Feeder tap within an MCC

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GWB

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We have a situation where lights in an MCC room need to be kept on when the main disconnect is off. This creates a problem because the bucket that feeds the lighting panel is fed from the main in the MCC. So, what I would like to do is feed the breaker that supplies the lighting from the line side of the main breaker.
I have read 240.21 ..my feeder taps are going to be less than 25 feet and will be sized for the breaker they will feed. They will be within the MCC . As far as I can tell, this is permissible but would appreciate any help.
Thanks
 
GWB said:
We have a situation where lights in an MCC room need to be kept on when the main disconnect is off. This creates a problem because the bucket that feeds the lighting panel is fed from the main in the MCC. So, what I would like to do is feed the breaker that supplies the lighting from the line side of the main breaker.
I have read 240.21 ..my feeder taps are going to be less than 25 feet and will be sized for the breaker they will feed. They will be within the MCC . As far as I can tell, this is permissible but would appreciate any help.
Thanks
Click to expand...

The picture I have in my head is a pair of #12s tapped into paralleled 4-750 on a 2000A CB turned down to 1900A*. The OC protection on the #12s is whatever is feeding the 4-750.

Perhaps a bit more information on the system would help. Here are some suggestions:
Which part of 240.21 are you using?
Is there OC protection upstream of the tap? Maybe the MCC feeder is tied right to an xfm secondary.

*Using CLX cable of course ~:)-)

ice
 
iceworm said:
The picture I have in my head is a pair of #12s tapped into paralleled 4-750 on a 2000A CB turned down to 1900A*. The OC protection on the #12s is whatever is feeding the 4-750.

Perhaps a bit more information on the system would help. Here are some suggestions:
Which part of 240.21 are you using?
Is there OC protection upstream of the tap? Maybe the MCC feeder is tied right to an xfm secondary.

*Using CLX cable of course ~:)-)

ice
Click to expand...

Thanks for the reply ice. Yes, what you have in your head is about right. There is a breaker feeding the main (upstream of the tap) but it is 800 amp.

My tap will be #10 wire feeding a 25 amp breaker.
 
GWB said:
... my feeder taps are going to be less than 25 feet and will be sized for the breaker they will feed. They will be within the MCC ....
Click to expand...

GWB said:
... There is a breaker feeding the main (upstream of the tap) but it is 800 amp.

My tap will be #10 wire feeding a 25 amp breaker.
Click to expand...

I'm still pretty short on information to see what you are doing.

Where does the 25 foot limit come from? What you described does not meet 240.21.B.2. Still curious as to what partof 240.21 are you using.

So with the limited information we have, here is my current guess:
480V, 3phase, grounded Y, 800A CB at feeder end, 800A (to 1200A) CB at MCC, conductors are parallel 2-500.

Existing lighting panel transformer is fed from a 25A CB in an MCC bucket. xfm and panel are in the MCC or separate frm MCC - doesn't matter.

So far this would be a common install, but there are some others it could as easily be - and you are the only one that knows. If you wish some help you really need to tell.

Continuing on:
You want to disconnect the CB in the bucket from the MCC bus and wire it straight to #10s tapped somehow into the 2-500 on the lineside of the MCC 800A main.

You wish to do this because it is a lot cheaper (and easier) than bringing a conduit from the nearest separately fed MCC to feed the lighting xfm.

GWB - I'm going to quit guessing now. Your turn.

ice
 
I don't think you can use the 25 foot tap rule unless your conductors are at least 1/3 the ampacity of the 800 amp protected conductors you are tapping so they would need to be 267 amp conductors to use the 25 foot rule.

To use the 10 foot rule you might be Ok, but read condition #4 240.21(B)(1):
For field installations, if the tap conductors leave the enclosure or vault in which the tap is made, the ampacity of the tap conductors is not less than one-tenth of the rating of the overcurrent device protecting the feeder conductors.
Click to expand...

One tenth rating still requires 80 amp conductor if supplied by 800 amp device. Is leaving the mains compartment but not leaving the MCC the same thing as leaving the enclosure or vault in which the tap is made? I really don't know. But that will make a difference as to whether or not this applies.

Also consider the fact that the reason you are attempting to supply the lighting ahead of the main is likely becasue you don't want any live conductors in the MCC when the main is open.
 
iceworm said:
I'm still pretty short on information to see what you are doing.

Where does the 25 foot limit come from? What you described does not meet 240.21.B.2. Still curious as to what partof 240.21 are you using.

So with the limited information we have, here is my current guess:
480V, 3phase, grounded Y, 800A CB at feeder end, 800A (to 1200A) CB at MCC, conductors are parallel 2-500.

Existing lighting panel transformer is fed from a 25A CB in an MCC bucket. xfm and panel are in the MCC or separate frm MCC - doesn't matter.

So far this would be a common install, but there are some others it could as easily be - and you are the only one that knows. If you wish some help you really need to tell.

Continuing on:
You want to disconnect the CB in the bucket from the MCC bus and wire it straight to #10s tapped somehow into the 2-500 on the lineside of the MCC 800A main.

You wish to do this because it is a lot cheaper (and easier) than bringing a conduit from the nearest separately fed MCC to feed the lighting xfm.

GWB - I'm going to quit guessing now. Your turn.

ice
Click to expand...

Correct.
 
GWB said:
Correct.
Click to expand...

No more guesses from me until you answer the questions (all of them) and tell us about the system. One word answers are not covering it. I've got to go to work - I'll check back later today.

ice
 
kwired said:
I don't think you can use the 25 foot tap rule unless your conductors are at least 1/3 the ampacity of the 800 amp protected conductors you are tapping so they would need to be 267 amp conductors to use the 25 foot rule.

To use the 10 foot rule you might be Ok, but read condition #4 240.21(B)(1):


One tenth rating still requires 80 amp conductor if supplied by 800 amp device. Is leaving the mains compartment but not leaving the MCC the same thing as leaving the enclosure or vault in which the tap is made? I really don't know. But that will make a difference as to whether or not this applies.

Also consider the fact that the reason you are attempting to supply the lighting ahead of the main is likely becasue you don't want any live conductors in the MCC when the main is open.
Click to expand...

Thanks kwired. Its looking like I will need to use #4 wire to feed a breaker mounted next to the lighting transformer, not more than 10 foot from the tap point. This would prevent live wire from being present in the MCC when the main is off, something I'm not comfortable with anyway.
 
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