electrical Box fill

Discussion in 'Electrical Forum discussion & Blog' started by brosnt1, Jul 23, 2007.

1. brosnt1New Member

Joined:
Feb 10, 2006
How many number 12 ga NM romex cables can enter one of the blue plastic single gang outlet/switch boxes? also is pig tailing outlets code?

2. jwelectricElectrical Contractor/Instructor

Joined:
Jun 14, 2007
Occupation:
Instructor
Location:
North Carolina
the number of conductors in a nonmetallic box is dependant on the cubic inch of the box.

Look at the inside back of the box and see how many cu. in. the box is.

Joined:
Feb 9, 2006
Location:
Springfield, IL
Single gang outlet, you can go with three, four is just pushing it. Personally, I hate working on a job where it was not done right, and the wires are cut way too short (key point to remember).

5. jwelectricElectrical Contractor/Instructor

Joined:
Jun 14, 2007
Occupation:
Instructor
Location:
North Carolina
Most single gain non-metallic boxes come in 18, 20 and 22 cubit inches sizes.

314.16 is the section in the code that is used for box fill calculations. Here we are told that every conductor that enters the box counts as one conductor and all the grounds count as one conductor and the device counts as two conductors.

Using 12/2 with ground NM cable and a receptacle we would total all the conductors and multiply this by 2.25. Table 314.16(B) is where we get the multiplier from.

If there were two cables in one box then we would have a total of five conductors, two whites, two blacks and both grounds count as one. The device would be two more bringing our total to seven. 7 x 2.25 = 15.75

If we had three 12/2 with ground we would have a total of nine or 9 x 2.25 = 20.25.

This tell us that a 18 cu. In. box could be used if only two cables were installed but if three cables are installed then the larger 22 cu. In. box would be required.

6. brosnt1New Member

Joined:
Feb 10, 2006
Thanks for the information, what about the pig tailing of recepticles and switches?

7. jwelectricElectrical Contractor/Instructor

Joined:
Jun 14, 2007
Occupation:
Instructor
Location:
North Carolina
They don't count for anything