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This question is similar to others, however similar questions suggested are over 5 years old. The work has not yet begun, but here is what I am thinking. I am running a 50A 240V circuit from my main panel to a subpanel in my attached garage, for a new garage heater. I do understand that I really don't need a subpanel for a single dedicated circuit, but I want to be able to shut down power to the heater at point-of-use. The garage floor level is below the level of the first-floor floor joists. From the main panel in the basement, I am running 6/2 NM-B by drilling through each first floor joist. The run is less than 25 feet. The last joist I drill through let's me go straight into the garage at the shared wall. I plan to surface mount the subpanel in the garage on the wall that separates the garage from the house, approximately 5 feet up from the garage floor. Once I am on the garage side, I plan to run the wires in an appropriate sized conduit. My question is, do I need to put in an additional junction box on the last joist in the basement to simply change over from NM-B to THHN/THWN-2, so that I am running the THHN/THWN-2 in conduit instead of NM-B when I get into the garage, or can I run the NM-B all the way from the main panel to the garage subpanel (with or without conduit)? I am totally open to other solutions or ideas.

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  • Is the garage/house separation a frame wall, or...? Feb 28, 2022 at 2:14
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    You can also run 6-6-6-6 or 2-2-2-4 SER cable. Which is 50A and 90A wire respectively, and about 1/3 the price of #6NM copper. There is nothing wrong with aluminum wire at these large sizes. Since you're going to a garage, bring extra amps for EV charging; not for you, for home buyers. It really helps resale value. Feb 28, 2022 at 5:36
  • @ThreePhaseEel Frame wall
    – user149104
    Feb 28, 2022 at 15:08

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With an attached garage you can use NMB all the way to the panel. You need a 4 wire feed and the neutral must be isolated from ground. No additional ground rods are required since the garage is attached.

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  • The wire in the garage, if run on the outside of the wall (exposed to potential damage), must be run in conduit to protect it from damage. NM-B can be run through conduit, it's just more difficult to pull. Run oversized conduit to ease the pull - one 5' section of conduit, even if huge, won't be all that expensive.
    – FreeMan
    Feb 28, 2022 at 15:00
  • @FreeMan - Can I run NM-B in conduit from the subpanel to the heater? The heater will be hardwired. I thought NM-B shouldn't be run in conduit, or can it be in this situation? It will be 15-20 feet from subpanel to heater.
    – user149104
    Feb 28, 2022 at 15:14
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    @user149104 I'm sure Ed will confirm, but there's nothing at all wrong with, nor code that prevents, running NM-B in conduit so long as you don't exceed conduit fill limits. Cable requires larger conduit than the 3 individual wires inside it would, and pulling cable through conduit is more difficult, so it's usually recommended not to. However, for your short, 5', straight run, it shouldn't be too difficult. Whether you choose to pull it to the heater is entirely up to you - if you've already got suitably sized wire, that would make the pull easier and require smaller conduit.
    – FreeMan
    Feb 28, 2022 at 15:18
  • There is nothing wrong with nm in conduit the conduit has to be appropriately sized and you have to be careful pulling NMB in conduit because cutting / scraping the sheath can fail an inspection. The sheath must be intact, can not be removed to pull into the conduit as the individual wires are not labeled. But a straight run is not hard. Inside the building envelop can’t go outside with NMB even in conduit for most locations.
    – Ed Beal
    Feb 28, 2022 at 18:20
  • @ freeman, @EdBeal - I've been thinking about this, and kind of still have my original question. So I have conduit on the garage side of the shared wall to protect the cable. On the house side there isn't any conduit. So how do I go from my 2-2-2-4 (or NM-B) cable that isn't in conduit to a point where it IS in conduit? What goes on the end of the conduit at the start point? Is there a junction box on the end on the house side, or do I simply have an open end/stub of conduit that goes through the last floor joist and the cable is just fed into it?
    – user149104
    Mar 1, 2022 at 23:29

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