Have you considered moving one of the 14-2 cables to the new box and installing a 14-2 between the boxes, thus not affecting the box fill of the original box.

You know, that is a good idea. I should be able to do that with no problems, given that one of the cables is "passing through" the circuit to the next box. I think if I can find which of them has the most slack, I can just relocate that one. The good news is that I am not concerned about cutting out drywall if I have to.

Thanks for the suggestion!

If you don't care about the drywall. Cut out a hole, mount a 4&11/16" by 4&11/16" square box with a 1/2" deep single gang mud ring. Then repair the sheet rock cutting out a hole only around the mud ring. That will give you a massive box with only a single gang opening.

Spencer, thank you for the suggestion. I will be able to use that approach in another room where I also have a box-fill issue.

Spencer is giving away a lot of secret tricks.

This is how you get minimum depth when needed.

A 1.25 inch deep square box with a plaster ring.

Meets fill, a little bit of trouble to work in through the plaster ring.

I appreciate having folks like both of you who take the time to help others out. I try to "pay it forward" by posting helpful things on other sites and I enjoy doing it, but when it comes to electrical I'm more of a "trainee". (Though I bought one of those NEC summary books a while back which has been really useful.)

I used the square box-plastic ring combo some time ago when I was doing some different work (on an exposed joist): I wanted to mount a receptacle directly on the square box and that seemed the best way to do it. It never occurred to me to use that arrangement behind drywall, but I can see that it will work well.

Thanks again.

When you have a tight space metal boxes shine.

The plastic boxes are enough larger at the same volume (thicker walls) that they sometimes just will not fit.

I have gone as far as drilling a hole for the box bonding screw into the material behind the box.

Old work requires a lot of tricks.

... receptacle, two 14-2 cables, and a 14-3 cable... I believe that already puts you over the max fill limit.

Normel: I thought the math was: 2" per conductor except all grounds count as one, plus 2" for the device, so:

4 x 2 and 3 x 2 for the current carrying conductors = 14
All grounds = 2
Device = 2
Total = 18, which fills the 18" box.

Is the correct math different?

Better late han never... I wanted to get back to you guys and thank you for the great suggestions. I used the "square box with plaster ring" method and it worked nicely. Not only is it necessary for code compliance not to exceed box fill, but it sure is easier to work when you have all that volume to fit the wires!

Anyway, thanks again.