When installing a 100A sub-panel with larger wire (3-3-3-5 copper SER), is there a specific size of wire clamp I need for this? Do I use the same type of wire clamp as I would for NM cable, just larger (like the type that go into the knockout with a nut and have some screws to tighten the cable)? Is there a specific size of clamp I need?

I think the codes I've seen (which I haven't search through exhaustively) just say the wires need to be secured and follow the manufacturer guidelines or something. Would appreciate any specific parts I should buy.

  • Is this in conduit or just loose?
    – Machavity
    Dec 10, 2019 at 21:15
  • 1
    Go to the electrical supply house and they'll set you up. It's just so typical of the big-box stores to cheerfully sell you the wire and not discuss box entry. Dec 10, 2019 at 21:21
  • @Harper-ReinstateMonica, they know how to measure wire, not what to do with it. It's a DIY store after all!
    – JPhi1618
    Dec 10, 2019 at 21:28
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    Just loose, in-wall (the main and sub are mounted in the drywall so I didn't want to run conduit behind the drywall or go out/back in if I didn't have to). I guess I'll just make a trip one of these weekday mornings to an electrical store to ask.
    – Matt
    Dec 10, 2019 at 21:29
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    @Machavity And the online UI will totally help you buy the wrong thing :) Dec 10, 2019 at 21:37

1 Answer 1


Yes, you're looking at a larger NM clamp here

Looking at some sizing data, a 3-3-3-5 copper SER is 0.91" in diameter. This fits neatly into the .790-.980" cable range supported by an Arlington 843S or equivalent 1" trade size NM/SE clamp.

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