Transformer Secondary Protection

[jbs948]

Quick Question:
Art. 450.3B Note 2 permit primary protection only for transformers with the primary OCP does not exceed 125%?

Detailed Question:
I have an existing 75kVA 480v:208/120v transformer. The Primary Protection is a 100A 3P Breaker. The secondary is connected to a MLO Panel, with a few misc. breakers. Is secondary protection for the transformer not required, because the 100A breaker is under 125% of the primary-rated current. I believe the existing MLO panel to be in violation because there is no OCP for the conductors.

 

[augie47]

I read Note 2 as allowing you to have up to six circuit breakers in the secondary panel as long as the combined ampacity does not exceed the allowed value for a single device. Looking at Art 240, since you have primary OCP, it appears that you would be compliant if your secondary conductors are not over 10ft long (25 long industrial)

 

[jbs948]

450.3B Note 2 begins with "Where secondary overcurrent protection is required,...". What determines when/where secondary OCP is required? Is secondary OCP only required when the primary OCP exceeds 125% of the transformer primary-rated current?

I believe that when the primary OCP is less than 125% of the transformer's primary-rated current, then secondary OCP is not required.

I am only referring to the transformer, not the primary or secondary conductors.

 

[jim dungar]

You are looking in the wrong place.

You are looking in the wrong place.

450.3(B) allows the primary protective device to be larger than 125% if proper secondary protection is provided. There is nothing in 450.3(B) that requires secondary protection.

You need to be looking in 240.21(C). The secondary conductors need protection. Hopefully, you probably fall under the 10' or 25' tap rule.

And don't forget 408.36(A)if you are in NEC2005 or earlier, and simply 408.36 if you are using NEC2008.