Parallel Feeders

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PMBunbury

New User
Location
Santa Rosa
Occupation
Electrical Project Engineer
I have over 50 years in the industry and many associates and former associates call on me for clarification of the NEC from time to time. I was a Journeyman Electrician IBEW for over 10 years then an electrical C10 contractor for over 10 years but graduated with a BSEE in 1983 and became a full time electrical engineer. I never had this question before and I hope someone in the inspection authority can answer.
It is not unreasonable to think that a parallel feeder with at least 3 90-degree bends will have a slightly less than perfect exact same length of feeders. Code does not provide a plus or minus tolerance. Is there an acceptable tolerance?
 
PMBunbury said:
It is not unreasonable to think that a parallel feeder with at least 3 90-degree bends will have a slightly less than perfect exact same length of feeders. Code does not provide a plus or minus tolerance. Is there an acceptable tolerance?
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You're correct the NEC is silent on the tolerance between the different lengths. IMO they could easily assign a percentage like 5% which would keep everything working properly but they do not. With long runs we never bother to count bends or factor in the conduits on the outside of the rack being longer than the inside because at the end of the day in the real world it just doesn't matter. In answer to your question they're required to be the same length. How you ensure that they are is up to you. You can leave extra conductor length at the terminations for the shorter runs of raceway. If the parallel runs are 5' long then the length is more critical then if they're 150' long. For 150' of raceway length we wouldn't bother to measure the conductor lengths.
 
HAH! You're same as me! I graduated in 1982 from RIT.
I think parallel conductors are all considered the same length. At least that's how I'd call it for a big-box data center, which is what I'm doing now in Oregon/Washington area. We're updating the Power Study for the 5-year 70E recommended frequency.
Peace,
John
 
jim dungar said:
And if 5% was acceptable how long before someone wants 10% using all the same arguments?
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As retired said in most cases it's impossible to determine if the parallel conductors are the same length especially with long runs with several elbows. In many of those installations the lengths vary by 5% or even more yet we don't find these parallel conductors burning up all over the place. The textbook theory that says parallel conductors have to be the exact same length is a fallacy in the real world so the code should reflect that. I just used 5% as an example.
 
Remember that the conductors themselves have diameter tolerance. Additionally to maintain the exact same characteristics the ferrous conduit would need to be the same length.

IMHO the industry ignores this issue rather than having a well defined allowed tolerance...but I don't think I've seen any reports of a problem from this issue.
 
infinity said:
The textbook theory that says parallel conductors have to be the exact same length is a fallacy in the real world so the code should reflect that. I just used 5% as an example.
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My point is, if you publish a tolerance people will eventually want it increased.
Without some technical support (i.e. text books) for the proper % tolerance isn't better to leave it as it has been for almost 100 years?

How often do people have to repull conductors? How did the AHJ know they were different?
 
jim dungar said:
My point is, if you publish a tolerance people will eventually want it increased.
Without some technical support (i.e. text books) for the proper % tolerance isn't better to leave it as it has been for almost 100 years?
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For one we live in the real world. So IMO for 100 years they've gotten it wrong. The code should reflect real world installations not something that is completely theoretical.

This is right up there with the stupidity of the straight and tapered thread nonsense. We've been threading SE cable connectors into meter hubs for at least 75 years without issue yet the code still doesn't permit it.
 
You are supposed to measure them out ahead of time to get the same lengths. It is a pita but if you want close to perfect that is about the best you can do and don't cut any excess in the enclosures unless you cut the same amount from each one.
 
Dennis Alwon said:
You are supposed to measure them out ahead of time to get the same lengths. It is a pita but if you want close to perfect that is about the best you can do and don't cut any excess in the enclosures unless you cut the same amount from each one.
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When your pulling 300' of 750's off of a reel there's no way you measuring anything before they're pulled in. :giggle:
 
infinity said:
When your pulling 300' of 750's off of a reel there's no way you measuring anything before they're pulled in. :giggle:
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I realized that but the only way to get it exact is to do it that way. I guess you can pull the wire leave enough extra and then put a wire distance meter on it. I don't know how accurate they are.
 
do you know why i like the NEC handbook vs the paperback? because the handbook includes blue notes explaining the INTENT and the purpose of the code articles. from everything i've learned over the years, the INTENT and the purpose of the "same length" on parallel runs is to ensure that each parallel has the same ampacity, and we're not running one over the others.

now, you're the engineer, so i'm confident you can do the calculations on what an extra 12" of wire is going to be on a 300' pull. and then do the resistance changes of the conductor calculated by ohm's law with voltage applied.... long story short, when is the last time you saw the voltage from a phase to b phase to c phase actually within .03333% tolerance (300/1)?

you're the big guy with the magic stamp. whatever YOU say is the correct tolerance, is what is correct. kinda like how inspectors get to determine what is "subject to physical damage."
 
infinity said:
... If the parallel runs are 5' long then the length is more critical then if they're 150' long. For 150' of raceway length we wouldn't bother to measure the conductor lengths.
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Another thing with very short lengths of parallel conductors is that the variation in contact resistances between different crimps and bolted connections can start to become a factor in the sharing of currents between conductors. And so good connections become more critical in this case.

For example, a connection that is 60 micro-ohms would be 10% of the resistance of 5 feet of 1/0 copper. At 150A the connection would dissipate (150)² x 60 x 10E-6 = 1.35 watts, which is not likely to create a temperature problem but it would have a noticeable impact on current sharing.
 
Just do a simple calculation based on the idea of current division in a parallel circuit using the lengths of the conductors as the resistance. Unless the run is very short and heavily loads, a 5% length difference will not be a real world issue.
For example, a 600 amp load on two parallel 350 kcmils with one set 20' long and the other 21' long (5% difference). The short set would carry 307 amps and the longer set 292 amps. The ampacity of 350 is 310 amps.
If we make the difference 10%, 20' and 22' the current division is 314 and 289.
 
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