Step Down Transformer Secondary Conductors

[BRYAN AMBROGIO]

A 112.5kVA transformer with a 150A primary breaker with 3#3/0 primary conductors - feeding a 400A MCB panel with #500 secondary conductors.
Per 240.4(B) we can use #500's on the secondary side good for 380amps, jump to the next breaker size @ 400amps.
An inspector has rejected this under 240.21(C) - Transformer secondary conductors are not permitted to be sized under 240.4(B)
However, 240.21(C) specifically calls out "without secondary overcurrent protection". This is not our case.
Reading further into 240.21 transformer secondary requirements are defined but only for specific circumstances as follows:
A) Without Overcurrent Protection
B) Length of Conductors
C) Primary Overcurrent Device
D) Industrial Installations
E) Outdoor Secondary Conductors

We believe that since this installation doesn't fall within any of the specific parameters of 240.21 we are allowed to use 240.4(B) for breaker sizing on the secondary.

Can anyone point to anywhere in the code that addresses this issue "WITH OVERCURRENT PROTECTION" or if anyone can give me any input on this issue that may shed some light?

Thank You

 

[iwire]

The conductors do not have overcurrent protection, they are connected to the transformer secondaries.

 

[don_resqcapt19]

You have to understand that there are two sets of rules that you have to comply with. The rules in Article 450 apply to the protection of the transformer itself. The rules in Article 240 apply to the protection of the transformer secondary conductors.

In your case, you have complied with the rules in 450.3 that permit primary only protection for the transformer itself, you still have to comply with the rules in 240.21(C) for the protection of the secondary conductors.

First take a look at 240.4(F). This specifically says that the transformer primary protection cannot be used to protect the secondary conductors in your application. That means you have to provide protection for the secondary conductors based on 240.21(C).

 

[under8ed]

Here is a link where mike Holt himself explains sizing conductors for this transformer..
It's quite old, & even then he needed a larger secondary.
Your situation could be different on a couple of aspects.

http://www.mikeholt.com/mojonewsarchive/NEC-HTML/HTML/TransformerInstallation~20020516.htm

 

[infinity]

As Don pointed out your secondary conductors would require an OCPD less than or equal to the ampacity of the secondary conductors. The inspector was correct to reject the 500 kcmil (380 amp) conductors with a 400 amp OCPD.

 

[BRYAN AMBROGIO]

Thank You

Thank You

Thank You guys very much this helps