Greetings and question regarding parallel conductors

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ericsherman37

ericsherman37

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Oregon Coast
Hello everyone, this is my first post. I'm an apprentice and I'm in the process of preparing for my Oregon licensing exam. I have quite a while still, but it never hurts to prepare early.

Anyway, I've got a question regarding parallel conductors. I ran into the issue a while back on the job and wasn't quite sure how to proceed. I got a couple different opinions from a couple different journeymen and that of course lends to my confusion.

If you're paralleling conductors, say for a feeder circuit, do you first find the equivalent single conductor, and divide up the circular mil area by the number of parallel runs and then select the appropriate conductors to parallel? Or do you divvy up your parallel runs based on their ampacities in 310.16?

I am leaning toward the latter, but I have never had this question conclusively answered by anyone. I appreciate the input, thanks!
 
Greetings and welcome.

To answer your question, you divide the amperage by the number of parallel runs, then size the conductors by the amperage needed.

For instance, if you are running a 600amp feeder, and want to run 3 sets of parallel runs, you need 200a conductors in each run.
 
That is kind of what I figured.

The issue was actually in a movie theatre in my area undergoing an expansion. We're putting in a 300A 3 phase subpanel for a couple of new projectors, and there isn't very much room for a single big conduit. We figured it would have taken a set of 350 copper THHN conductors, which including a full-size neutral and whatever equipment ground it required, would have used a 2 1/2" pipe. Not enough room for an EMT 90 degree elbow of that size. We wound up running two sets of 1/0 copper (I think) in 1 1/2" conduit, and it worked out.

It's just been gnawing at me for a while. Thank you for enlightening me.
 
I agree with 480sparky and will add that if you where to do it by circular mills you would end up spending extra money that you do not need to.

For example lets you need to run a 500 amp feeder if you look at a 1000 kcmil copper conductor at 75 c you will see it is rated 545 amps.

If we wanted to run 2 sets to that based on circular mills we would end up with two runs of 500 kcmil with a total ampacity of 760 amps. A bit of overkill for a 500 amp feeder.

This happens because as the conductors get larger we get less usable ampacity for a given amount of copper.
 
iwire said:
This happens because as the conductors get larger we get less usable ampacity for a given amount of copper.
Click to expand...
Well, I'm a the-glass-is-half-full kinda guy. I prefer to look at it the other way: the smaller the conductor, the greater the ampacity/area. :D
 
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