I’ve looked around for an answer to this but keep finding split branch circuit topics which isn’t what I’m looking to do.

I’m planning to run 4 circuits to one side of my house. These runs would go into the attic. I also see where a few other circuits could also have a run along the same rafters/path before going a different direction/room. I’m trying to figure out what the best way to have about a 20’ run where these 4-6 circuits are next to one another before going their separate ways. It just seems a bit ridiculous and maybe a structural issue to bore 6 holes laterally in a series of rafters to run the 6 different circuits.

Hopefully this makes sense the way I described it. I’m not entirely sure if conduit or two conduits on the rafters would be the way to go with this or some other solution that meets code.

  • Don't know if there is a code for open air circuits. There is for the number of wires in each size of conduit. As long as you give each cable a tiny bit of space, you should be good. Holes in wood structural members need to be less than 40% of the wood, but a few holes can be spaced out.
    – crip659
    Commented Sep 26, 2023 at 20:07
  • Depending on power needs of these circuits, access, and specific topology of the distribution at the far end, it might be more appropriate to run a single larger cable through your attic and put in a subpanel with individual breakers for each area at that subpanel location. Then you just need to run a 40A line (or whatever you actually know you need from a load calculation) from your main panel to your subpanel. You could even use aluminum for the subpanel feed, since aluminum is a lot cheaper than copper, and as long as it's installed and sized properly, is ideal for feeding subpanels.
    – Milwrdfan
    Commented Sep 26, 2023 at 21:09
  • 1
    @Milwrdfan I thought about a sub panel but really it’s a pretty small footprint, around 1000 sqft house. The problem I’m trying to solve is having two rooms and a bathroom + other misc outlets all being fed from one breaker and poorly wired circuit. Effectively half the house is ran off one breaker currently, it’s a mess. Hence one circuit for each room, one for the bathroom light/fan and one dedicated to a Gfci outlet to the bathroom. Plus a couple of circuits that are nearby that could share the same main run from the panel. Commented Sep 26, 2023 at 21:26

1 Answer 1


There is no limit on number of conductors run next to each other. However, there is a derating factor applied based on the number of current-carrying conductors bundled for more than 2' (assuming you're asking under NEC). Note that these have to actually be bundled (or in conduit)* for the derating factors to apply. Running circuits next to each other, passing through the same holes in the joists but otherwise in free air, has no limitation. Drill 2 1" holes or whatever is convenient (following hole location/size requirements for your joists, obviously) and run 3 wires per hole.

*there is some debate about what bundled actually means, and if devices like this stacking clamp will derate your wires or not. If you read the NEC really closely, you'll also note that it has the wording 'without maintaining spacing,' and then never defines what 'spacing' is for this scenario. this previous post addresses that. your AHJ will have final say on what does and doesn't apply. NEC 310.15(B) wording was changed in 2014, I believe, to become less specific about the scenarios where derating factors apply.

  • I would read "bundled" as being more rope like/on top of each other.
    – crip659
    Commented Sep 26, 2023 at 20:29
  • @crip659 'bundled' is just my explanation of what would require derating in this type of scenario, not an NEC term. they only term they use for when derating applies outside of raceway/conduit, multiconductor cables, or installed in earth is 'without maintaining spacing.'
    – ickybus
    Commented Sep 26, 2023 at 20:33

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