45kVA inrush and disconnect rating

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Rock86

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new york
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Electrical Engineer / Electrician
I had an electrician call me up the other day and ask about the effect of inrush current on a 45kVA transformer and nuisance tripping. He was curious if he could up size the circuit breaker 250%.

After some research and reading an article about inrush current in transformers by Eaton, I came to conclusion that he would be safe with an OCPD at 125%, but if he provided OCP on the secondary side, he could go up to 250% on the primary. Because of the building design, we concluded with his solution to use an OCPD of 250% on the primary at the panel, a fused disconnect within the 25ft tap rule, and an OCPD of 125% on the secondary panel.

Any thoughts about this? I know the extra fuse disconnect sounds redundant, but it was to meet tap rule. Anyone experience nuisance tripping from transformer inrush?
 
Sounds good. People will often size transformer primary OCPD to be closest to the highest allowed due to inrush issues . It's a real thing.
 
david luchini said:
Why the tap? Why not just run the larger primary feeder directly to the transformer?
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240.21(B)(3). The primary OCPD is located elsewhere in the building, and the feeders are supplying a separately derived system therefore, it is our interpretation that the tap rule applies. If this is incorrect, I would appreciate the clarification.
 
Rock86 said:
240.21(B)(3). The primary OCPD is located elsewhere in the building, and the feeders are supplying a separately derived system therefore, it is our interpretation that the tap rule applies. If this is incorrect, I would appreciate the clarification.
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There is no requirement to supply the transformer with a feeder tap.

A non-fused disconnect would meet the transformer local disconnecting means requirement.
 
I think the disconnect he is referring to is the one on the secondary side, not the primary. Yes that disconnect is needed anyway, not only for the tap rule but also 408.36 (if using panel boards).

The big disadvantage of using 250% for the transformer primary protection is you need conductors to match. Also then you may be into, say, a 400 amp disco instead of a 200, so the cost jumps quickly. One needs to weigh the extra cost with the likelihood of tripping on start-up, and think about how big a deal it is if a breaker is used and it trips sometimes upon start up.
 
david luchini said:
There is no requirement to supply the transformer with a feeder tap.

A non-fused disconnect would meet the transformer local disconnecting means requirement.
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After some careful review by your influence, I understand it better. Thanks.
 
electrofelon said:
I think the disconnect he is referring to is the one on the secondary side, not the primary. Yes that disconnect is needed anyway, not only for the tap rule but also 408.36 (if using panel boards).

The big disadvantage of using 250% for the transformer primary protection is you need conductors to match. Also then you may be into, say, a 400 amp disco instead of a 200, so the cost jumps quickly. One needs to weigh the extra cost with the likelihood of tripping on start-up, and think about how big a deal it is if a breaker is used and it trips sometimes upon start up.
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I was talking Primary side.
 
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