Transformer Protection

[augie47]

Scenario: 240v single phase two wire system supply transformer "A" with 480 v secondary.
Primary OCP at 125%. Two-wire Two-wire system with adequately sized secondary conductors to meet 240.21(C)(1) (no secondary OCP). Secondary of transformer "A" feeds transformer "B" which steps 480 back down to 240/120. Transformer "B" has 125% secondary protection.
Does Transformer B need overcurrent protection and does it need a disconnecting means ?

 

[electrofelon]

Scenario: 240v single phase two wire system supply transformer "A" with 480 v secondary.
Primary OCP at 125%. Two-wire Two-wire system with adequately sized secondary conductors to meet 240.21(C)(1) (no secondary OCP). Secondary of transformer "A" feeds transformer "B" which steps 480 back down to 240/120. Transformer "B" has 125% secondary protection.
Does Transformer B need overcurrent protection and does it need a disconnecting means ?

Awesome question. I have pondered this. Let me confirm that you are thinking what I am thinking: 450.3(B) does not seem to have any allowance for not having primary protection, but it seems logical to apply the principle in 240.21(C)(1) to the "B" transformer. Am I with you?

Regarding disconnecting means, I see no requirement.

 

[augie47]

Yes. Since 240.21(C)(1) allows the primary to protect the secondary conductors in this situation could that (primary on transformer "A") not also be transformer "B"'s protection ?

 

[dkidd]

450.14 Disconnecting Means. Transformers, other than Class
2 or Class 3 transformers, shall have a disconnecting means
located either in sight of the transformer or in a remote location.
Where located in a remote location, the disconnecting
means shall be lockable in accordance with 110.25, and its
location shall be field marked on the transformer.

 

[dkidd]

but it seems logical

 

[augie47]

450.14 Disconnecting Means. Transformers, other than Class
2 or Class 3 transformers, shall have a disconnecting means
located either in sight of the transformer or in a remote location.
Where located in a remote location, the disconnecting
means shall be lockable in accordance with 110.25, and its
location shall be field marked on the transformer.

understand.. and there is a lockable disconnect on transformer A which could serve as "B's" disconnect. For conversations sake, assume we do need to install a disconnect at "B", can it be non-fusible ?

 

[dkidd]

450.12 does not require overcurrent protection, just a disconnect.

 

[electrofelon]

understand.. and there is a lockable disconnect on transformer A which could serve as "B's" disconnect.

Right. That is what I meant by my "no disconnect requirement" comment. Should have said, "I dont see any requirement for a transformer specific disconnect."

As far as the protection goes, The more I think about, I would say the proposal is perfectly acceptable. The fact is the transformer is protected, simple as that. 450 doesnt give a specific requirement for where this protection must be or that it has to be 'dedicated' protection, and I think the laws of physics are fairly reliable :angel: