Conductors in parallel

[drg]

Last year I had to parallel some 500 MCM feeders, can remember cutting all the lenghts to exactly 22ft. when time came to terminate the "C" phase they were hard to hook up because the "A" and "B" phases were hooked up and used up a lot of working room in the CT panel,
I was told to cut the 4 feeders on the "C" phase but make sure that they were all cut the same amount, the 4 wires were cut exactly 8" a piece and terminated on the lugs.
I have read article 310.4 and understand that the work I was told to do was correct, but looking back and getting more involved in this I really am curious what would be to long or to short in this instance when cutting parallel conductors?

Anyways when cutting these conductors I always use a tape measure and put tape around the area to be cut , then they are stretched out in alike groups, phase taped and doubled checked.

I was told recently that the longer the conductor the less critical this becomes but I really would like to know "some type of numerical amount" that would be applied to this, such as a 1/2" difference over 10 ft is no good, or is o.k. someting on that order.

John

 

[bennie]

Re: Conductors in parallel

This can be expressed in a ratio and proportion equation in algebra terms. I don't have or know the terms.

The ratio of resistance change is directly proportional to the change in length.

A 50 foot cable with a 100 foot cable will have 1/2 the resistance, or 50%

A 5 foot difference will be a 1/10 resistance change or 10%.

A 5 foot difference in a 100 foot cable is a change of 1/20th, or 5%.

In otherwords the amount of resistance change is inversely proportional to the change in length of the cable.

This is my opinion, the NEC does not deal with the math.

 

[drg]

Re: Conductors in parallel

I guess if you look at in proportion and ratio being off approx. 2.5 inches when trying to parallel 20ft lenghts you would only be off 1% . that sounds like a small amount .

In reality I would guess the lenghts to be within a 1/2" of each other and then you would only be looking at a small fractional percentage.

This helps me look at this in a different light than what I have been.

Thanks for the pointy.

 

[bennie]

Re: Conductors in parallel

John: I hope you understood my statements, I'm not sure I do after reading them again.

Remember that only the same phase conductors should be the same. Different phases can be different lengths.

 

[drg]

Re: Conductors in parallel

Bennie, I understood what you were trying explain to me , and your last statements even though understood beforehand, were appreciated cause I realize you do not want to see anyone get hurt.

P.S. I usually have someone watching or checking anytime I do something like this so even if I began to do something wrong I would feel the whip across my backside long before anything was ever hooked up, let alone energized.

Thanks again and have a good evening.

 

[tom baker]

Re: Conductors in parallel

And different phases can be different conducotor type. Each parallel set must be the same impedance, ie length and type conductor (copper or Aluminum)