240.21(B)(5)

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Bjenks

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East Coast of FL
I have an existing commercial client who has a 400A meter outside that feeds underneath the slab up to a 400A main breaker panel. They are now attaching a wing to that building and will require an additional 250A of service. Can I use 240.21(B)(5) to put an 800A fused disconnect outside supplied by 2 sets of 500 kcmil. Then connect the existing feeder of the existing panel (un-bonding the N-G and supplying a G) as a tap to the 800A disconnect. Then run a second tap from the 800A disconnect outside the building to the new wing underneath a new slab to a new 400A main breaker panel? Since both breakers would be 400A I would have to make sure we use a single set of 600 kcmil or 2 sets of 3/0.

It seems that I can run all types of unlimited length taps from the fused disconnect as long as I stay outside of the building and I feed into a breaker that is size greater than or equal to the rating of the cable.
 
Bjenks said:
I have an existing commercial client who has a 400A meter outside that feeds underneath the slab up to a 400A main breaker panel. They are now attaching a wing to that building and will require an additional 250A of service. Can I use 240.21(B)(5) to put an 800A fused disconnect outside supplied by 2 sets of 500 kcmil. Then connect the existing feeder of the existing panel (un-bonding the N-G and supplying a G) as a tap to the 800A disconnect. Then run a second tap from the 800A disconnect outside the building to the new wing underneath a new slab to a new 400A main breaker panel? Since both breakers would be 400A I would have to make sure we use a single set of 600 kcmil or 2 sets of 3/0.

It seems that I can run all types of unlimited length taps from the fused disconnect as long as I stay outside of the building and I feed into a breaker that is size greater than or equal to the rating of the cable.
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What you propose seems fine to me. You have the no "next size up" rule, youve got the seperate N-G... Will there be any issue getting any existing grounding electrodes to the new service location? One could be picky and say the conductors are not outside the building since 230.6 is addressing services and 240.21 B5 doesnt define outside or reference 230.6. Im sure its the intent but it should be codified IMO.
 
Outside taps can be unlimited in length, below the slab or encased in at least 2" of concrete and it is considered outside the building.

I have no problem with your proposal as written.
 
OK, thanks. While I have used the 10 and 25 foot tap rule many times, I don't remember ever noticing this rule for feeders. Maybe because I usually do new construction and not upgrades. Also many times I read a code rule and interpret it one way while everyone else sees it differently so it is good to confirm before use.
 
Bjenks said:
OK, thanks. While I have used the 10 and 25 foot tap rule many times, I don't remember ever noticing this rule for feeders. Maybe because I usually do new construction and not upgrades. Also many times I read a code rule and interpret it one way while everyone else sees it differently so it is good to confirm before use.
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The more i think about it, i would get clarification from your ahj to make sure they would accept those conductors as being outside. say that you do what you propose and are cited for 240.4. You respond with 240.21B5. He says they are not outside. You cite 230.6. He responds with 230.1. I think he would win in court. I dont see outside defined anywhere else. Is anyone with me on this?
 
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