Tap conductor feeding primary side of transformer

[lpfam11]

I have a situation and am having a hard time coming up with a solution because of NEC 240.21.

I have a 2250A feeder. It is 297 volts (yes, 297 volts). I need to bring tap conductors off of it to feed a small transformer (also 297 volts) in a mini power zone center. There small transformer has a 40A breaker on the primary side.

The problem I have is that according to NEC 240.21, my tap conductors coming off of the 2250A feeder must be no less than 1/3 the rating of the over current protection device protecting the feeder which in my case is a 2500A fused bolt switch. That tells me that my tap conductors must be suitable for no less than 825 amps which means running (at a minimum) parallel 600 kcmil cables as my tap conductors.

This will not work for me because the breaker that feeds the small transformer is only 40A. You cannot fit 600 kcmil cables into the lugs on a 40A breaker.

My question is, does anyone have a way around this particular code or is there something else I can do that will allow me to run smaller cables over to the 40A breaker?

Any help is much appreciated.

 

[augie47]

You might look at 240.21(B)(1).
If you keep the taps to 10ft, they can terminate in a 225 amp panel with a 40 amp breaker.

 

[DARUSA]

I have a 2250A feeder. It is 297 volts (yes, 297 volts). I need to bring tap conductors off of it to feed a small transformer (also 297 volts) in a mini power zone center. There small transformer has a 40A breaker on the primary side.

The problem I have is that according to NEC 240.21, my tap conductors coming off of the 2250A feeder must be no less than 1/3 the rating of the over current protection device protecting the feeder which in my case is a 2500A fused bolt


You should have a 2500 amp feeder in order to have a 2500 amp fuse!!

 

[lpfam11]

240.21 (b)(1)

240.21 (b)(1)

240.21(B)(1) would allow me to feed a 225A panel with a 40A breaker but I am feeding a transformer with a 40A breaker on the line side. 240.21(B)(3) refers to feeding transformers with tap conductors.

I have thought of a possible solution but still cannot find clarification of whether or not it would be acceptable.

What if I came off of the feeder bus into a fused power block before going to the transformer breaker? Could I then go by 240.21(B)(1) for the taps from the feeder to the fuse block and then from the other side of the fuse block come out with my smaller cables to the 40A transformer breaker?

 

[Smart $]

...

What if I came off of the feeder bus into a fused power block before going to the transformer breaker? Could I then go by 240.21(B)(1) for the taps from the feeder to the fuse block and then from the other side of the fuse block come out with my smaller cables to the 40A transformer breaker?

Is the feeder a busway that is capable of accepting a busway plug-in unit (aka "bus plugs")...

Busway and plug-ins in picture may be smaller than your installation... just using picture for demonstrative purpose.

If so, is there a plug-in capable of being fused or breaker'd at 100A or less?

 

[jim dungar]

Have you thought about putting in a 40A protective device located within 10' of the 2250A feeder see 240.21(B)(1)?