Transformer Protection

[tommyve]

Hello,
I have a system that consist of 3 pole,150A circuit breaker on Panel A with a 125A rated feeder for 75' of run to a service/maintenance disconnect adjacent to a 75kVA 3 phase step down transformer (480:208/120V) which has 225A rated feeders coming off the secondary side for another 75' of run length to a 100A Main Circuit Breaker Panel B.


I am intending to use the MCB on panel B as my secondary over current protection, however I'm wondering if because it is over 25', I must include a local OCPD. I don't see how table 450.5(B) requires that, but I don't understand if 240.21(C) is relevant.

Do I need local adjacent (within same room) secondary protection or is the MCB on the panel B enough?

 

[infinity]

I am intending to use the MCB on panel B as my secondary over current protection, however I'm wondering if because it is over 25', I must include a local OCPD.

Yes as you've stated the secondary conductors are limited to 25' before an OCPD so you would need an OCPD within that 25'.

Welcome to the Forum.

 

[jim dungar]

. I don't see how table 450.5(B) requires that, but I don't understand if 240.21(C) is relevant.

These two sections are independent, even though they are related.

450.3 is about primary protection of the transformer. It basically says if your primary side OCPD is over 125% you need a secondary side device no greater than 125%.

240.21(C) is about the location of the secondary side OCPD.

 

[augie47]

Note the rules are different IF the secondary conductors are outside.