See any problem with this design?????

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chevyx92

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Want to set a 112.5 KVA stepdown XFMR 3P 480/208. Primary will be protected at not more than 125%, so 175A 480V breaker feeding transformer. On the secondary side there will be (2) 200A 208V 3P 4W coming out of transformer. Each 200A feed will power a 200A Main Breaker panel for a total of (2) 200A panels being fed from transformer. Anyone see a possible inrush problem on the primary breaker of 175A? Also if needed, can't I increase the primary breaker to 250% if needed which would mean a 350A breaker on primary side? BTW calculated load for both 200A panels would total 290A.
 
Two points to keep in mind. The 250% rule in 450.3 does not permit "next size up" so IF you elected to do so the breaker would need to be a 300 amp.
Also keep the 240.21 (C) transformer secondary conductor rules in mind in determining the length of those conductors.
 
chevyx92 said:
Want to set a 112.5 KVA stepdown XFMR 3P 480/208. Primary will be protected at not more than 125%, so 175A 480V breaker feeding transformer. On the secondary side there will be (2) 200A 208V 3P 4W coming out of transformer. Each 200A feed will power a 200A Main Breaker panel for a total of (2) 200A panels being fed from transformer. Anyone see a possible inrush problem on the primary breaker of 175A? Also if needed, can't I increase the primary breaker to 250% if needed which would mean a 350A breaker on primary side? BTW calculated load for both 200A panels would total 290A.
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At first glance it seems that 2-200 amp MB on the secondary side would be compliant for this transformer. I'm not sure that inrush should be a problem and 175 amp primary breaker would be typical for this transformer. If you used the 250% value on the primary, that would require larger primary conductors than for the 175.
 
texie said:
At first glance it seems that 2-200 amp MB on the secondary side would be compliant for this transformer. I'm not sure that inrush should be a problem and 175 amp primary breaker would be typical for this transformer. If you used the 250% value on the primary, that would require larger primary conductors than for the 175.
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Correct.
 
I think augie makes a good point that when use the 250% value you don't get to use the next size up rule. So yes, it would be a 300 amp max primary breaker (unless in the unlikely event you meet the requirements of Note 3) . As I said, unless this is some unusual application I don't see inrush being an issue. Am I missing something?
 
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texie said:
I think augie makes a good point that when use the 250% value you don't get to use the next size up rule. So yes, it would be a 300 amp max primary breaker. As I said, unless this is some unusual application I don't see inrush being an issue. Am I missing something?
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No. Panels will feed a bunch of wood cutting machinery.
 
I think I would give some consideration to adding an individual breaker or fused disconnect for each of the 15HP motors and feed them from your transformer secondary (this would be allowed with the 125% primary protection).
To me this reduce the possibility of having your 200 amp breakers tripping by the 15HP motor inrush in the event the panels are carrying other loads at the time the 15HP motors start.
(you might even be able to eliminate one of the panels)
 
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