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I'm installing a subpanel on the outside of my house. It is a "Siemens SN Series 125 Amp 12-Space 12-Circuit Outdoor Main Lug Plug-On Neutral Load Center". The main lugs are rated for 2/0 to 4 AWG wire. I'm feeding the panel with #8 THHN protected with a 50A breaker in the main panel. The THHN is running through 1 1/4" Schedule 40 PVC. As I understand it since the 50A breaker and lugs on the subpanel are rated for 75C, I can use #8 with a 50A breaker.

The question is how to connect the #8 wire to the lugs on the subpanel. Are there connectors I can crimp onto the wire? Should I still use the 110 Lb-Ins torque setting?

Would it make sense to add a 50A breaker to the subpanel and feed it that way (not using the main lugs)?

Thanks

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  • Wow, too bad you couldn't have used #4 or #2 aluminum. Save some money and they fit the lugs. May not have fit in your conduit though. Commented Nov 3, 2022 at 2:08
  • @Harper-ReinstateMonica 2 1/4" Schedule 40 PVC can handle pretty much anything. Commented Nov 3, 2022 at 2:09
  • Oops, I meant 1 1/4" conduit, just edited my post. Still could have fit #2 or #4 aluminum, but it is a long run ~90 ft and I didn't want to have to wrangle those thick conductors. Commented Nov 3, 2022 at 2:21
  • Switching to aluminum is probably worth it for the cost savings, but it might just move the OP’s problem to the other end, if the feeder ends up too big for the feed breaker’s terminals.
    – nobody
    Commented Nov 3, 2022 at 2:30
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    Harper's Rule: Buy the wire last. Commented Nov 3, 2022 at 3:44

2 Answers 2

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You should have enough room for Polaris connectors

You should have enough room for a pair of Polaris connectors to adapt the 8AWG copper wires coming into the panel to short lengths of 4AWG aluminum wire that then connect the connectors to the actual panel lugs.

Or, you can just upsize the feeder

That said, your conduit is large enough that you should be able to fit a 2-2-4-6 MHF cable down it, which gets you 90A for not much more than your 8AWG copper THHN cost.

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  • What if I add a 50A double pole breaker and feed the panel through the breaker instead of the lugs? Commented Nov 3, 2022 at 2:22
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    You can do that. Breaker must be bolted down. Commented Nov 3, 2022 at 2:30
  • Thanks, is that a different kind of breaker or a hold-down kit? Commented Nov 3, 2022 at 2:40
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    @ChristianK -- for your Siemens panel, that's an ECMBR2 Commented Nov 3, 2022 at 4:21
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I doubt you can crimp connectors on. Your options, generally speaking, are:

  • Pigtail 8 AWG to 4 AWG

This should be easy. Just a matter of finding the right wire nut or other listed connector. Checked the Ideal catalog (the big name brand in wire nuts) and nothing larger than 6 AWG. Actually, hard to find anything but Polaris which are not cheap.

  • Change to Aluminum Wire

Yes, that sounds radical. But 4 AWG or even 2 AWG Aluminum will likely cost less than 8 AWG Copper. Then you don't have to do anything special, and you get higher capacity for future expansion (65A for 4 AWG, 90A for 2 AWG).

  • Backfeed a Smaller Breaker

A backfed breaker also needs to be bolted down, but that's not a big deal - typically costs just a few $ for a little piece of plastic, but will vary by breaker/panel manufacturer. As long as you don't need the spaces (you will be left with 10 spaces - that's 10 120V circuits or 5 240V circuits or a mix) that's OK.

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