Bifurcated transformer secondaries

mshields

Senior Member
Location
Boston, MA
The wording in 240.21(C) makes clear than one can have multiple sets of conductors coming off a transformer secondary. The application at hand is an outdoor customer owned pad mounted transformer so 240.21(C)4 applies.

The secondary is 480V so FLA times 1.25 is 338Amps. Next size up per 240.6 is 350Amp.

But I'm going to have two secondary breakers each equal in size. So that's easy. two 175A-3P OCPD's.

So a) anything wrong with the above and b) what if they were different sizes. Is that permissible? And if so, where is this made clear in the NEC? Or is it simply silent on the matter?
 

topgone

Senior Member
The wording in 240.21(C) makes clear than one can have multiple sets of conductors coming off a transformer secondary. The application at hand is an outdoor customer owned pad mounted transformer so 240.21(C)4 applies.

The secondary is 480V so FLA times 1.25 is 338Amps. Next size up per 240.6 is 350Amp.

But I'm going to have two secondary breakers each equal in size. So that's easy. two 175A-3P OCPD's.

So a) anything wrong with the above and b) what if they were different sizes. Is that permissible? And if so, where is this made clear in the NEC? Or is it simply silent on the matter?
No problem. Please remember that you are allowed to protect your ttransformer at the primary only. As long as you only have a dedicated primary OCPD set no more than 125% of primary FLA, you're good to go. Your secondary breakers should protect your respective conductors, though.
See 240.21(C)(1) thru (6)
 

mshields

Senior Member
Location
Boston, MA
in answer to your question

in answer to your question

Ron - This is indeed a 3 phase, delta wye.

Don - it's actually a 4160 to 480Y/277 so 450.3(A) would be applicable.

Thanks,

Mike
 

mshields

Senior Member
Location
Boston, MA
regarding using 125% on priamry of a transformer - IF IT WERE less than 1000V

regarding using 125% on priamry of a transformer - IF IT WERE less than 1000V

How does one get away with using a 125% primary breaker on a say 480 to 208Y/120V dry type transformer. With inrush of 12 on the first time and 4 every time after that, wouldn't a typical thermal mag primary OCPD sized at 125%, trip on start up every time?

Thanks,

Mike
 

ron

Senior Member
How does one get away with using a 125% primary breaker on a say 480 to 208Y/120V dry type transformer. With inrush of 12 on the first time and 4 every time after that, wouldn't a typical thermal mag primary OCPD sized at 125%, trip on start up every time?
The curve for the breaker typically has an inverse current characteristic, so the instantaneous portion is 10-12xtrip
 

topgone

Senior Member
The curve for the breaker typically has an inverse current characteristic, so the instantaneous portion is 10-12xtrip
Yep, breakers are designed to trip that way. But, because newer transformers are designed to exhibit very low impedances (high-efficiency cores), instead of the normal 6-12 times FLA, we can expect to have 8-17 times FLA inrush!:(
 
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