Transformer Secondary Disconnecting Means

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Dale P

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I am having a hard time understanding this subject. I have been recently told that ALL transformers need a disconnecting means on the secondary. When questioned where in the code this information lies the response is 240.21. I see how this applies to taps but I'm having a hard time understanding how a transformer secondary fits in. The information I have been given is if the conductors are over 10' a disconnect is needed. The primary is protected according to 450. They are not stating over current protection on the secondary but just a disconnecting means. I would appreciate any help on this subject or a point to where I can find the information. Thank You All.

Dale
 
Welcome to the forum.:)

Transformer secondary conductors are required to be protected from overcurrent by one of the methods in 240.21(C)(1) through (6).

This is most likely result in a breaker or fused disconnect that will also serve as a disconnecting means.

Chris
 
Dale P said:
I am on board with over current protection. I get confused with the 10' limit and just a disconnecting means.
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There is no requirement for a disconnecting means for secondary conductors over 10' long. Only "Outside Secondary Conductors" (240.21(C)(4)) requires a disconnecting means for the transformer secondary, and these conductors can be of any length.
 
Dale P said:
I have been recently told that ALL transformers need a disconnecting means on the secondary.
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You were told wrong (Corollary to ?Charlie's Rule?). You did well to question the basis for that (alleged) requirement, and it is sad that you were given no better reason than the one you received. But the code is what the code is (Restatement of ?Charlie?s Rule?), and the code has no specific requirement for a secondary disconnecting means. As it turns out, I can?t think of any way to achieve overcurrent protection for both the transformer?s secondary windings and the secondary conductors, without that device also being able to serve as a disconnecting means. But there is no specific requirement to be able to disconnect a transformer?s secondary conductors within a specific distance.
 
Welcome to the Forum.
Folks often misunderstand that Art 450 is convened about protection of the transformer and Art 240 for protection of the conductors. Though related, they address different issues.
My suggestion is that, after satisfying Art 450, you look at 240.21(C)(1) to see IF your situation requires secondary protection for conductors. If it does, then 240.21(C)(2) thru (6) will help determine the location of the OCP device.
 
augie47 said:
My suggestion is that, after satisfying Art 450, you look at 240.21(C)(1) to see IF your situation requires secondary protection for conductors. If it does, then 240.21(C)(2) thru (6) will help determine the location of the OCP device.
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And I always recommend starting with 240.21(C), then going to 450.3(b).
 
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