NEC 310.4 conductor in parallel

[Manny.M]

NEC 310.4 "Conductors in Parallel" states that the paralleled conductors in each phase shall be the same length.
Of course, sometimes, for different causes, the length of the conductors can not be the same. What percent of the conductors length is permissible to be larger or shorter complying with NEC 310.4?
Just right now we have 6 sets: 600kcm each with a average length of 45 FT but one of the sets is 48 FT and the inspector has concern about.

 

[iwire]

A great question and one the NEC provides no answer for.

You could try to calculate the loading to show if any conductor could be overloaded and present the results to the inspector.

 

[GoldDigger]

Of course, sometimes, for different causes, the length of the conductors can not be the same.

Even if you allow space to "store" equalizing extra cable length to bring the geometrically shorter runs up to the same length?
(Sometimes the problem lies in the layout of an existing duct bank or set of raceways, but other times it can be handled in the initial design.)

Calculating the exact resistances from the lengths and considering the worst case of no other resistance in the circuit (very conservative) you can calculate the current each run would carry assuming that the voltage drop on each run would be identical. For 3 feet out of 48, it should not be more than a 6% difference in current.
If you also take into account termination resistances and assume that they are all equal, the imbalance will be even less.

 

[qcroanoke]

Have a look at this thread: http://forums.mikeholt.com/showthre...city+of+different+length+parrellel+conductors

 

[GoldDigger]

Have a look at this thread: http://forums.mikeholt.com/showthre...city+of+different+length+parrellel+conductors

And when you look at it, take iceworm's demonstration with a grain of salt. The thread is closed, so I cannot comment there, but here is the problem I see:

iceworm calculates that for a 1% difference in length, the current in one condutor is .4975 and in the other it is .5025. Absolutely right. But he then says that the difference in current, .5, is .5%. That is not correct. The imbalance between the two wires is .5A out of 50A, which is 1%, just like the length difference.
If you want to refer to the difference as a percentage of the total current, then his characterization is correct. But one way or another, the short conductor is carrying 1% more current than the long one.

This is particularly important when you start looking at more than two conductors. The difference as a percentage of the total current will change as the number of conductors involved changes. But the percentage difference in current between any two of those conductors will still match the percentage difference in their lengths.

This whole thing is an approximation, which is perfectly good up to about 10% difference, at which point the current percentage difference starts to get noticeably worse than the length percentage difference.

PS: A different way of looking at it is to say instead that the difference in length between 100 and 101 is actually only .5% of

 

[iceworm]

iceworm calculates that for a 1% difference in length, the current in one condutor is .4975 and in the other it is .5025. Absolutely right. But he then says that the difference in current, .5, is .5%. That is not correct. The imbalance between the two wires is .5A out of 50A, which is 1%, just like the length difference.

I found an old laptop I can use for this website. So I can be back occasionally. Bummer, you didn't get rid of me totally :weeping: :happysad:

GD -
First, I'm pa2b - so ....

I looked through my math. I think I was saying that two conductors 1% different in length would share at 50.25% in the shorter and 49.75% in the longer. So, out of a 100% total load the difference is .5% -- .5A difference for a 100A total load.

Or, the shorter is .25A higher than the 50A each one should be carrying. The longer is .25A lower than the 50A each one should be carrying.

I think the math is good -- however, I have been all wet many times before. This would not be the first.

ice

 

[iceworm]

The cables can never be the same length. There is always a tolerance.

I like Iwire's answer in post 2. The physics doesn't lie.

ice