Parallel Conductor Length Differential

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sandsnow

sandsnow

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Southern California
The Code requires parallel conductors to be the same length.
My question is how much of a difference in length is realistically acceptable?
A matter of inches or feet or nothing? I seem to remember something about a certain percentage difference dependent on length.

Is there a way to reliably measure conductor length using an ohmeter?

The electrician did not keep his true tape measurements which would allow me to add the length of exposed wire to verify length.

These are six runs of 4 x 1,000kcmil each.
 
Re: Parallel Conductor Length Differential

There is no answer to this question in the code. It has been discussed several times before in this Forum, but I do not recall there being a consensus amongst those who posted their opinions. Some might say to defer to the AHJ, but that would not help you, would it? ;)

My own opinion is that it should not be approached from the perspective of measurements or analyses. If it looks to you to be ?about the same length,? and if it looks like the conductors go directly (i.e., in a workmanlike manner) from the conduit to the point of connection, then I would call it good. If it looks like one conductor is obviously much longer, by virtue of being coiled several times around the bottom of the panel, I would call that bad. But where is the dividing line between good and bad? I hate to say it, Larry, but I think that really is your call.
 
Re: Parallel Conductor Length Differential

Originally posted by sandsnow:
The Code requires parallel conductors to be the same length.
My question is how much of a difference in length is realistically acceptable?
A matter of inches or feet or nothing? I seem to remember something about a certain percentage difference dependent on length.

Is there a way to reliably measure conductor length using an ohmeter?

The electrician did not keep his true tape measurements which would allow me to add the length of exposed wire to verify length.

These are six runs of 4 x 1,000kcmil each.
Click to expand...
Based on the discussion we had here, my personal approach,if necessary, would be to do a current divider calculation based on the relative lengths and assure myself the ampacity of the "shortest" run wasn't exceeded.

If the general run length is known, determining the total lengths from the differences in the exposed parts should be enough.

If it isn't enough, I believe something more on the order of a Whetstone bridge would be necessary to compare the resistances. Even the best ohmeters aren't that accurate.

[ April 15, 2005, 12:18 PM: Message edited by: rbalex ]
 
Re: Parallel Conductor Length Differential

I agree with Bob.

My personal opinion is I don't see any reason why the difference would exceede 10% for anything except the shortest run. A 10% difference in length should give a 10% difference in current.

I think we decided that the load doesn't matter. If there is a 10% difference in length, then there will be a 10% difference in current.

So if the length is different by 10% and the load calculation is only 600A on a 800A set of conductors, then that should be safe. But if they cut the load calculation down to the last amp, they might have to get the length down to the inch.
 
Re: Parallel Conductor Length Differential

Originally posted by steve66:
I agree with Bob.
...
Click to expand...
Thanks.

I should make clear, my "if necessary" qualifier means I also agree with Charlie :D . Unless, there's a real obvious reason for concern "eyeballing" should probably be sufficient.
 
Re: Parallel Conductor Length Differential

I would ask to see a reasonable constant load, then use amp meter.If readings stayed within 5% i think it is good enough.What ever the readings the would be equally out of balance at any load
 
Re: Parallel Conductor Length Differential

I don't have a problem with that approach, Jim. But wouldn't Larry have to make his decision before allowing the system to be turned on?
 
Re: Parallel Conductor Length Differential

Unfortunately these are service lateral conductors (customer owned and installed), so I want to make the call before we release the service to the utility.

You guys have answered my core question. Eyeballing it works for me unless it is or looks way off.

That previous thread wasn't that old, my mind is slipping.

Anyway, thanks for the input.
 
Re: Parallel Conductor Length Differential

Originally posted by charlie b:
I don't have a problem with that approach, Jim. But wouldn't Larry have to make his decision before allowing the system to be turned on?
Click to expand...
For the purposes of the inspection, it could be passed as a temporary, right?

I know I would holler moan and scream about having to put up with such a process, and it would be a pain in the AHJ's rear to inspect for temp, wait a week, re-inspect under load, and I'm sure the GC would throw fits, and then it would fail, resulting in the POCO being called to discontinue service...need I continue? :D
 
Re: Parallel Conductor Length Differential

If other electric utilities are like mine, it is impossible to impress on our people to leave the service entrance conductors the same length. Our guys will just go out and trim the service entrance conductors down so "they look good", never mind that what they are doing is detrimental.

