Parallel Conductor Length Differential

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Re: Parallel Conductor Length Differential

I haven't been called in to investigate any fires where I could document it, because the fire dept usually documents the cause as something they are familiar with. It takes a knowledgeable investigator to zero in on the ignition source if it is electrical. A detailed, accurate assessment of the fire scene is essential. It is very clear that the shorter conductors will carry more current than the longer one, because their resistance is lower.
 
Re: Parallel Conductor Length Differential

Originally posted by john m. caloggero:

It is very clear that the shorter conductors will carry more current than the longer one, because their resistance is lower.
Click to expand...
I am curious, as the current goes up on the shorter conductor, how much does the capacitance reactance direct more current to the other conductors. I would not think a significant amount, but in theory, some?
 
Re: Parallel Conductor Length Differential

Larry
I think you should be concerned. Like Ryan, we were always made to keep our conductors very, very close to the same length. The answer is simple, because the math does not lie.
A good mechanic can handle this without a problem.

Also, waiting to test this under load is bunk. I have not been able to locate that in the NEC anywhere ;) .
 
Re: Parallel Conductor Length Differential

i always argued this issue with a fellow worker who went to extreams in his wire measurements! then we had a 4000 amp service blow up and had to run eight 500 mcm parallel open conductors into a vault to provide temporary power to a large building. we layed the cables on the tile floor and ran them about 100 feet into the vault. all were the same length from the supply house. we did not realize the power company cut the cables as they made them up???? i was suprised --- it made a large difference in ampacity -- the shorter cables were drawing current at peak hours of near 500 amps, while the longest cable of that same phase was drawing about 200 amps!!! i would guess there was maybe eight feet difference at most. we covered the cable run with plywood and installed two fans at one end to remove the heat generated by the cables. my therory that the cables would automatically balance out due to heat and wire resistance "went out the window"!
it does make a difference!!! the next time we had a situation like this we made the cables the same length and the current equalized between cables.
 
Re: Parallel Conductor Length Differential

I haven?t had to rethink such a fundamental concept in a long time.

Izak?s post concerns me the most. Where there are multiple turns in a duct-bank the probability of mismatching the lengths is fairly high. In any case, I refer to my orignal "current divider" post.

Assuming all variables except the length are constant it is fairly simple to create a ?hack? formula to determine the current dividers.

For each length Lk, for k = 1 to N:

LE = 1/((1/L1)+(1/L2)+...+(1/LN)) [Similar to computing parallel resistances]

Then Ik = Itotal x LE / Lk

Please for give the lack of true subscripts

I did compute different values for current sharing than John did.

For the ?short? run, the 10? conductors had ~560.5A each and the 12? had ~467A for a total of ~3270A. John?s calc would indicate 5x 590 + 536 = 3486A, 216A more than the original total.

For the ?long? run, the 100? conductors had ~546.8 each and the 102? had ~536A for a total of ~3270A. John?s calc would indicate 5x 554 + 543 = 3313A, 43A more than the original total.
 
Re: Parallel Conductor Length Differential

I guess the only solution is to forbid the use of parallel conductors, and get the cable manufacturers to start making 10,000 MCM conductors. :D
 
Re: Parallel Conductor Length Differential

A good mechanic can handle this without a problem.

Out of all the answers I think this sums it up COMPLETLY
 
Re: Parallel Conductor Length Differential

One more comment. How you terminate, the arragement of the bus, there are other variables than just the conductor lenght.

Which takes me back to my last post comment

[ April 18, 2005, 06:52 PM: Message edited by: brian john ]
 
Re: Parallel Conductor Length Differential

HI Bob: Sorry gentlemen for the confusion I created: let me start over. Where the conductors are shorter, they will have a lower impedance and therefore conduct more current than the longer conductors. In the case I cited, 5 conductors were 10 ft and one was 12 ft. The amount of current on each 10 ft conductor will be 527.42A and on the 12 ft it will be 632.09A. The current is split up among the shorter conductors. 5 x 527.42 = 2637.1A plus 632.09A = 3270A.
For the 100 ft and 102 ft runs, 543.19A carried by the 100 ft and 554.05A by the 102 ft conductor.
5 x 543.19A = 2715.95A plus 554.05A = 3270A. The shorter conductors are sharing the the excessive current among them and dumping the rest on one conductor. I set up the formulas in excel. They are the same as for parallel resistors as Bob indicated.
 
Re: Parallel Conductor Length Differential

John,
Where the conductors are shorter, they will have a lower impedance and therefore conduct more current than the longer conductors. In the case I cited, 5 conductors were 10 ft and one was 12 ft. The amount of current on each 10 ft conductor will be 527.42A and on the 12 ft it will be 632.09A.
Click to expand...
How is the current less on the shorter conductors?
 
Re: Parallel Conductor Length Differential

John: Given five runs (10 foot each) and one run (12 feet), and given a total current of 3270 amps, I calculate that the 10 foot runs would each get 560.48 amps and the 12 foot run would get 467.28 amps.
 
Re: Parallel Conductor Length Differential

Hi Charlie, Boy am I glad this question was posted, I double checked my excell program and found some errors in the program. I corrected them and arrived at 560.57A for five 10' and 467.14 for one 12'. thanks again. Glad u can't see the red face.
 
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