Secondary overcurrent protection of transformer

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Joe Gauthier

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Recieved a drawing of a 225 KVA transformer 480 to 208/120 3P4W.
Secondary conductors are 2 sets 0f 500 mcm spliced in a trough to 4/0 per phase feeding 4-200 amp main circuit breaker panels.
Do the 4-200 amp MCB comply with the code requirement as secondary overcurrent protection.
 
Joe -

There isn't an easy answer. Take a look at 240.21 and carefully examine all of the twists, turns, and kinks. There are lots of installation specific requirements to consider.

carl
 
Joe,
I don't see any way that you can leave the transformer with parallel 500s, tap off 4/0s to the panels and have a code compliant installation. 240.21(C).
Don
 
Joe to answer your question NEC 450.3
Transformer seconady protection: for CB only
any location = 6% impedance or less= 300% of FLA
= Greater than 6% impedance =250% of FLA

Supervised loation = 6% impedance or less= 300% of FLA
6% or greater = 250% of FLA
 
Code exhibit

Code exhibit

Exhibit 450-4 indicates that I can use more than one circuit breaker for secondary overcurrent protection, this exhibit shows no more than 6 circuit breakers. My concern is the splicing and reduction of wire size.
 
ryan_618 said:
I agree with Don. Taps from a transformer secondary are not permitted.
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So how would he make this code compliant with his two sets of 500 kcmil? If he installed a 800 amp disconnect fused at 700 amps and then fed all 4 panels from the load side of the disco?
 
Trevor,

This was design is shown on a drawing. I agree it does not look right, I do however need to find the article that suppors my concern.
 
Ryan,
Ah *^&%) I hate it when I do that. You are correct I did use over 600Volts.
I meant to use 240.4, 240.21, 240.100 & 240.101
Aren't taps allowed under 240.21 (1) and (2) ?
 
Trevor,
So how would he make this code compliant with his two sets of 500 kcmil? If he installed a 800 amp disconnect fused at 700 amps and then fed all 4 panels from the load side of the disco?
Click to expand...
The easiest way would to run the 4/0s directly from the transformer to each of the panels assuming that the conductors would be 25' or less in length. If the 4/0s are run in the wireway from the transformer to the panels, derating of the 4/0s would not be required based on 376.22.
Don
 
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