Oversize transformer

[teco]

Hello,
I've been asked by our customer to use a spare existing 500 kva transformer, rated 480 delta primary and a 208/120 wye secondary to feed
a 800 amp circuit breaker located in a PDU 15 feet away. I have been asked to supply only the required 800 amp raceways and wiring .
My question is does this violate 240.21c3?

Thanks

 

[iwire]

Hello,
I've been asked by our customer to use a spare existing 500 kva transformer, rated 480 delta primary and a 208/120 wye secondary to feed
a 800 amp circuit breaker located in a PDU 15 feet away. I have been asked to supply only the required 800 amp raceways and wiring .
My question is does this violate 240.21c3?

Thanks

You can do that but on the primary side you may not be able to trim the feeder down and still be able to energize the transformer. The inrush remains the same.

 

[augie47]

It seems to violate 240.21(C)(3)(2) but you should be able to meet 240.21(C)(6)

 

[teco]

You can do that but on the primary side you may not be able to trim the feeder down and still be able to energize the transformer. The inrush remains the same.

Iwire,
I am feeding the primary side with 600 amp feeder. I your opinion this does not violate 240? I have been told by an in house guy it violates 240.21C31. You don't agree?

Thanks

 

[teco]

It seems to violate 240.21(C)(3)(2) but you should be able to meet 240.21(C)(6)

Augie,
Do you mean 240.21(C)(6)1?

 

[don_resqcapt19]

I agree with Augie. You could make this installation under the rules in (C)(6).

 

[teco]

I agree with Augie. You could make this installation under the rules in (C)(6).

Thanks Don, That is what I'm hanging my hat on. If I have it right the primary to secondary ratio is 4/1 (480/208/120). One third of the primary OPCD is
200 (600 amp breaker) 4x200= 800A secondary conductors.

 

[david luchini]

Thanks Don, That is what I'm hanging my hat on. If I have it right the primary to secondary ratio is 4/1 (480/208/120). One third of the primary OPCD is
200 (600 amp breaker) 4x200= 800A secondary conductors.

Your primary to secondary ratio is 2.3/1.

You could use a 800A or 1000A primary c/b and still use the 800A secondary conductors per 240.21(C)(6), if tripping the primary on inrush is a concern like Bob mentioned in post 2.

 

[don_resqcapt19]

To follow up on David's post. The minimum ampacity of the secondary conductors for this application would be (480/208)(600)/3 or 460 amps.

 

[teco]

Thank you all.