to get something going here and there and to see the various answers/comments.

The primary breaker seems small. The primary fla should be 180 amps and protection could be as much as 250% of this per Table 450-3(b). It may be more cost effective to provide 2 225 amp breakers on the secondary at the transforemer in place of the 400 amp fused disconnect and the 225 amp main breakers in the panels. The secondary of the transfromer will require a grounding electrode system per 250-30(a)(3). The common grounding grid may not meet this requirement.
Don(resqcapt19)