Tapping a 4000A bus.

[EEJaime]

Gentlemen,
I previously posted a question regarding the tapping of branch panelboard feeders on an airport project we have under construction. The following issue was also raised by the Electrical Inspector, although in this case, I feel he may be mistaken, (difficult to convince an AHJ that they are ever mistaken, I know).

We have an existing indoor 12kV-480Y/277V,3ph,4w,4000A substation supplying the normal side feeds to several Automatic Transfer Switches. Unfortunately the 480V Distribution section is full and we needed to add a feeder for another ATS. The designer called for a new feeder to be tapped to the existing bus and routed to an enclosed 1600A/3P circuit breaker which feeds the Normal side of the new ATS.

The inspector has quoted the feeder tap section of the NEC 240-21(b)(2) because the tap conductors are 37ft in length which he states exceeds the 25ft maximum allowed by code.

My assertion is that this provision of the NEC is not applicable because we are not tapping a "feeder", we are making our connection at a switchboard. Furthermore, we are protecting the feeder with the circuit breaker and the feeder is contained within the electrical room. (There is really no other location in this extremely full electrical room to mount the breaker, (aside from the fact it is already installed)).

Does that argument have any validity in your opinions? I would be interested in hearing your views. Are there any Inspectors in the forum?

Thank you and regards,
EEJAIME

 

[BJC]

I think the inspector is right.
This is the designers problem, get him on site and tell him you need a solution now.
The other people to call is the vendor rep for who ever made the switchboard and see what they can do. Even if your just "tapping" the bus I wouldn't do it without the manufactures OK.
The solution might be to replace it with a new one. A newer one or a differnet manufacture might have more breaker spaces ( you can't have more than six.)
Tell the owner the design may be terminal.

 

[EEJaime]

Thank you BJC, the manufacturer participated in the tapping operation and was aware it was being done. The original design had the breaker closer to the switchboard, but due to space constraints, it was moved slightly. Additionally, because of the quantity of existing switchgear, feeders, bus ducts, cable trays, etc..., the feeder routing was forced to make a slight detour with an added 90deg bend, and to go over an existing conduit rack.

It is impossible to replace the switchboard without an astronomical impact to the project both operationally and monetarily, as it also feeds major equipment and distribution boards all over this 24 hour operating international airport terminal. Quite a mess!

 

[davidbeach]

I agree with BJC, the 25 foot rule applies. You could always run 4000A worth of conductor for your 37ft (bus duct might be easier if not less expensive).

 

[jschwei314]

That is a good question. I never liked this idea to add just one more feeder. I have seen this with then a second feeder added.

I have used this example to get a discussion going.

One fella then asked what if one used cable protectors, cable fuses. Put these rascals on the cables when the cable attaches to the main 4000 amp bus.

All I could say, That is a good question again.

 

[wareagle]

Posted by EEJ "My assertion is that this provision of the NEC is not applicable because we are not tapping a "feeder", we are making our connection at a switchboard."
That does not matter.240.21 says that the conductor shall have overcurrent at the point where the conductors receive their supply.

Posted by EEJ "Furthermore, we are protecting the feeder with the circuit breaker and the feeder is contained within the electrical room."
I am confused by this statement. Are you saying your are protecting the new circuit by the 4000 amp main breaker?

 

[dpc]

I also must agree with the inspector (a rare event). As David Beach says, you can run a 4000 A tap more than 25 feet, but that will be a big tap.

 

[EEJaime]

Wareagle,
I was not inferring that the tap would be protected by the 4000A main, I was just stating that the tap conductors terminated in a ciruit breaker so that they were protected from an overload condition from the load side. I know the concern for tap conductors is for a fault as well as overload, but I just don't know what makes 25' such a magic number. I am sure that it is based on something, I just don't know what factors into that figure so that an "equal accomodation" might be attempted to satisfy the AHJ.

DPC,
I know that full sized tap would work as well, it is just that there is absolutely no room in the board, or on the bus, or to add a bus section. We have an extremely physically challenged condition. I was just searching for some way to request a variance from this code section since the installed tap conductors from terminal to terminal are 37'-0". I was trying to determine how the additional 12' make this an unsafe installation and see if there was a way to mitigate the inspector's/code's concerns.

