What size aluminum ser do I need to feed 100 amp subpanel? Ten feet from main panel.

  • 1
    What is a "ser"?
    – jwh20
    May 17, 2019 at 22:26
  • A 4 wire cable.
    – user101687
    May 17, 2019 at 22:32
  • 2
    Are the existing and new panels flush mounted, surface mounted, or outdoors? Is there a wall or other divider between the two panel locations? (I ask because SER might not be the right choice for this application to begin with) May 18, 2019 at 0:39
  • homeline surface mounted type
    – user101687
    May 18, 2019 at 0:49

2 Answers 2


Why SER?

With surface mounted panels in what presumably is an unfinished space, there is 0 reason to run an exposed SER cable here. Instead, I would run a conduit between the two panels (2" EMT will handle anything you want to do here), and then simply run 3 XHHW-2 conductors (hot, hot, neutral) in the conduit between the two panels, using the conduit as a grounding path.

As to inner conductor sizing...

I would use 1AWG Al wires here; spending the extra for copper is a waste, and you can't get away with 2AWG for this because you'd exceed the 75°C termination limit that is normal for breakers and loadcenters if you did. This limitation is a consequence of NEC 110.14(C)(1), by the way.


One essential thing with this job, no matter who does it, is that all lugs will need to be tightened to the torque specified on the corresponding labels using an inch-pound torque-wrench; this is required by 2017 NEC 110.14(D), and is a good idea anyway, lest your electrical system go the way of Greg Biffle's infamous lugnuts.

  • Code states you can use aluminum 310-14 it is allowed,may not be a fan of it Then i went to 310-15 ,6 It sent me to 310-15(b)(6) says #2 allowed. i dont have new code book. Have there been changes?
    – user101687
    May 20, 2019 at 0:14
  • 1
    @RobertMoody Aluminum's still fine (as long as it's the AA-8000 series stuff), but I suspect you are looking at the wrong column in the NEC ampacity tables (either that, or they've changed since your Codebook was published) May 20, 2019 at 0:51
  • I have a 2008 code book 310-14 has seu and ser at AA-8000, What about table 310-15(b)(6) that change is dwelling sub panel only. Plus can you fix my last comment stuff about fan in it .
    – user101687
    May 20, 2019 at 1:18
  • 1
    @RobertMoody -- you're misinterpreting what is now 310.15(B)(7) -- the conductors in question have to be carrying the entire dwelling unit load in order for that part of the Code to apply; since you have part of your house on the main panel and another part on the subpanel from the sounds of things, then you can't use the 83% adjustment factor (or table 310-15(b)(6), for that matter) May 20, 2019 at 3:23
  • Looked for .310-15(b}(7) was not in 2008 See you state,entire load.So now table 310-15(b)(6) Allows #2 alum for main panel only seu . And when comes from main panel to a sub panel (b)(7) factors in, and you cant use #2 ser. So now to new code #2 seu can be service riser to a 1oo amp meter disconnect. t. Witch now make that main panel..(b){7)ser has to be changed to now feed sub panel? #2 not aloud you have to increase the wire size.Is (b)(7) a table?
    – user101687
    May 20, 2019 at 16:49

Why aluminum?

4 copper or #2 aluminum per NEC Table 310-15(b)(6). Note that sub-panels must have the neutrals and grounds completely separated, with the neutral bus isolated from (not bonded to) the steel enclosure. If you have any questions about this please let us know.

  • 1
    Why aluminum? Because it is often cheaper. May not matter much for 10'. May 19, 2019 at 15:15
  • 1
    I hate aluminum and it's tendency to corrode and cause problems with dissimilar metals, unless your budget is that tight I'd use copper everywhere. The cost difference here at this length is minimal and the peace of mind that comes with not having to slop antiox all over everything and knowing it will last forever. May 19, 2019 at 15:51
  • 1
    @Bobthebuilder corrosion isn't the problem -- if your connections are corroding, then you've done something really bad technique-wise as a proper mechanical lug or crimp connection forms a gas-tight seal. (Thing is, without a torque wrench, it's pretty trivial to get a mechanical setscrew lug connection wrong, and mere luck when you do get it right) May 19, 2019 at 22:56
  • @Bobthebuilder -- and p.s., copper wire connections can fail from bad torquing too, it's not a problem limited to aluminum wire! May 19, 2019 at 22:58
  • 1
    @Bobthebuilder -- furthermore, you are actually calling out an undersized wire in this answer! (you need to use the 75°C column when doing panel to panel runs as that's all the terminations can take!) May 19, 2019 at 23:07

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