Tap Rules with new service

[ResSparky]

I am a residential electrician who has the chance to do a very straight forward commercial job, but I have a question that I could kindly use some input on.....

A large building is divided into 5 office/warehouse spaces. The main service is 800 amp 120/208v three phase four wire terminating into a 800 amp main breaker panelboard (just with the one 800 amp main breaker). To the right of this panel is a very typical inter-connecting gangable metering system (with 100 main breaker with each meter) that are connected to the main panel load side busbars for the single phase power to each office subpanel for general lighting/use.

To the left of the panel is an enclosure that is also connected to the main panel load side busbars which have terminal lugs for cable that is feeding (thru a very large gutter and with 6' in distance from main panel) a 400 amp "termination box" to provide three phase 4 wire power to any office that requires a three phase panel. Two office/warehouses have existing three phase 200 amp services from the lugs in the termination box, into a meter socket, into a fused main disconnect, and running quite a distance into the panel located in their offices.

The office directly on the other side of the electrical room will now need a three phase service for a small three phase motor. I understand the need for the disconnects on the existing three phase services due to the distance between the meter socket in the electrical room and the panel inside the offices. But if i wanted to install a new meter an additional 6' from that termination box (so roughly 12' total from main panel), can I simply go "back to back" from the meter socket and straight into a main breaker panel directly behind it on opposite side of the same wall? so meter inside electrical room and panel located on otherside of electrical room wall......or is there an issue of since I am using that tap to feed my new meter socket, will I need a fused disconnect prior to leaving the electrical room and into the panel in the office?

For some reason I was concern about doing this without the fused disconnect?!?!

Any input will be very helpful.

 

[augie47]

A flaw may occur due to 240.21(b)(2), the 25 ft tap rule, requiring the conductors to have an ampacity 1/3 of the over-current device protecting them. If I read your scenario correctly, the protection would be the 800 amp main which would then require conductors rated 208 amps or more. If you meter socket and gear will accept that conductor size, I see no other problem although I may be overlooking something.

 

[Smart $]

... feeding (thru a very large gutter and with 6' in distance from main panel) a 400 amp "termination box"...

How are you determining this "termination box" is rated for 400A? That is, are there fuses or a breaker somewhere, and if so where?

What size/number/type wires connect the busbar lugs to this "termination box"?

Two office/warehouses have existing three phase 200 amp services from the lugs in the termination box, into a meter socket, into a fused main disconnect, and running quite a distance into the panel located in their offices.

As I currently understand the situation, putting the meters ahead of the fused disconnects is a violation of 240.21(B).

 

[Smart $]

A flaw may occur due to 240.21(b)(2), the 25 ft tap rule, requiring the conductors to have an ampacity 1/3 of the over-current device protecting them. If I read your scenario correctly, the protection would be the 800 amp main which would then require conductors rated 208 amps or more. If you meter socket and gear will accept that conductor size, I see no other problem although I may be overlooking something.

Seems to be within 10', so 240.21(B)(1) would apply.

 

[augie47]

Seems to be within 10', so 240.21(B)(1) would apply.

You are correct, the "12 ft" from "12 ft from the main panel" stuck in my head and I failed to re-read.
Your questions on the tap box seem to be the pertinent ones now.

 

[ResSparky]

A flaw may occur due to 240.21(b)(2), the 25 ft tap rule, requiring the conductors to have an ampacity 1/3 of the over-current device protecting them. If I read your scenario correctly, the protection would be the 800 amp main which would then require conductors rated 208 amps or more. If you meter socket and gear will accept that conductor size, I see no other problem although I may be overlooking something.

thank you for the reply, the protection would be the 800 amp main.

the "termination box" (manufacturer's label on cover is calling it a "400 amp rated termination box" which has 4 sets of lugs for a 3 phase 4 wire service) which is tapped from the 800 amp main busbars via lugs, is exactly 6'. i would then be only able to install a new meter socket 6'-7' from the lugs inside the "termination box".....plus the few feet between the meter socket and the new panel.

so i would have to count the 10' or 25' tap rule "from the point of supply" being the 800 amp main or from the "termination box" ? And thank you again for the reply, i am learning something here! and I have learned that I must re-study my tap rules.

 

[ResSparky]

How are you determining this "termination box" is rated for 400A? That is, are there fuses or a breaker somewhere, and if so where?

What size/number/type wires connect the busbar lugs to this "termination box"?



As I currently understand the situation, putting the meters ahead of the fused disconnects is a violation of 240.21(B).

thank you for the reply.

this "termination box" has a manufacturer's label on the cover which says "400 amp termination box" and contains inside sets of lugs for 4 wire 3 phase power. the "termination box" is being feed from the right side with what i could tell is 500mcm off the busbar lugs in the 800 amp main 6' away. the only location to install a new meter for new three phase service would be another 6' from the "termination box" on the left side. and of course the few feet of cable "back to back" between the new meter socket and the new panel.

wow, that is what is existing.......previous contractor installed meters ahead of the disconnects BUT was able to have them located much closer to the "termination box" than what I am left with wall space and clearances.

 

[Smart $]

thank you for the reply.

this "termination box" has a manufacturer's label on the cover which says "400 amp termination box" and contains inside sets of lugs for 4 wire 3 phase power. the "termination box" is being feed from the right side with what i could tell is 500mcm off the busbar lugs in the 800 amp main 6' away. the only location to install a new meter for new three phase service would be another 6' from the "termination box" on the left side. and of course the few feet of cable "back to back" between the new meter socket and the new panel.

wow, that is what is existing.......previous contractor installed meters ahead of the disconnects BUT was able to have them located much closer to the "termination box" than what I am left with wall space and clearances.

From your explanation and some presumption on my part, what you want to do would not be compliant... mostly because the existing installation is already non-compliant.

First, the 500kcmil conductors from MDP to termination box are considered taps themselves because they are only rated for 380A and supplied directly by an 800A load bus in the MDP.

Connecting to a termination box at the other end isn't a problem if circuit-continuation conductors have a 380A ampacity (e.g. 500kcmil). However, I presume the existing conductors going to the existing two 3? meters are probably less than 500kcmil in size. The problem here is, that constitutes the tapping a tap conductor, which is not permitted. Additionally, the first device a tap conductor connects to must be an ocpd or a single piece of utilization equipment. As mentioned previously, the meters being ahead of the disconnects is also non-compliant.