 

[dpc]

Ah, a variance - a horse of a different color. If it were me, I'd base a variance request on the fact that the entire length of the tap exposed, is contained within the electrical room and this room is kept LOCKED and is accessible only to qualified persons. If the room is sprinklered or has fire detection of some kind, and has fire-rated walls, doors, etc, those are other points in your favor. If you have a local contractor who works with the inspectors all the time, I'd let them handle it instead of the "know-it-all" consultant, (especially if you're from out of town).

But I wouldn't request it on the basis that it's a stupid, arbitrary rule and 37 feet is just as safe as 25 feet.

 

[EEJaime]

DPC,

Thank you, I needed a chuckle. I have just submitted the "REQUEST FOR HARDSHIP EXEMPTION OR EQUVALENT FACILITATION" with the Department of Building and Safety citing the exact points you've outlined,(sans the last one of course). We will see what that buys us-hopefully we can get this thing moving forward.

Construction in a 30 year old building which was constructed in a hurry for political reasons back in the 1980's and which is in the middle of a political tug-of-war today, and which of course is a twenty-four hour a day operation is such fun! Especially when it is making the local news on a monthly basis for cost overruns and delays.

 

[gepman]

I think that the primary concern is fire since it is the National FIRE Protection Association especially since they allow outside taps of unlimited length so the fire rating features mentioned by dpc is important.

The two faults that you are worried about are a phase to ground fault and a phase to phase fault. You could set the ground fault protection on the 4000A main to be less than the tap ampacity (in fact it should be anyway). It is hard to imagine a phase to phase fault that wouldn't very quickly get larger than 4000A on those size of conductors. The tap rules cover all taps including those without ground fault protection and this is one of the reasons that you see the rules for the ratio of the tap conductor ampacity to the tapped conductor ampacity for the 10' and 25' taps.

 

[BJC]

EEjamies
Are you designing this or woring for the contractor?
Your dealing with standby and emergency circuits in a age of heightened consern about security and safety.
So far we've seen two band-aids.
I can understand the AHJ position. It just don't sound like the place or time for band-aids.

 

[LionelHutz]

Last time I looked I recall the tap rule giving a minimum conductor size for a maximum conductor length. I would assume this is so that a short at the end of these conductors would have a low enough impedance to generate enough fault current that the main protection device would trip before thermal damage to the tap conductors occurs.

There are time vs current damage curves for copper conductors. I had one around here but I'd have to dig to find it again. But, using this curve to show that the 4000A breaker trip curve would protect the 1600A conductors may also help you get your variance. If not the 4000A breaker then CT's and a protection relay as already suggested in your other post.

 

[waross]

Have you considered a 4000 amp tap 12 feet long, followed by 25 feet of reduced size conductor? The Canadian code refers to the point at which the ampacity is reduced rather than the point of supply. Would this solution be acceptable under the NEC?
respectfully

 

[davidbeach]

waross, your suggestion would work under the NEC also; a tap begins at the point of reduced ampacity, so you couldn't really have a 4000 amp tap from a 4000 amp bus, but would instead have a 12 foot extension of the bus, even if it was made of cable rather than bus bar.

 

[gepman]

I think that having a 12 foot 4000A "extension or tap" followed by a 25 foot 1600A tap would be a case of meeting the code but being less safe than a 37 foot 1600A tap. Whatever the mechanism to make the 4000A cable (most likely) or bus to 1600A cable connection would be more prone to problems than the smaller 1600A unspliced conductors, especially if a mechanical lug or crimp lug with a bolted connection was used. If cable is used for the tap or extension then there would most likely be more parallel runs for the 4000A run than the 1600A run making it more likely to be exposed to damage.

I would also like to caution EEJaime that if you tap a UL (or other listed) switchboard you are basically nullifying the listing unless your tap is listed for field installation on that particular switchboard. If you have a stickler of an inspector he may not approve the means and method of the tap even if he approved the concept of the 37' tap. A 1600A or 4000A tap is not a trivial matter especially if there is not adequate room to make the tap. A listed transition section to cable out connections is probably better.

I have attached a photo of a 2000A bus extension that a contractor tried to substitute for a transition section to a new switchboard section. Needless to say this attempt by the contractor is no longer there.

 

[itsmoked]

Ahhh.. It's a shame he was forced to use tie wraps instead of duct tape. He must have felt out of his element.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[dpc]

That's scary. I hope someone lost their journeyman's license.