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Looking to update the electrical setup currently in my detached garage - read through many posts on here of similar projects, but nothing with exactly the same specs. Hoping to find some right answers / make sure what I'm doing makes sense.

Currently, the 12 AWG THHN being fed through conduit out to my detached garage (50 ft. from house) is being powered via a 12 AWG Romex line indoors, in a junction box on a basement wall. This is also tied to a kill switch that has a separate line, ran up to a kitchen receptacle.

1950's house, so much of the awful wiring throughout is grandfathered in I believe, but the garage is on the same circuit as many other major lines - to my kitchen lights/outlets, basement lights, etc. I currently am trying to put the garage on its own circuit, using a 40 amp double-pole breaker that is not being used in my breaker box. I'm not certain if this is a true 240V breaker, though I believe it is based on the plug in the kitchen (the breaker used to be for an electric range)

My plan is to put a subpanel in the garage with 2 breakers, a 15A for the lights and 20A for outlets (power tools will be used) I know everyone always says the bigger the better for a garage subpanel - I don't need it. Lights and those outlets are the most I'd need. Just trying to make sure I get the correct wire (8 AWG THHN-2 / THWN) but don't know specifics as far as how many hots/neutrals/etc. needed for this sub-panel, if I actually need to keep a kill switch in the house for power sent to the garage (and if so, how is this done since the current setup of Romex to THHN is not ideal?) Most of what I'm seeing in looking at THHN-2 in 8 AWG are sold as single wires, so I want to make sure I order correctly and get the wiring done well. Or is there a better solution altogether? Any help is appreciated!

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This is actually a good plan. 40A is fine on 8 AWG aluminum. That is the max on 8 AWG aluminum, so depending on conduit size (because if your existing conduit is usable and big enough you don't want to have to replace it) you may want to bump up a size or 2. It generally costs relatively little extra for larger aluminum wire. Depending on where you get it, you might find bumping up a size or two costs you in the neighborhood of $0.10 per foot per wire - 4 wires x 50 feet x $0.10 is all of $20.

Note that you can use larger wire than you currently need. But check your conduit size first - it is possible, for example, to have conduit that can take 3 x 12 AWG (hot, neutral, ground) but can't handle 4 x 8 AWG (hot/hot/neutral/ground - actually, the 4th wire for ground may be able to be smaller - there are charts showing the requirements). And it may be large enough for 8 AWG but not for 6 AWG.

The breaker can also be a factor. It varies by manufacturer, but some max. out at 8 AWG wires and some can use larger wires, and you must use the correct breaker for your panel.

Back to the size of the breaker. It must be no larger than the wire can handle (8 AWG minimum for 40A). You do not need a separate kill switch for the circuit in the main building - the breaker serves that purpose. You do need a separate kill switch (officially: disconnect) in the subpanel, which can either be by using a "main" panel as a subpanel (but keeping ground and neutral separate because it is a subpanel) or by using a locked-down backfed breaker as a disconnect switch (effectively a "main breaker" for the subpanel). Actually, you may be able to avoid that altogether based on Rule of 6, but I am not sure.

I highly recommend putting in a modest size "main panel". For example, in a quick search at Home Depot I found a Siemens panel for ~ $90 that includes a 100A main breaker, 20 spaces (not "circuits", full size spaces), 3 20A single breakers and 1 30A double breaker. And in fact that would work great for your setup. It doesn't matter that 100A is larger than the 40A feed. You would use 2 of the 20A breakers for your light and receptacle circuits, with 1 more available for something else in the future. You could then use the 30A double breaker for when you get an EV. 30A @ 240V is plenty for typical charging needs. The 20A circuits will need GFCI (well, definitely the receptacle circuit - lighting will depend on local code adoption), which can be done least expensively at the first receptacle, and you're all set.

You will need to add ground rods. As I understand it, a single circuit (as you have now) does not require ground rods, but a subpanel does. 2 rods, several feet apart, connected to a ground wire which connects to the subpanel ground bar.

If your conduit is rigid metal and in good condition then you don't need a ground wire in the feed. Otherwise you need hot, hot, neutral and ground. Again, your existing conduit size is a key factor to be checked before doing any of this. If your existing conduit is not large enough for 8 AWG wires then you need to rethink this plan as there are various options for conduit or for direct burial cable (not individual wires).

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    "several feet apart" being minimum 6 feet apart. If not being a cheapskate 10 or more feet apart is better, but 6 feet is code minimum ground rod separation.
    – Ecnerwal
    Nov 12, 2023 at 13:48
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    @manassehkatz - thanks so much for the response, this is really helpful and exactly what I needed. I'll look to roll with this setup now, the 100A panel is a good idea and I like that combo pack that has the breakers included. I'm not sure specifically what size the conduit underground is, there is a run of about 3' coming straight up the garage wall into the junction box that's about 1" but I'm not certain what's underground is the same size. Again, really appreciate your helpful response on this!
    – kraesheo
    Nov 15, 2023 at 23:27

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