75KVA transformer protection

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jamism

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Hello Everyone,

I have a 75KVA 3 phase transformer (480V to 208V) 90A on the 480V side. I know I'm supposed to protect it up to 125% which turns out to be 112A. I want to use a 110A 3 pole breaker but is a 100A breaker still OK to use. I am worried about nuisance trips and the inrush current tripping breaker. What do you guys think is 100A good?

Thanks.
 
can't you go to the next higher standard size breaker? I am a big fan of installing the largest OCPD the code allows.

i might even be inclined to use the rule that allows for 250% in some cases, but in this case it might create an expensive solution.
 
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In this part of the country, historically many 75KVA transformers have been protected at only 100A on the primary.
 
Where I am if an EE designs it the 75 KVA will have a 150 on the primary.

If an EC designs it there will be a 100 on the primary, I have never seen a problem with the 100.

The benefit of using a 100 is the fact a 100 amp breaker will fit in a panel where a 125 or 150 will not.
 
The make and model of the breaker and should also be considered. There are differences on trip characteristics, that inrush may factor in.
 
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