Reply to tommom:
Each of the two 12.5kV-480V substation transformers have a bus connection between the transformer neutral and the substation ground bus; in addition, there are two MCCs with neutral buses which are cable connected to the substation ground bus. From what we can tell, the "300'...
Before considering current limiting breakers, review NEC 240.86(C). I had a similiar problem and was going to use Cutler-Hammer DSL breakers to reduce the fault current to the downstream MCCs, but was prohibited by this requirement. This requirement first appeared in the 2002 NEC apparently...
I want to thank everyone for their comments on this issue. We checked the various items that were mentioned in this forum. The reply that we have a 300' long current transformer is correct. We installed a 4/0 jumper from one of the MCC ground buses to building steel with no effect on the...
We used 1% reactors at 12.5kV to reduce fault current on the 12.5kV-480V substations from 70,000 amps to 64,000 amps. The power factor also dropped 2% with these reactors.
We are measuring 200 amps in the ground conductors from the 480 volt substation ground bus to the MCC ground buses. The 480 volt sub is a double-ended substation with two 3000 kVA 12.5kV-480 volt transformers. Each half of the substation powers three 1600 bus MCCs, each with 1000 amps of load...
