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jonny1982
Guest
Is it legal to feed a 112.50 KVA transformer with 480 volt power using a 150 amp breaker (primary) and then come out to a 120/208 panel that has a 125 amp main (secondary)?
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petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- Semi-retired engineer
why not?
112.5 KVA is 135A primary and 312A secondary current. The transformer is well protected.
iceworm
Curmudgeon still using printed IEEE Color Books
- Location
- North of the 65 parallel
- Occupation
- EE (Field - as little design as possible)
As others posted, sure it's legal. I recommend reading 450.3 and table 450.3B.
Keep in mind that just because it is legal, doesn't mean it will work. Don't feed bad if you get nuisance trips.
ice
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jonny1982
Guest
Thanks, but still a little confused. I just read something that said if continuous load, must multiply by 1.25. If Primary is 135 then 135 X 1.25 is 165. So Wouldn't I need a 175 amp breaker to feed primary and same for secondary (312 X 1.25 is 390 ) so 400 amp breaker???
Thanks
I suggest you go back to step 1. What is your load and is it continuous ?
From your initial post you indicated a 125 amp secondary panel.
As noted this is well below the capacity of a 112.5 kva transformer.
The transformer primary current will be proportional to the secondary current.
With the load you have on the transformer, 45 kva max based on your 125 amp panel, your 150 amp primary breaker might work (keeping iceworm's comment about nuisance tripping into account).
If you want to have the full capacity of the 112.5kva transformer, then the 150 is well undersized.
Your OP was "is it legal".. it is !
From your initial post you indicated a 125 amp secondary panel.
As noted this is well below the capacity of a 112.5 kva transformer.
The transformer primary current will be proportional to the secondary current.
With the load you have on the transformer, 45 kva max based on your 125 amp panel, your 150 amp primary breaker might work (keeping iceworm's comment about nuisance tripping into account).
If you want to have the full capacity of the 112.5kva transformer, then the 150 is well undersized.
Your OP was "is it legal".. it is !
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