Parallel circuit breakers feeding a transformer?

[Paul Falcone]

Can I feed the primary side of a step up transformer (208V to 480/277V) with parallel conductors from parallel circuit breakers?

It's a 112.5 kVA transformer. Primary CB is 400A, however distribution panel feeding transformer can only accommodate 225A branch circuit breakers. Can I use 2 200A, 3P CBs to feed the transformer if my parallel conductors are sized to handle the full 400A (500 MCM)? Will 500 MCM fit on a 200A CB? Is there a way to interlock the 2 breakers to use smaller conductors (3/0)? The distance from panel to transformer is less than 10 feet, so conductor cost is not much of a concern. Any info or other solutions would be appreciated. Thanks.

 

[augie47]

Can I feed the primary side of a step up transformer (208V to 480/277V) with parallel conductors from parallel circuit breakers?.

Not allowed ( 240.8)

It's a 112.5 kVA transformer. Primary CB is 400A, however distribution panel feeding transformer can only accommodate 225A branch circuit breakers. Can I use 2 200A, 3P CBs to feed the transformer if my parallel conductors are sized to handle the full 400A (500 MCM)? Will 500 MCM fit on a 200A CB? Is there a way to interlock the 2 breakers to use smaller conductors (3/0)? The distance from panel to transformer is less than 10 feet, so conductor cost is not much of a concern. Any info or other solutions would be appreciated. Thanks .

Is your transformer stepping 208 up to 480 or step-down ?

 

[Paul Falcone]

Not allowed ( 240.8)


Is your transformer stepping 208 up to 480 or step-down ?

Stepping up from 208V (pri) to 480V (sec).

240.8 is pretty clear ... won't be doing that!

Thanks!

 

[augie47]

Don't see any easy answer. There is no "next size" on the 250% rule so your 400 is too big.
Any change of reducing the OCP supply your 400 amp panel to 300 ?
Otherwise, you could install a feed thru lug kit (if available) and feed a fused disconnect or breaker ahead of your transformer.

 

[Paul Falcone]

Don't see any easy answer. There is no "next size" on the 250% rule so your 400 is too big.

I think my original information might not have been very clear. To size the CB on the primary (208V, 3P) side of my 112.5 kVA transformer I used:

112,500/208/1.732 = 312.28 x 125% = 390.35A = 400A CB (next size per 240.6). Am I correct?

The distribution panel is 600A and cannot accept the 400A CB for my transformer. I will investigate the feed thru lug option and then installing a fused disconnect ahead of the transformer.

Thanks.

 

[charlie b]

112,500/208/1.732 = 312.28 x 125% = 390.35A = 400A CB (next size per 240.6). Am I correct?

If you use both primary and secondary protection (and I always do), then the primary can be as high as 250%. That gives you a range of 312 amps to 781 amps. I would size this one at 400, as you have.

 

[skeshesh]

You can perhaps use an enclosed CB (400A) feeding it either with parallels or I think you may be able to directly tap the panelboad bus if you're close enough which it seems you are. Afterwards you can feed the transformer from the single enclosed CB with one set of feeders.

 

[augie47]

I think my original information might not have been very clear. To size the CB on the primary (208V, 3P) side of my 112.5 kVA transformer I used:

112,500/208/1.732 = 312.28 x 125% = 390.35A = 400A CB (next size per 240.6). Am I correct?

The distribution panel is 600A and cannot accept the 400A CB for my transformer. I will investigate the feed thru lug option and then installing a fused disconnect ahead of the transformer.

Thanks.

my error... I am so used to "step down" that even after you told me I still calculated backwards.
Your number is good for 125%. As noted, if you have secondary OCP at 125%, your primary could be as high as 781 amps, so IF you have a way to connect to your 600 amp panel as long as your conductors are sized properly you can use the 600 amp protection.

 

[Paul Falcone]

As noted, if you have secondary OCP at 125%, your primary could be as high as 781 amps, so IF you have a way to connect to your 600 amp panel as long as your conductors are sized properly you can use the 600 amp protection.

The secondary conductors feed the "normal" lugs of an ATS, then from the ATS to a panel with a 175A MCB (125%). I don't think that qualifies as secondary protection because they hit the ATS first. (correct?)

However, it might be cost effective to install a fused disconnect to serve as secondary OCP (125%) instead of a 400A primary OCP disconnect ... IF I can connect to the panel.

Thanks for all the help and advice!