Largest Reasonable Conductor Size to Specify

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jrohe

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Professional Engineer
I am looking for opinions on something.

I have worked at my company for 26 years and they have had a standard practice that the largest copper wire size we would ever specify would by #500 kcmil. As such, if a load was greater than 380 amps, we would use parallel conductors but never exceed the #500 kcmil wire size (even using 2 sets of #3/0 in lieu of one set of #600 kcmil).

The rationale behind this standard has been off-the-shelf availability and lug capacities for wire sizes larger than #500 kcmil. Maybe this rationale was valid before I joined the industry, but I am not so sure it is still legitimate.

I would like to change the maximum wire size we specify to #600 kcmil (possibly #750 kcmil) but before I do, I want to make sure I am not going to cause issues. So I have a few questions:

1. From what I've seen, if a lug will accommodate #500 kcmil wire, it will typically accommodate #600 kcmil wire. Is this true?

2. Is #600 kcmil or #750 kcmil wire more difficult to source than #500 kcmil wire?

3. Would anyone hesitate specifying or using #750 kcmil wire? I believe lugs could become an issue at #750 kcmil.

I appreciate anyone's input on this topic.

Jason Rohe, P.E.
 
I will tell you this. I have done the calculations a bunch of times. 1/0 is the cheapest copper you will get for the ampacity.

The problem with larger wires is that you get more copper but not more ampacity as you get bigger.

Look at the cost of the wire versus how much ampacity you get.
 
For instance
(2) 1/0 is 300 amps, even derated for more than 3 ccc. you have to go to 350 mcm to get 300 amps in a single conductor.

(2) 1/0 is about $7 a foot right now. 350 is more than $11 a foot.
 
I have used 750 CU and would use parallel conductors any time over 750. Using 600's was bad but depending on the situation could be alright. Overall I think your company is smart to stay with 500 being the max. If you do choose to go larger see if you can be a helper someday when you know 500's or larger are being installed and terminated.
 
I am not man enough to deal with larger conductors. I am happy with 4/0 max...LOL

The trouble with smaller conductors may be the space you have to deal with and the number of lugs needed for the equipment.
 
Dennis Alwon said:
I am not man enough to deal with larger conductors. I am happy with 4/0 max...LOL
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My thoughts, too, 4/0 is plenty.

It's not just the lugs and ampacity, it's also the handling, pulling, and making-up in the equipment. Oh, and it'll depend on how much larger stuff would be installed and the overall project size. If you have to add a couple of helpers and a forkliff+driver, any savings from using fewer larger conductors could fly out the window.
 
750 kcmil is the largest we install. For large equipment like 4000 amp service and switchboards smaller conductors don't make much sense.
 
Thank you, everyone, for your insights! I think we will stick with the #500 kcmil max.
 
petersonra said:
I will tell you this. I have done the calculations a bunch of times. 1/0 is the cheapest copper you will get for the ampacity.
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As the conductors get larger the cross section for conductivity increases faster than the surface area to dissipate heat.

Smaller conductors dissipate heat better. If you could parallel #14s, they would blow 1/0 out of the water in terms of amps capacity per unit copper. Of course code puts a hard cap on this.

Jon
 
infinity said:
750 kcmil is the largest we install. For large equipment like 4000 amp service and switchboards smaller conductors don't make much sense.
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Just curious. What does don't make much sense actually mean? Does that mean you've done a recent economic analysis of the cost of running smaller conductors and more of them? Or does it mean you've just done it that way for a long time and aren't willing to consider changing?
 
More or equally impactive than just actual cost of the wire per foot, you also deal with an exponential cost impact with labor to work with the larger wire. Pulling over a distance, maneuvering the cable, bending, terminating all become more difficult the larger the conductors are. Also cost of tools to work with the larger conductors exponentially increase with size. All that has to be accounted for when determining the max size you or your company is willing to work with, and the cost factoring overall might become such that it is cheaper to just pull 2,3, or 4 runs rather than a single larger pull. Just cost of conduit that would accommodate the difference between 1 pull of 700 vs laying seperate pipe for 2 pulls of 4/0 the 700 is almost triple the cost of the 4/0.
 
petersonra said:
Just curious. What does don't make much sense actually mean? Does that mean you've done a recent economic analysis of the cost of running smaller conductors and more of them? Or does it mean you've just done it that way for a long time and aren't willing to consider changing?
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No, it means that when your labor cost are $115/hour the less sets of conductors the better. That means less conduits, less terminations, etc. To pull 300's with a super tugger requires the same amount of set up as pulling 750's.
 
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