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Can I splice multiple wires in a larger box with smaller enclosed outlet boxes for each splice or do I need to keep each splice separate? I had a fire that burned most of the home runs to the attic and I have replaced all the full-length home runs that I can get to. There are 5 home runs that need to be spliced. Can I run a master larger splice box with 5 smaller boxes inside for each splice to keep all the runs in one place or do I need to keep the splice boxes separate i.e. 5 outlet boxes mounted to a truss? I'm trying to keep things tidy. (Edit) The splices will be 2 @ 12/2 romex and 3 @ 14/2 romex. The metal box pictured is for concept and is not UL listed.

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    I admire your commitment to neat workpersonship, but you'll struggle to achieve that while also installing cable clamps on each box and finagling the cables into them. I suggest cable retainers instead, if anything. Just bring the cables into the big box in a sensible arrangement and lay them in neatly and be happy.
    – isherwood
    Commented Jul 28, 2023 at 15:19

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You don't need "small boxes inside a big box" for splices (junctions.) One large junction box of adequate size (found via a box fill calculation, but pretty sure you have far more than is needed there) is all that's needed.

Remove the smaller boxes and use them elsewhere.

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  • Agreed. I've taken the same approach when extending home runs while replacing a fuse box with a service panel in a different room. The inspector was happy with me repurposing the fuse box as a junction box in a new location, in fact. Saved some cost.
    – isherwood
    Commented Jul 28, 2023 at 15:13
  • Technically this doesn't answer the question, though. If OP is hoping to do this to "keep things tidy", the matter of legality hasn't been addressed.
    – isherwood
    Commented Jul 28, 2023 at 15:17
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Is that large box UL listed as a junction box? With those mounting flanges inside it looks like the enclosure for a security alarm or something. I'm not certain that electrical code will accept that particular box as a junction box.

You haven't mentioned the gauge and quantity of conductors needing to be spliced. Another answer describes box fill calculations for 12/2/2 being joined with a couple of devices. Assuming that you're also working with NM-B ("Romex") cable, but in the 12/2 rather than 12/2/2, and with five cables being spliced, there are 10 conductors coming and 10 conductors going. That's 20 conductors, at a cost of 2.25 cubic inches each for 12 gauge, so 45 cu in just for those. We've got to consider the ground conductors too. There are 10 of those, which are counted 4-for-1 in fill units, so that's 2.5 units times 2.25 cu in, so 5.63 more cu in required. Total volume required is 50.63 cu in.

A single 4-11/16 square box at the common 2-1/8 depth accommodates 42 cu in. A deeper box could get you across the 50 cu in threshold, or you could use two boxes, or you could use a single box plus an extension ring. Or go for a giant box like you've pictured -- but one that's listed for the purpose.

The octagon boxes shown in your picture could likewise work. They're much smaller than a 4-11/16 square box, so you would need to use several.

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