My situation is essentially the same as this question, for reference. I am trying to accomplish the same goal: feeding a sub-panel in an outbuilding from my home's main service panel.

I have a 6ga NM wire (3 conductors #8 bare ground) and a set of 4 THHN 6ga (3 conductors w/insulated #8 ground) coming into a space where I need a junction box. What size box would that require, given that this is only for the purpose of splicing the two sets together, there are no devices present, and no other wires in or passing through?

To extend the question a bit, this is happening in the interior space, so a surface mounted box would be ideal. Is there such a thing as a box that fits the bill and doesn't look like it belongs in a factory?

  • Can you mount this where it wouldn't be as visible? Say, inside the attic?
    – Machavity
    Commented Jun 22, 2021 at 12:02
  • @Machavity At this point, no. The NM wire was run to an accessible point during a basement remodel. The THHN will run through a brick wall to this box. Commented Jun 22, 2021 at 12:06
  • @Machavity In fact, going up was never possible. It was always going to have to punch out the basement wall somewhere. Commented Jun 22, 2021 at 12:08
  • 1
    You can install the box in an accessible attic. In basements many times I will make a picture frame and use Velcro to mount it to the wall. I recently had a new inspector require the UFER ground to have a sign on the frame so I used a ground symbol on the wood she was fine with that, I have had to put signage on service panels that I had cabinets so there are code compliant ways to hide the boxes. As long as the finish is not damaged and the panel is designed to be removable the box is accessible. See the answers for the size.
    – Ed Beal
    Commented Jun 22, 2021 at 13:15
  • Psst... to splice #6 or smaller, "MAC Block Connector". Mini Polaris connectors at a fraction of the price. Commented Jun 22, 2021 at 16:56

2 Answers 2


5 cubic inches per 6 Ga conductor terminating in the box (30)


3 cubic inches for up to 4 8 ga grounds. (3)

33 cubic inches.

If the NM clamp is external, no additional allowance. If the NM clamp is interior, add another 5 cubic inches (38).

A 4-11/16 x 2-1/8 steel box will have room to spare. You can dress it up however you like, paint it, etc. Or you might find a 6x6x4 PVC box more to your taste? That part really becomes a decorating question, and a cabinet with a door that opens without tools is an option to hide any box you don't like the look of.

  • Is there a particular NEC section (or other source) that spells out how to derive these numbers? Perhaps a chart for quick reference? Commented Jun 22, 2021 at 13:13
  • 1
    NEC 314.16 14Ga 2, 12Ga 2.25, 10Ga 2.5, 8Ga 3, 6Ga 5 - look up box fill calculator for an easy way on a computer. Grounding conductor rules changed with 2020 code (after 4, must add 1/4 per )
    – Ecnerwal
    Commented Jun 22, 2021 at 13:57

There's a couple of ways to play this. Neither is terribly aesthetic.

  1. Deep metal junction box. You'll need clamps, but these are cheap and easy to find. Add a lid
  2. A large PVC box. You can buy these in varying sizes (4x4, 6x6) and they come with no holes (you'll need to drill some). Make a hole and add a clamp to fit it

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