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I am combining a bunch of underutilized circuits, pulling them out of the breaker panel into an 8x6x4 junction box above it where I will combine about 10 cables into 3 circuits fed to the breaker panel through one conduit and stranded wire.

Questions:

  • Any issue with using 1/2 inch smurf tube between the new box and the panel? Why? Because I have some from a previous project, I have connectors, it's easy.
  • I believe I can run one ground wire through the tube for everything, is that right? Same size as the largest wire in use? (I think one size lower is allowed but I have no reason to do that, it's only four feet of wire).
  • I believe I should run one neutral per breaker through the tube, right? I think I can combine two neutrals into one as an MWBC, but there's no good reason to do that and it creates assorted problems.
  • Box fill: I'll have about 10 cables in, another few going through the box that I haven't talked about, and the conduit to the panel... total about 30 conducting wires, biggest is #10, 12 grounds, no devices ... I need about 90 cu in and I'll have almost 200. So that's good right?
  • Conduit fill: Maybe 8 #12 stranded wires and a ground in a 1/2 inch NM tube .. somewhere around 35% fill I think so also good?

Is there anything else I should be aware of?

Why am I doing this (not part of the question but in case it matters): It's a 40-space panel and it's completely full. All the cables are coming in the top side, it's a terrible rat's nest above the panel that I can't tidy up because the cables are intertwined with little slack. And a solid wall of copper inside the panel above the main breaker with cables doubled and tripled up through every single knockout. This little project will significantly improve the mess both above and just inside the breaker panel.

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    Sounds like fun. I suggest that you put a number on each wire, and a list on the inside of the cover plate of where each number goes. You will thank yourself later. I would also write a J{x) or some such on the inside of the box of every receptacle so that you later know that this is that cable in your junction box. Nov 22, 2023 at 4:32
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    Just for clarity: reduction of ground size is allowed for greater than 30A and 8 conductors is the max #12 THWN allowed in 1/2" ENT per table C2, Nov 22, 2023 at 17:28

1 Answer 1

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In general this sounds good. A few things to watch for:

Required Circuits

Code now requires dedicated for circuits for a bunch of things:

  • Bathroom receptacles - generally one per bathroom, though multiple bathrooms can be combined but not other rooms
  • Kitchen - two "countertop receptacles"
  • Laundry Room - one receptacle circuit

and there may be some others. So if any of the affected circuits include any receptacles in those locations then it is a bit of a problem to combine them. If code did not require the dedicated circuits at the time they were installed then you don't have to install dedicated circuits. But you can't make things worse. For example, if a bathroom receptacle circuit is shared with receptacles in another room then you can't combine that circuit with any others, because that would make the bathroom part even less dedicated.

Multiple Circuit Derate

If you have a short (I believe the magic number is 2 feet) connection between two panels, and I think that applies to "panel" + "junction box", but I am not 100% sure, then you can put as many wires through conduit between them as you can fit (i.e., just subject to conduit fill rules). However, for any longer distances there is a derate based on the number of current carrying wires. But it gets more complicated than that because the derate is from the 90 C ampacity column instead of the usual 75 C, so 12 AWG gets 30A as the starting point. If I understand the rules correctly, 3 circuits should be 6 wires and a 20% derate, but even with 8 wires and a 30% derate (not sure why you have 8 wires but 3 circuits if these are all 120V circuits) but worst case that is 30% and 30A x 0.7 = 21A, which is still higher than the 20A that you can use for the breaker so you're OK for this one. But something to keep in mind if you add another circuit or two.

Conduit Fill

I had trouble finding a conduit fill calculator for ENT/"smurf tube", but it looks like you are right at the edge between 1/2" and 3/4" depending on the number of wires (7 vs. 8 vs. 9). If it is really 3 circuits (2 wires each) + 1 ground then you're probably OK. If it is more (or some are 240V/120V two hots + neutral) then I don't think so.

I would recommend running two conduits, with one or two circuits each.

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  • Great answer that addresses the general cases for the things I need to worry about. Some vague aspects of my question don't matter, I can address them as I go, depending on exactly what I end up doing, with this information.
    – jay613
    Nov 24, 2023 at 12:17
  • Consolidating useful info from comments here in accepted answer: 1) Label cables before removing from panel and keep them labeled in JB, 2) Can't downsize grounds for #10,12,14 wire
    – jay613
    Nov 24, 2023 at 12:17
  • 3) An implied next step is rearranging breakers in the panel. There are MWBCs that are not handle tied or labeled. Since rearranging breakers, now is the time to identify all of them, move pairs together if any have been separated and add handle ties.
    – jay613
    Nov 24, 2023 at 12:20

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