I have a 3.5 inch metal octagon junction box with 5 12/2 wires coming in. This is how they are wired. 1 is spliced and goes out but the other has 2 splices and both go elsewhere. Both are on their own breaker)

Is this pass through allowed with two separate circuits? Am I overfilled?

enter image description here

All grounds are connected too iirc.

  • How deep is your octagon? Commented Feb 11 at 6:15
  • 1.5 inches maybe? Havent measured. @ThreePhaseEel
    – cde
    Commented Feb 12 at 0:01
  • 1
    Yes 1.5 inches.
    – cde
    Commented Feb 13 at 17:21

1 Answer 1


Let's clarify the "5 12/2 wires" as 5 cables with 15 wires total.

Box fill is defined in NEC 314.16. The number of circuits is not relevant. Likewise, where they go to or come from is not relevant, because all of the wires in this scenario "terminates or is spliced within the box".

You have 10 "conductors" at 12 AWG = 10 x 2.25 = 22.5 cu in.

You have 5 "equipment grounding conductors" = 1.25 x 2.25 = 2.8 cu in.

Assuming you have internal cable clamps = 2.3 cu in.

Assuming no other device or equipment in the box, the total is 27.6 cu in.

Your octagon box needs to be at least 27.6 cu in.

  • Thanks on the fill part. What about the two independent circuit part?
    – cde
    Commented Feb 12 at 0:02
  • 1
    @cde -- as long as you make sure that the neutrals from the two circuits don't get joined together inadvertently, you should be good Commented Feb 12 at 3:15

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.