Oversize grounded conductors

[sandsnow]

Grounded Conductors are increased in size by designers for non-linear loads. Usually doubled in size by paralleling.

In this case the designer used #1 Copper in parallel.

I see this as a violation of 310.4 or did I miss something??

 

[don_resqcapt19]

Re: Oversize grounded conductors

Larry,
You didn't miss anything, the designer did, unless this is an existing installation.

310.4 ... Exception No. 4: Under engineering supervision, grounded neutral conductors in sizes 2 AWG and larger shall be permitted to be run in parallel for existing installations.

Don

 

[physis]

Re: Oversize grounded conductors

I've never had a need to paralell conductors so I don't know what all it is that you people do that for.

But reading 310.4 and stuff nearby I only see permission to paralell 1/0 and larger. I don't see anything that prohibits paralelling smaller conductors.

 

[infinity]

Re: Oversize grounded conductors

310.4 states that conductors 1/0 and larger shall be permitted to be connected in parallel, therefore conductors smaller than 1/0 cannot unless they fall under the requirements of one of the exceptions.

 

[tom baker]

Re: Oversize grounded conductors

Conductors are run in parallel as the ampacity does not increase proportionally with the size. Example ampacity of 250 kcmil is 215 at 60 deg and 500 kcmil is 320 at 60 deg. For larger services its much less expensive to run conductors in parallel.

 

[physis]

Re: Oversize grounded conductors

310.4 states that conductors 1/0 and larger shall be permitted to be connected in parallel

I agree with that.

But the "therefore" part is an inference rather than a code.

90.5(B) defines the language as a permissive rule. Permissive rules don't cause anything to be required or prohibitted.

90.5(A) covers manditory rules and doesn't apply to the language of 310.4

Edit: Needed a liitle touch up.

[ June 08, 2005, 09:51 PM: Message edited by: physis ]

 

[George Stolz]

Re: Oversize grounded conductors

Good point. That looks like a small whoops.

 

[infinity]

Re: Oversize grounded conductors

But the "therefore" part is an inference rather than a code.

90.5(B) defines the language as a permissive rule. Permissive rules don't cause anything to be required or prohibitted.

90.5(A) covers manditory rules and doesn't apply to the language of 310.4

So what you're saying then is that any size conductors can be run in parallel according to the wording of 310.4? If this is true than why would we need any of the exceptions that permit smaller than 1/0 conductors?

 

[George Stolz]

Re: Oversize grounded conductors

I'd say technically, the way it is written now, Sam is correct. The intent is obvious, but the wording is unintentionally flexible. It is up to the AHJ to approve conductors smaller than 1/0, their approval just isn't guaranteed.

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).

It should be reworded:

Aluminum, copper-clad aluminum, or copper conductors of size smaller than 1/0 shall not be connected in parallel. Such 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).

Proposal time?

[ June 09, 2005, 07:04 AM: Message edited by: georgestolz ]

 

[George Stolz]

Re: Oversize grounded conductors

Originally posted by tom baker:
Conductors are run in parallel as the ampacity does not increase proportionally with the size.

To add to this, a little bit of homespun knowledge I've not exposed to the forum, it could be true or false. It's what I was taught.

AC current doesn't travel so much through the body of the conductor, it travels along the outer surface of the conductor, the "skin effect." This phenomenon creates the ampacity effect that Tom referenced.

Is this true or false?

Sam wrote:
I've never had a need to parallel conductors so I don't know what all it is that you people do that for.

That sucks, man. It's kinda fun (in a boring way) seeing what different combinations of parallel conductors you could use to feed a large service.

The price per foot of large conductors exceeds the price of several runs of smaller conductors, and the smaller ones are easier to handle.

I have a six-plex with a 400 amp service. If I were to try to do it with one run, I'd need 900 MCM AL! I think that stuff's pricey! As it is, I run two parallel runs of 250 MCM AL, which is way cheaper, and easier to terminate.

 

[petersonra]

Re: Oversize grounded conductors

Exception No. 4: Under engineering supervision, grounded neutral conductors in sizes 2 AWG and larger shall be permitted to be run in parallel for existing installations.
FPN: Exception No. 4 can be used to alleviate overheating of neutral conductors in existing installations due to high content of triplen harmonic currents.

This seems to be permission to add a conductor in parallel in an existing installation, but not in a new one.

 

[sandsnow]

Re: Oversize grounded conductors

Thanks, that's what I thought.

 

[physis]

Re: Oversize grounded conductors

Out of curiousity Larry, you being an agent of the AHJ, what will your decision be?

 

[iwire]

Re: Oversize grounded conductors

Originally posted by georgestolz:
I have a six-plex with a 400 amp service. If I were to try to do it with one run, I'd need 900 MCM AL! I think that stuff's pricey! As it is, I run two parallel runs of 250 MCM AL, which is way cheaper, and easier to terminate. For what it is worth 600 bends easer than 500 due to stranding differences.

Don't forget that if your two sets of 250 Kcmil AL are installed in one raceway will only have an ampacity of 368 amps.

One more note, if this 'sixplex' has more than one service disconnect the service conductors only need to be sized to the calculated load not the total of the disconnects.

[ June 09, 2005, 04:54 PM: Message edited by: iwire ]

 

[rcarroll]

Re: Oversize grounded conductors

2 questions iwire, if the 250 kcm were installed in the same pipe & rated 368 amps could you still protect them with a 400a breaker per 240.4(B) & I'm really going out there, if 2 sets of conductors are in the same pipe & terminated properly for parallel conductors, are the wires still to be derated or could they be considered 1 set of wires? I hope I didn't confuse anyone with this, but I had a debate with an electrician over this a couple years ago.

 

[George Stolz]

Re: Oversize grounded conductors

Originally posted by iwire:
Don't forget that if your two sets of 250 Kcmil AL are installed in one raceway will only have an ampacity of 368 amps.

Originally posted by rcarroll:
...if 2 sets of conductors are in the same pipe & terminated properly for parallel conductors, are the wires still to be derated or could they be considered 1 set of wires?

That's a good question. 310.4 uses the phrase "To form one conductor" as though the parallel conductors are effectively one conductor. But then again, it goes on to reiterate the need to derate.

I wonder what other's thoughts are?

I think they're not one conductor, and the language is just to make the function of the parallel conductors clearer.

 

[iwire]

Re: Oversize grounded conductors

Originally posted by georgestolz:
2 2" PVC's, and one disconnect. None of that crazy MDP stuff.

 

[iwire]

Re: Oversize grounded conductors

It would look something like this.

 

[sandsnow]

Re: Oversize grounded conductors

Originally posted by physis:
Out of curiousity Larry, you being an agent of the AHJ, what will your decision be?

The installation was rejected. As Bob pointed out, Ex 4 is pretty clear. I just like to double check myself sometimes.