IF the AHJ were to turn a 1200 or 1600 ampere service down for violation of 310.4 and then the EC being made to correct the situation, then a law suit against us would stop this kind of stuff. :mad:
 
Re: Parallel Conductor Length Differential

Originally posted by georgestolz:
Originally posted by charlie b:
I don't have a problem with that approach, Jim. But wouldn't Larry have to make his decision before allowing the system to be turned on?
Click to expand...
For the purposes of the inspection, it could be passed as a temporary, right?

I know I would holler moan and scream about having to put up with such a process, and it would be a pain in the AHJ's rear to inspect for temp, wait a week, re-inspect under load, and I'm sure the GC would throw fits, and then it would fail, resulting in the POCO being called to discontinue service...need I continue? :D
Click to expand...
The problem here is the electrician was not prepared to show his conductors were the same length. You kind of make it sound like I'm out of line here for making sure the installation is in compliance.
Yes if push came to shove it could be turned on as temp.
My aim is to try and approve things as we go and not leave things for deferred approval. Also to keep the job moving. Hold it up I will if warranted, BUT not without carefull consideration.

Thus the purpose of my original post, to gather some practical wisdom, to make my decision.
 
Re: Parallel Conductor Length Differential

Originally posted by charlie:
.

IF the AHJ were to turn a 1200 or 1600 ampere service down for violation of 310.4 and then the EC being made to correct the situation, then a law suit against us would stop this kind of stuff. :mad:
Click to expand...
Confused here
If the EC installed the conductors, why would you get sued????
 
Re: Parallel Conductor Length Differential

I must admit till this thread opened i never gave thought to them trimming as well as us.On a short run of 50 feet or less this just might get out of balance with a foot of differance.That would be 2%
 
Re: Parallel Conductor Length Differential

Originally posted by jimwalker:
he would get sued if his men trimmed the wires at there end and caused one to be shorter.
Click to expand...
Ok,I understand that part. The utility end is out of my control, and I would not be looking there.

Now I get it. If the conductors melted down, then whoever cut them to the wrong length would be to blame.

So I will have the EC put a marking tape on each conductor to indicate equal length point for the utility to base their cuts. Hopefully they will cut equal length off each.
 
Re: Parallel Conductor Length Differential

on parallel runs where you have like, 6 to 10 pipes coming out of the ground on either end, they are NEVER in a straight line with each other - rather you will have 3 and 3, 4 and 4, 5 and 5, and so on...

when this happens, in the duct bank, the nineties coming up in the 'back' will be longer by AT LEAST the outer diameter of the pipe TIMES 2 At Both Ends.... on 4 inch pvc, that is at the Very Least, 18 inches on the conductor that falls on the 'inside' of the 90... as much as 36 inches overall on the conductors that lay at the 'back' of the 90, and that is assuming that each phase conductor falls in the same place in EACH PIPE

the code says that parallel conductors must be the 'same length' but i have NEVER seen it installed that way..


and that if each phase conductor falls in
 
Re: Parallel Conductor Length Differential

The NEC clearly states that parallel conductors are required to be the same length. The exact lengths are more critical when the length of the parallel conductors not the same length. For example, six 1000 kcmil conductors in parallel in separate raceways have a current-carrying capacity of 3270 amps. If five conductors are 10 ft long and one is 12 ft long, the ten ft conductors will carry 590 amps and the 12 ft one 536 amps. The ampacity of 1000 Kcmil is 545A for termination on a 75 degree F termination. One conductor will overheat. OOPS see following post for a complete one.

[ April 16, 2005, 11:30 PM: Message edited by: john m. caloggero ]
 
Re: Parallel Conductor Length Differential

The NEC clearly states that parallel conductors are required to be the same length. The exact lengths are more critical when the length of the parallel conductors not the same length. For example, six 1000 kcmil conductors in parallel in separate raceways have a current-carrying capacity of 3270 amps. If five conductors are 10 ft long and one is 12 ft long, the ten ft conductors will carry 590 amps and the 12 ft one 536 amps. The ampacity of 1000 Kcmil is 545A for termination on a 75 degree F termination. One conductor will overheat. Where 5 conductors are 100 ft and one is 102 ft, the current will be 554 amps on the 100 ft conductors and 543 amps on the 102 ft conductor. Length is a BIG factor. DRAW YOUR OWN CONCLUSION.
 
Re: Parallel Conductor Length Differential

John,

Since length is a big factor, how many cases of overheating or damage have been documented?

Besides measuring installed conduit length and then exposed wire length, what other ways or methods are available to inspectors to ensure compliance with this?
 
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