Transformer Protection

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augie47

augie47

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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 ?
 
augie47 said:
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 ?
Click to expand...

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.
 
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 ?
 
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 said:
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.
Click to expand...

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 ?
 
augie47 said:
understand.. and there is a lockable disconnect on transformer A which could serve as "B's" disconnect.
Click to expand...

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:
 
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