A 75 kVA 480-120/208Y step down transformer protected on the 480V primary side by a 100A fused disconnect switch feeds two 120/208 panelboards. These are two 42 circuit 225 Amp Main Breaker panels loaded to the gills (and contain mini breakers also) from the secondary of the transformer. The low voltage side current rating of the transformer is 203 Amps. As I interpet the code, with the 100 Amp fused disconnect switch on the primary, secondary protection on the transformer is not required.
There is probably not 450 Amps of connected load on the panels, but could someone please refernce what code sections either allow or disallow this installation.
WireStu, You are correct that the transformer itself does not require overcurrent protection on the secondary side, however the conductors feeding the panels may require overcurrent protection. See 240-21(c)(2). Also this installation would be in violation of 384-15 if all 42 spaces are filled and some of the spaces have 1/2 space breakers in them. Don(resqcapt19)
Re: Too Many Breakers?#76689 03/15/0102:50 PM03/15/0102:50 PM
Glenn, Yes, 240-3(f) requires CONDUCTOR overcurrent protection for 3 phase 4 wire systems, but you go to 240-21(c) to find out how to protect these conductors. There is no requirement to protect the trasformer secondary if the primary OCPD is rated at 125% or less of the primary current. The transformer overcurrent protection requirements are all in 450-3.
Stu, The 100 amp OCPD on the primary will protect the transformer from the 450 amp connected load. A load calculation need to be done to see what the actual load is. The transformer may be too small.
Re: Too Many Breakers?#76692 03/15/0106:35 PM03/15/0106:35 PM