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Parallel SER Cables

Posted By: George Little

Parallel SER Cables - 05/02/19 07:06 PM

Just had someone ask me about running SER Cable in parallel such as 4/0 Al. 3 conductors w/ ground. The overcurrent would be rated at 400a. so the question comes up about the proper size ground. Table 250.122 would require a #1 as ground. I don't know if this is doable Look at 250.122(F). I think it's fine. What say U Greg? or anybody?
Posted By: gfretwell

Re: Parallel SER Cables - 05/03/19 02:42 AM

I think your guidance is in 250.122(F)(2) and it looks OK to me as long as they get grouped on a bus or common connected pair of lugs on each end.

(2) Multiconductor Cables.
(a) If multiconductor cables are installed in parallel, the
equipment grounding conductor(s) in each cable shall be
connected in parallel.
Posted By: George Little

Re: Parallel SER Cables - 05/03/19 02:52 PM

The issue becomes that of the size of the equipment grounding conductor being the correct size to satisfy the size of the overcurrent device protecting the conductors, per 250.122. But, as I see it the contractor can parallel SER cables if he complies with the paralleling rules such as same length, size, material, etc.
Posted By: George Little

Re: Parallel SER Cables - 05/03/19 03:00 PM

Added thought: The equipment grounding conductor in each cable must be full sized based on Table 250.122.

(d) Except as provided in 250.122(F)(2)(b) for raceway
or cable tray installations, the equipment grounding conductor
in each multiconductor cable shall be sized in accordance with
250.122 based on the overcurrent protective device for the
feeder or branch circuit.
Posted By: gfretwell

Re: Parallel SER Cables - 05/03/19 05:48 PM

I usually assume the EGC in a cable is sized to 250.122. That may not be "full sized" and is significantly less in anything but the "small conductors" using 250.122 table. I haven't really analyzed the numbers to see how that would work when you double the ungrounded conductor size vs the EGC sizes in all cases.
Just looking at the easy example, 3/0 is good for 200a (@ 75c) and can have a 6ga ECG. Paralleling 2 of them gets you 400a and the required EGC is a #3. Table 8 says a #6 is 0.027 sq/in and a #3 is 0.053. Two #6s are 0.054 so it is pretty much the same with a little to spare. I still have not seen an issue.
Posted By: HotLine1

Re: Parallel SER Cables - 05/03/19 06:05 PM


FWIW, as Greg did the math, I do not see an issue with it. Although it is not a common practice to parallel SER.
Posted By: gfretwell

Re: Parallel SER Cables - 05/03/19 07:58 PM

I did wonder about that myself but it does look legal. Each set of conductors will go through the same KO and stuff like that. As long as they are 1/0 or greater and have equal characteristics I don't see the problem. This may just be a situation of "what I had on the truck". wink
Posted By: George Little

Re: Parallel SER Cables - 05/03/19 08:10 PM

The thing that was bothering me is it's pretty clear to me that the EGC in each cable must be sized per 250.122 and I'm not sure what size that wire is in the SER cable. I'm sure it's sized to match the requirements lets say for a single cable but you might have to order it special if you start paralleling them together. But if you look at Table 250.122 it tells me what size that conductor needs to be in relation to the overcurrent device on the circuit or feeder.
Posted By: gfretwell

Re: Parallel SER Cables - 05/03/19 09:03 PM

I believe that the EGC in any cable is sized to the requirements of the ungrounded conductors but what I can't find is if it is sized to 250.122 or 250.66. Since this is "service" cable it might actually be sized to 250.66 which exceeds 250.122. I am going to dig around in the shed and see what I have in the way of SE cable and see what the EGC looks like to me twisted up round.
Posted By: gfretwell

Re: Parallel SER Cables - 05/03/19 09:29 PM

OK I looked at the only piece of SER I have and it is 2-2-2-4 (2ga with a 4ga EGC) that exceeds 250.66 and assuming 2ga is worth 100a 250.122. You can't even get to #4 using 310.15 (B)(6) which lets you use a 125a breaker on your #2 and 250.122.
Looking at the Southwire site, that oversizing continues up through the larger sizes. For a 1/0 they sell you a #2 EGC and even with the 175a breaker you can use for that residential main feeder only wants a #6 as would 250.66.

I think you are more than safe
Posted By: George Little

Re: Parallel SER Cables - 05/03/19 09:42 PM

The cable we are talking about is 4/0-4/0-4/0-2/0 and as long as we only use two cables we'd be okay.
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