I am installing an upgraded breaker panel and I need to extend the existing branch circuit wires from the existing panel to the new one. I would like to know if it is permissible to run 2 romex 12/2 w/g through a single box connector. I will be using handy boxes as splice boxes and my space is somewhat limited so I would like to make 2 splices in each box and have pairs of romex cables enter the new breaker panel through the same opening.
Welcome to the marvelous world of "box fill calculations". Like hotrodding, it's all about the cubic inches.
You are extending 2 circuits, so you have a total of four 12/2 Romex entering the box. That is 8 wires being joined on 4 wire nuts, not including grounds. Here's how the math works on that:
- Each wire end (which is <12 inches) counts as one wire, so 8 total #12.
- All grounds together count as one wire (of the largest size if mixed) so 1 #12.
- All cable clamps together count as one wire (of the largest size if mixed) so 1 #12.
Total: 10 #12 "wires for box fill calc purposes".
A #12 wire is allocated 2.25 cubic inches. 10 of them need 22.5 cubic inches.
Oddball sized boxes must be stamped by the manufacturer with their volume in cubic inches. Standard sized boxes can use the standard sizes assigned by Code in 314.16(a).
My googling of "Handy box" turned up a 2-1/8" x 4" box which has 10.3 cubic inches. That would be shy by half.
A 4x4x1-1/2 square box just almost makes it at 21 cu.in. Check with your AHJ and see if he'll let it slide. That box won't be terribly overcrowded.
Otherwise, you can go several ways.
- Use a 4x4x1.5 box (21) then add a domed cover good for 6.5 cu.in. This will give you a 5 c.i. surplus. Some of those covers have cutouts to mount a GFCI, deadface or AFCI. You could mount one there, and use it to protect one of the circuits. A receptacle costs 2 wire counts (4.5 cu.in.) so you just make it.
- Use a taller (>10.5 cu.in.) domed cover that gives you the cubic inches for two GFCIs or AFCIs, and protect both circuits.
- Use a 4x4x2-1/8 box (30.3 cu.in.)
- Use a 4-11/16 square box (42 c.i.) and splice four circuits in that (16 wires + 2 = 18 x 2.25 = 40.5 c.i).
For larger boxes and odd covers, buy those at the real electrical supply or trusted lumberyard. Big-box stores notoriously overcharge for them.