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Friday, March 27, 2015
Question:
Charlie,
I have submitted code questions in the past and very much appreciate the service that you provide the industry.
I have been asked to install an isolation transformer in line with a feed to an existing panelboard. The panelboard is a 250-amp main lug only, fed from a 250-amp circuit breaker in a switchboard in the same room. The system is a 240/120-volt, 3-phase, 4-wire, Delta B-phase high leg. The isolation transformer is a 240-volt primary and secondary, and as such the neutral will be established on the secondary side of the transformer. This is not an industrial facility and the secondary conductors will be approximately thirty feet. My question is does the existing panelboard now require a main circuit breaker?
Thank you, Dennis DiPasquo
A
Answer:
Hey Dennis thanks for your question, we are glad you are happy with the CQD. Based on my understanding of your installation, yes. In general, panelboard protection must be provided on the secondary side of a transformer as stated in 408.36(B). Also the thirty foot length of secondary conductors is a problem - if you are using the rules in 240.21(C) for the location of the transformer secondary conductor overcurrent device.