Parallel Taps?

[pwest]

I have a question regarding taps of parallel feeders.I am working in a shopping center with an existing 800Amp service. There are 2 sets of 500MCM in a trough fed from an 800Amp Main Breaker (They are joined together at both ends,same length, same insulation etc. in compliance with article 310-4).There are multiple taps to multiple disconnects, in compliance with article 240-21(taps not over 10ft long). The taps are connected to 1 set of 500MCM. The local inspector states that the tap must be connected to BOTH sets of 500MCM.In other words a connection with (2)500mcm run and (1)#2 tap. He states that the 1 set of 500MCM is only rated for 400Amps with over current protection at 800Amps and the connection of the 2 sets of 500mcm at the end of the trough is insignificant. Is this so???? If so, where does it appear in the NEC

 

[petersonra]

Re: Parallel Taps?

An interesting question. My intial thought was that the inspector has to be right on this one, but I did not find anything in the code to back up this thought.

I can see where it would be a whole lot more convenient to do it your way though, and did not see a prohibition on it in my brief look.

My second thought is this. look at a single tap say midway. the current division is not equal between the 2 conductors because you have 3 times as much length of conductor on one cable as the other because on one conductor the current has to go all way to end to get to the tap.

Seems like a bad idea to me.

 

[charlie b]

Re: Parallel Taps?

Good question!

I agree with the Inspector. But I concede that there is no clear code statement on the topic (or at least none that I can find). My basis is the physic of the situation. The two parallel phase conductors will have different amounts of current going through them. That creates a different amount of voltage drop from one end to the other end, due to a connection (to only one of them) somewhere in the middle. That will cause some bizarre currents to flow (i.e., from the untapped conductor to the tapped conductor at the load end, and from that point backwards along the tapped conductor to the point of the tap).

If I were pressed to cite a code article for a violation, I might have to hang my hat on 310.4, item (5): The conductors shall be terminated in the same manner. That phrase may appear to mean terminated at the beginning and again at the end, but I would include terminated at any tap point along the line as well.

{Edit: Looks like Bob and I had about the same response, but he beat me to the draw.}

[ January 05, 2005, 02:26 PM: Message edited by: charlie b ]

 

[jtester]

Re: Parallel Taps?

I agree with the inspector also. The Handbook has explanatory notes in 310.4 thet indicate parallel conductors are considered 1 conductor and tapping just one would create an imbalance as noted in the other posts.
Jim T

 

[tom baker]

Re: Parallel Taps?

Ilsco makes a parallel tap conductor. As the NEC considers parallel conductors to be one, then IMO they both need to be tapped, otherwise why is this product available?
http://ebusiness.ilsco.com/webapp/wcs/stores/servlet/CategoryDisplay?storeId=10001&langId=-1&catalogId=1&categoryId=174

 

[crossman]

Re: Parallel Taps?

I agree that it could be construed as a violation of 310.4. The lengths of the parallel paths to that single tap are now different.

On the other hand, I bet it would work okay for 1000 years.

 

[rbalex]

Re: Parallel Taps?

Originally posted by jtester:
I agree with the inspector also. The Handbook has explanatory notes in 310.4 thet indicate parallel conductors are considered 1 conductor and tapping just one would create an imbalance as noted in the other posts.
Jim T

I think the Code itself is clear enough.

310.4 Conductors in Parallel. Aluminum, copper-clad aluminum, or copper conductors of size 1/0 AWG and larger, comprising each phase, neutral, or grounded circuit conductor, shall be permitted to be connected in parallel (electrically joined at both ends to form a single conductor).
...
The paralleled conductors in each phase, neutral, or grounded circuit conductor shall
(1) Be the same length
(2) Have the same conductor material
(3) Be the same size in circular mil area
(4) Have the same insulation type
(5) Be terminated in the same manner

 

[petersonra]

Re: Parallel Taps?

The code does not define terminated. where it is used, the context clearly suggests that it means at the end of a wire (or whatever), and not in the middle, so I am not convinced the 'terminated" argument against this installation flies.

However, the 2 paralleled conductors are no longer acting as a single conductor if anything is tapped off just one of them. So I would agree with Bob that its in violation of how the code defines paralleled conductors.

[ January 05, 2005, 03:37 PM: Message edited by: petersonra ]