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I'm in the planning phase of upgrading a 30 breaker panel to a 42 breaker panel (and re-doing all the circuits), and I'm trying to decide if it would be a better move to run conduit from the garage where the panel is up through the attic (about a 30' run), terminate at a large junction box, and then continue the rest of the runs with NM. My reasoning is I don't want to go ripping through all the walls to pull existing wire, so I has planned on cutting the existing circuits up in the attic and either running new NM or re-using the existing NM (assuming it's long enough to reach the location of the junction box).

This isn't commonly done where I'm at, but I'm anticipating adding more circuits in the future, and I think it would be easier to deal with conduit than chase NM all the way through the panel. I'm aware of conduit fill limits and derating; I'd probably need a variety of 1/2", 3/4" and 1" conduit for my needs.

I'm looking for some input to see if others have done this, and whether it's worth it. I'm sure it's personal preference and how much work I'm willing to put into this, but maybe I'm overlooking something.

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  • Why not keep using the old NM? Unless you have a bad mice/rat problem, NM does not go bad. If using conduit, you don't use wire in cable anyway. Right now it does seem like more of a make work project that can probably be done much easier. Have you thought of just adding a good size sub panel, instead of replacing your panel for just 12 more breakers?
    – crip659
    May 6, 2023 at 0:58
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    @crip659 I do intend to re-use the old NM. The current circuits are all over the place, with one breaker controlling multiple outlets and lights. The NM for these circuits drops through the top plate and comes back up through the top, so I had intended to use the accessible NM to create new circuits, or drop new NM if the wire happens to continue horizontally through a stud. I also intend to add a subpanel for handling backup power to critical circuits during outages, so I figured I'd just upgrade the main panel while I'm at it. EDIT: I should clarify that I whould use THHN and terminate to NM
    – tripleblep
    May 6, 2023 at 1:03

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Sounds great until you meet the derating problem. But you say you are aware of that.

Since the conduit is longer than 24" you have to use vastly oversized wire once you have more than 3 circuits in it. Even 3 circuits are derated, but the derating can use the 90°C column so it ends up not being an actual derate for 15 & 20A circuits on 14 & 12 gauge THHN if you have 3 circuits or less.

The math works much better for multiple smaller conduits, each having 3 or fewer circuits.

Or a single conduit feeding one big circuit to a sub-panel that feeds your multiple circuits, rather than a junction box, assuming you have the required working space for a sub-panel there. That can be much more efficient and simplify the wiring a lot.

I run all my wiring in EMT or FMC or as AC/MC cable. I've seen too much rodent-modified NM/B in remodelling projects and I know how long it will take my rural fire department to arrive. So I'd consider this an upgrade for the portions that end up in conduit, and it does simplify any future additions (but the sub-panel option simplifies them more - or you'll need some "spare" conduits in your run to the "large junction box.")

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