Installing a 100 amp subpanel approximately 160 feet from my main panel to a new garage. 4-4-4-6 copper I understand will be more than sufficient for a 240v/60 amp load at the subpanel and have less than 3% voltage drop. The problem I am having is finding 4-4-4-6 copper cable anywhere on line. I prefer copper to aluminum. I can find 4-4-4-6 SER, but as I understand it SER is not to be direct buried. Any direction on a place to order the copper cable, or advice on other substitutes is appreciated.

closed as off-topic by mmathis, Lee Sam, ThreePhaseEel, Ecnerwal, Daniel Griscom May 22 at 1:52

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    Copper at that size and length is going to be an expensive "preference". – JPhi1618 May 21 at 19:15
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    May be better off with conduit (sized to allow future upgrade if needed) and individual wires. – manassehkatz May 21 at 19:28
  • I understand it is more expensive, but won't matter in anycase if I cannot find any. – Mark Grosvenor May 21 at 19:29
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    @LeeSam There are enough on-topic issues here. – Harper May 22 at 1:17
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    Hello, and welcome to Home Improvement. Unfortunately, "shopping" questions are off-topic here, although you may be able to edit out the shopping portions. Please take our tour so you'll know how better to contribute here. – Daniel Griscom May 22 at 1:52

That choice of copper wire is going to be even more expensive when the inspector makes you rip it out and replace it with the 3-3-3-5 you should be using. You need to use the 75C column in 310.15b16 and can't use the 83% bonus derate on subpanels.

There's an urban myth that 4Cu/2Al is allowable on 100A, that was due to a misunderstanding of the applicability of 310.15b7, which in fact only applies to service entrances that serve the entire dwelling unit. E.g. from meter to weatherhead or from main shutoff to main panel. NOT subpanels.

I agree SER is not made for direct burial.

I concur that at this distance, 3Cu or 1Al will contain voltage drop to <3% at 60A. Expect about 4% at a full 100A, which I wouldn't lose any sleep over. You are correct not to design for 3% at breaker trip current.

Regardless expect to spend $5-7 per foot for #3Cu and about $1.60-$2.00/foot for #1Al.

The price you will really pay is decided by your local electrical supply house, at least the one willing to carry oddball cable like this. The size isn't oddball, the use of copper when just about anyone else would use aluminum is oddball.

Some houses in the 1970s had a problem with 15/20A circuits using powerline grade aluminum (AA-1350). But there is nothing wrong with aluminum feeder at #4 size and up, even the obsolete AA-1350 alloys proved reliable. Regardless, out of an abundance of caution, AA-1350 was outlawed and a new alloy, AA-8000, was invented specifically for household wiring. The lugs will be aluminum, which means aluminum feeder does not introduce a "dissimilar metal" issue - but copper does! Ironically.

Given your desire for copper, you are probably better off laying conduit e.g. sched. 80 PVC, and running individual copper wires (THWN or XHHW) therein. They definitely make those in copper. The downside is theft risk, but it's a lot harder to steal buried cable.


It's unlikely anyone makes or carries such a thing in copper. By the time you get to cables this big, most people are going SER. Your best bet is to buy 4 gauge THHN and bury it in conduit (can't be direct buried). Not only is that a lot easier to find, it gives you exactly what you're looking for.


I believe you can use USE-2 (Underground Service Entrance) wire for this purpose. You would need to get individual wires instead of a cable, but otherwise it's the same. See 2017 NEC Article 338 for the rules about using it.

One source is here; you can probably find others as well now that you know the right keyword to look for. I've never bought from them so it's not a recommendation, it's just the first place I found. https://www.wireandcableyourway.com/xlp-use-2-rhh-rhw-2/

Note that you would almost certainly be better off using aluminum wires at this size. Most of the problems with aluminum come from connecting it to copper, but the lugs on your panelboards and breakers will be aluminum anyway, so that's a non-issue in this application.

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