Tap Rule

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scottbrown

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I'm reviewing a set of drawings for a building remodel. The main switchboard in the building is a 3000 amp board with no room for additional switches. The designer wants to tap directly to the busses in the switchboard and extend conductors sized for a 600 amp load to a fused switch nearby and then to a down stream load.

My thinking is that since the feeder ampacity is less than 1/3 the main board ampacity, that 240.21.B.1 (using 2002 NEC) would apply. Where there is a problem is that in order to maintain proper workspace clearances, the switch will need to be set so that it is more than 10 conductor feet from the point where the conductors are connected to the busses. Is there any relief for this type of a situation written into the code (I could not find any)?

Thanks in advance,

Scott
 
Re: Tap Rule

scottbrown said:
Is there any relief for this type of a situation written into the code
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None that I know of. Sounds like you are into 240.21(B)(2) and you upsize your conductors.
 
The designer wants to tap directly to the busses in the switchboard and extend conductors sized for a 600 amp load to a fused switch nearby and then to a down stream load.
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I would say absolutely not. The switch is a UL tested piece of equipment
as is. You should not modify it in any way.
 
I agree with what is being said here. One point though. If taking the approach Pierre mentions, without seeing the acutal location, but considering that the conductors will be longer than 10', I wonder if the AHJ would see it as a violation of 230.70(A)(1) in that it would not be "nearest the point of entrance of the service conductors".

To put it another way, if the NEC does not consider feeder conductors safe when installed this way and protected at, in this case, 5 times their ampacity, would service conductors be safe, installed the same way, but protected only by the utility cut out on the other side of the transformer?
 
Feeders in general have overcurrent protection at their supply, hence the tap rules in regards to overcurrent protection and sizing.

Service conductors in general do not have overcurrent protection at their supply, hence there is no requirements for the sizing of the conductors in reqards to the service supply.
That is why we do not see the same requirements for service taps as for feeder taps.
 
Why not make the tap 1000 amps? Now your 10' of conductor can be 25'.
 
First of all, in my opinion, you can not tap a switchboard bus." Bob said it first, and I completely agree. I assume the plan was to drill the bus, and bolt on lugs. I would not only say no, but heck no.

(BTW, I do know someone that did this in the past. And once they put one tap on a switchboard bus, two or three more were sure to follow).


Also, I completly agree with what you are saying about "nearest the point of entrance". If you have a place to tap service conductors before the overcurrent protection, I don't think you can claim the OCP is nearest the point of entrance. (Even if you get past that, the 10+ foot to the new service disconnect is going to be a problem. And you're disconnects won't be grouped.)

I was about to claim service conductors can't be tapped, since the tap rules in 240 only apply to feeders and branch circuits. However, I have just learned that 230.46, and 230.33 do allow service conductors to be tapped. I wouldn't mind this so much if the taps originated outside, but the thought of tapping service conductors inside worries me. But I still think "nearest the point of entrance" places a severe limit on these taps.

Steve
 
P.S. Is it possible to add another service or service entrance per 230.2(C) and 230.71?

Steve
 
Guys we tap busses all the time, it goes with the job.

If it is a violation someone better pass the word to the inspector and all the electricians. :lol:

Most times this will not involve drilling the buss as there will be extra holes.
 
iwire said:
Guys we tap busses all the time, it goes with the job.

If it is a violation someone better pass the word to the inspector and all the electricians. :lol:

Most times this will not involve drilling the buss as there will be extra holes.
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I totally agree. It's done all of the time. Many times large switchboards will have extra sets of holes for adding lugs as Bob said.
 
Re: Tap Rule

scottbrown said:
The main switchboard in the building is a 3000 amp board with no room for additional switches.
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Excellent discussion on service taps, if SEC cost & routing through building is practical.

Another idea, can some of that 3000A panel be relocated to a subpanel, clearing room for subpanel feeders. (No tap rules apply).
 
I stand corrected about tapping panels. I didn't realize there were already holes for this. I guess that means the manufacturer also provided some space for lugs and bending wire.

I realize that services are tapped all the time, but I usually think of those being made by the POCO outside the building.

Steve
 
iwire said:
Guys we tap busses all the time, it goes with the job.

Most times this will not involve drilling the buss as there will be extra holes.
Click to expand...

"Most of the time..." Bob - are you saying that drilling a hole in the buss [in the field] does not affect the manufacturer's listing or warranty?

I hired an electrician who did drill the bus for a tap... the electrical inspector asked for a letter from the manufacturer stating the bus was listed for such field drilling....we didn't bother trying...just replaced the entire service [ouch!!]
 
dana1028 said:
I hired an electrician who did drill the bus for a tap... the electrical inspector asked for a letter from the manufacturer stating the bus was listed for such field drilling....we didn't bother trying...just replaced the entire service [ouch!!]
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You should have asked for the letter.

I bet they would have told you things like maximum hole size and spacing requirements.

By the way I am talking about large switch gear not a typical panel bus which would not normally have room for such 'extras'.

I will in the near future be making a 1600 amp tap from a 4000 amp board.

Due to space constraints we will be hiring a UL switch board shop to produce some new bus sections to give me a place to tap.

These new bus sections will fit the factory holes and be supported by insulting material.



Bob
 
Pierre C Belarge said:
Tapping an existing buss is one thing.

Even though you are using a UL shop for this additional buss, do you need to call a lab such as UL to field inspect this installation?
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That will be up to the shop that is making the parts.

We are contracting them to provide a solution.

I doubt highly UL will be out to field inspect.

The shop makes replacement parts for all sorts of switchgear and panels for us and others, I have never seen a UL field inspection.

Regardless I don't expect the AHJ to have any issues. :)

MA Code
90.4. Revise the first paragraph to read as follows:
90.4 Enforcement. This Code shall be used by the
authority enforcing the Code and exercising legal
jurisdiction over electrical installations. The authority
having jurisdiction of enforcement of the Code
shall accept listed and labeled equipment or materials
where used or installed in accordance with instructions
included with the listing or labeling. The
authority shall have the responsibility for deciding
upon the approval of unlisted or unlabeled equipment
and materials, and for granting the special
permission contemplated in a number of the rules.
Click to expand...
 
eric stromberg said:
Bob,
Will the total length on the tap conductor be less than 25 feet?

Eric
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As drawn by the EE it was going to be longer than 25'.

I had to submit an RFI to ask if I could relocate the new breaker enclosure closer to the tap location.

The move was approved but it will still be close, I will have to run my raceways carefully, I may have to go 'as the crow flies' instead of parallel with the building to keep it short.

The EE also showed running an EGC from the 4000 amp service gear to the 1600 amp breaker based on the 1600 amp breaker.

I submitted another RFI basically telling him that I would be sizing the EGC based on the 4000 amp OCP that the tap is fed from.
 
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