ECN Forum Forums General Topics of the Trade General Discussion Area Parallel conductors

How important is the rule for parallel conductors to be equal length?

Is there any leeway?

I am installing a set of 4 (phase A, B, C & Neutral) in a 4" PVC conduit, 6 times, for a 2000amp service.

How would you handle the length issue in this installation? Cut all the same length, based on the longest, and loop where needed?

What method is best to help train this wire? Has anyone used that new device that fits on a ratchet? I saw one at a trade show, looked like a good idea.

Any hints on how to do this?

Thanks in advance.

Rick Miell

Rick,
The shorter the length of wire and the higher the load, the more critical the length becomes. I normally lay the conductors out and cut them all the same length before installing them. Finding space for the excess wire is sometimes a problem.
If you look at your installation and assume 4 sets of 600 kcmil with a calculated load of 1680 amps and multiple service disconnects and cable lengths of 20', 21', 22' and 23', we will find that the two shortest cables will be carryign more than there rated 75 degree ampacity. The 21' would be at 452, the 21' at 428, the 22' at 409 and the 23' at 391 amps. The rated ampacity of 600 kcmil is 420 amps.
Don(resqcapt19)

Rick,
It is important to get them close, but reality says that there will be some difference because all of the conduits don't enter the service equipment in the same place. I can't speak for the ratchet device. I've always used a hydraulic wire bender.
I used to have a problem from a class that I took that showed in theory how important it is to have the conductors the same length.
Good Luck, and let us know how the installation turns out.

Regards,
Doc

I need to find out how inspectors are treating this rule that all conductors need to be the same length. Is there any standard percentage length difference that they allow?

I find that if there is a 10% difference in length between two conductors, there is approximately a 10% difference in the current through each conductor, with the shorter conductor carrying the larger portion of the current.

Also, do inspectors look at the whole installation as to how important it is for conductor length to be exactly the same.

For example, if you have 2 parallel - 1/0 conductors, 75C rated 150A feeding a 300A load. If there is any difference in length between the conductors, the shorter conductor would exceed it's ampacity rating.

But, if those same two conductors are feeding a 250A load, the ampacity of the shorter conductor is not exceed until it there is over a 30% difference in wire length.

Can anyone help me here?

Thanks,

Laura Jenkins
IBEW 48

for the length i have always worked that the shortest should be within 2%of the longest,

Are you basing that 2% off of anything in particular?

Laura

i am a canadian electrician and was taught this many moons ago,but if i have time i will look through the code book to see if i can locate this rule.