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· Registered
48 Posts
Discussion Starter · #1 ·
Because of wall depth issues, I need to use gangable metal box to create a "shallow" 3-gang old work box. I am hoping someone can verify a couple things related to box fill for the gangable boxes...

1. Each gang has 2 internal NM cable clamps on them. My interpretation is that when they are ganged together as 1 3-gang box, all 6 internal clamps count as a single conductor. Correct?
2. Total volume is simply 3 times the volume of a single box.

Therefore, the below should be correct assuming the above statements are accurate:

3x2x2.75" gangable box has a 14 cu in capacity based on 314.16a, so total capacity is 42 cu in of 3 ganged together.

I have 3 14/2 NM wires and 3 12/2 NM wires entering the box. Each gang has 2 internal NM cable clamps on them, and I will have 1 switch connecting with 14/2 and 2 switches connecting with 12/2.

For box fill I am looking at:

6 14 conductors
6 12 conductors
1 12 conductor for all cable clamps
1 12 conductors for all grounds
2 14 conductors for 1 switch
4 12 conductors for 2 switches

Therefore in total, I have the equivalent of:

12 12 conductors or 27 cu in
8 14 conductors or 16 cu in

and I come up 1 cu in short of being able to use this box within code.

However, if I remove all of the internal cable clamps and use external NM connectors in the knockouts, I need 40.75 cu in and I am good to go.

Is this accurate?

I was kind of hoping that somewhere in the code it allowed some extra allowance for fill when the boxes were ganged together to pick up that 1 lousy cu in I needed.

any feedback would be appreciated.

· Registered
48 Posts
Discussion Starter · #2 ·
I should have mentioned that I ask this question because when I fairly accurately measure the inside of the box fully assembled, I get dimensions of 5.4375 (5 7/16) x 2.8785 (2 7/8) x 2.75 which calculates out to 43 cu in.

· A "Handy Husband"
15,055 Posts
The conductor factor is based on the largest wire size. Use 2.25 times 20 = 45 cu in.

· Super Moderator
18,311 Posts
The ground is based on the largest in the box as well as the clamp allowance. The device count is based on the conductor size attached to the device.
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