150kva transfomer

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mbrooke

mbrooke

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150kva dry type 480 to 208/120 delta wye transformer. 200amp primary breaker with 3/0 cu. Secondary conductors are 500mcm. Secondary goes over to a tap box that splits into 2 panels each with a 200amp main. This is an older exiting installation.


Does this sound correct from a code standpoint?
 
mbrooke said:
150kva dry type 480 to 208/120 delta wye transformer. 200amp primary breaker with 3/0 cu. Secondary conductors are 500mcm. Secondary goes over to a tap box that splits into 2 panels each with a 200amp main. This is an older exiting installation.


Does this sound correct from a code standpoint?
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It sounds wrong in terms of 240.21 and 240.21(C).
 
iwire said:
I would say common but non-compliant.
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:lol::lol::thumbsup: Took the words out of my finger tips. :D


As of late I have been looking as a lot of dry type transformer installations and I have to say just about everyone has peaked my interest code wise.


Would this have been code complaint going 30 years back by chance?
 
Looking at this the FLA of a 150kva secondary is 416 amps while 500mcm is rated 380amps? Or this is correct?



Looking at 240.21, my take is that splitting the secondary is not allowed even if the same size, however, if it came out of the transformer terminated from its own lugs it would then be complaint?
 
I see nothing in the '99 Code that prohibits it.
In '08 it is prohibited by 240.21(C) in a wording change.
 
mbrooke said:
Looking at this the FLA of a 150kva secondary is 416 amps while 500mcm is rated 380amps? Or this is correct?



Looking at 240.21, my take is that splitting the secondary is not allowed even if the same size, however, if it came out of the transformer terminated from its own lugs it would then be complaint?
Click to expand...
I think you are correct. And according to 240.21(B)(3)(5), if the 500 were to be landing in a single overcurrent device it must be protected no more then the conductor ampacity 380 is between standard devices - so you would need 350 amp overcurrent device or larger conductor.

As far as splitting - even if you ran 600 instead of 500, transformer secondary conductors are tap conductors and they must land at an overcurrent device, you can not tap a tap. However running 3/0 to each disconnect is acceptable, as long as the ampacity isn't less then 1/3 the protection of the feeder - which it isn't in this case.

Not sure just what changes were made that Gus brought up in 99, but tapping a tap may still have prevented this install from being compliant, as that has been a general no-no for a long time.
 
I think the change was made to the '99 to clarify the situation.
240.21(C) covers "Transformer Secondary Conductors" which, when you take the definition of tap conductors into consideration, are not technically
"tap conductors".
I have seen the arrangement the OP described in many older installations leading me to think it was an well documented violation.
 
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