Electrical Code Question

[Dave in NM]

I am tying to clean up the spaghetti electical wiring in my garage and need someone versant with home electrical requirements to calculate the size junction box I need to install. Into the box is the feed, off the load side of a 20 amp GFI receptacle, (3 No.12 THHN stranded .) Branching from this box, which also contains a 20 amp single pole switch feeding porch lite (No.12 NMB), are a circuit to one end of the garage (2 no.12 THHN stranded and a solid 14 THW ground), and another circuit to a receptacle on the other end of the garage which is two No. 10 THHN stranded and a No.12 standed THHN ground. The city wanted the bigger wire to that end of the garage because of the heat in the attic. All of the wiring is in EMT conduit save the short run up to the porch light. So let’s recap: I have a box with a 20 amp switch, three 12’s in, two 12’s out with a 14 out, a 12 NMB out, 2 10’s and a 12 out. And the switch will be fed with hot 12 pigtail. Whew. Question is, how much volume is required for this switch box. (Current box is 41.5 cu. in. ) Any help would be appreciated as the city makes the Dept of Motor Vehicles look like service at its best. ie, you can’t get there from here. Thanx, Dave

[LJ in TX]

But I would recommend a box larger than the one currently there. MY thought is at least one that is about twelve inches by four inches deep by six inches. that should give you plenty of room to safely route all of this wire coming in and out. I like to be conservative and overdo the amount of volume needed. hopefully others will offer a more educated opinion. good luck LJ

[Ron]

You would need a box big enough to handle 29.75 cu. in. capacity. A 4″ sq X 21/8″ deep box is rated for 30.3 cu. in’s. This is the minimum size box required and is based on there not being any clamps inside of the box. My recommendation would be to go to a 411/16 X 21/8″ deep box and raised cover to allow for the size of the GFI. You won’t have to cram. The fill is based on the following:

9 #12 wires x 2.25 cu. in. each

2 #10 wires x 2.50 cu. in. each

1 GFI Receptacle = 2 #12 or 2 x 2.25

1 20A switch = 2 #12 or 2 x 2.25

The ground wire is only counted once but you must keep in mind the room the others take up. (hence the 411/16″ box. This will give you a nice clean installation. And if this is all on a 20 Amp breaker you must replace the #14 ground with a number 12.

Ron

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