Electrical Box Fill - Calculations


Old 09-19-05, 10:46 AM
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Join Date: Mar 2001
Location: Kansas City Area
Posts: 215
Electrical Box Fill - Calculations

Need to figure out if I am within the fill limits for some of my boxes. I cannot seem to locate what the ##'s are in order to add it up myself. Here are my most packed two boxes. All wire is #14 of the romex type. Let me know what to count & what not to count. I think I read that only 1 ground wire per circuit is counted vs potentially 3. What about wire nuts & pigtails?

Box #1. Box size 22.5 cu in. Items in box - 4 #14 romex & 1 GFCI outlet. 3 of the 4 lines are wirenutted together, plus a pigtail to the GFCI, the remaining line is on the GFCI load side.

Box #2 - 4 gang switch box, have not bought yet but will get the biggest / deepest I can. 4 incoming #14/2 romex for power, 4 outgoing 14/3 to other switch, 4 outgoing 14/2 to lights, 4 3-way switches. Black, Red, Whites, & ground connected accordingly (correctly).

Other questions.
When connecting the ground wire to a light fixture, does my wirenut have to be Green colored? It is also a small wire size from the fixture, thus I have a #14 bare ground to either a #16 or #18 green fixture wire. Do I use a smaller wire nut, (smaller than yellow). If that box by the fixture is metal, does it (the box) need to be grounded with a Green ground screw?

Thanks Mike
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Old 09-19-05, 01:20 PM
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Join Date: Feb 2005
Location: Near Lansing, Michigan
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Okay, for #14 you use the figure of 2 cu. in. per conductor. You only count conductors that enter or leave the box; so, pigtails do not count. All of the ground wires in the box count as one conductor. The cable clamps (if internal to the box) also count as one conductor; single gang plastic boxes are exempt from counting the clamps as a conductor. Finally, count each switch and receptacle in the box as 2 conductors. Add these up and you have the total conductors for the box.

Box #1: (8) white/black conductors + (1) grounds + (2) GFCI recept. = (11) conductors * 2 cu. in. = 22 cubic inches of fill.

Special note here: if this is a metal box with internal clamps, your box is overfull. My calculations assumed a plastic single-gang box which is exempt from counting the clamps.

Box #2: (28) black/red/white conductors + (1) grounds + (1) clamps + (8) switches = 38 conductors * 2 cu. in. = 76 cu. in. box

I would consider re-designing the second box with the power feeds and runs to the light fixtures out of some of the other boxes. That box will be jammed full and very hard to follow or understand any of the wires in there.

No, the grounds wire nut does not need to be green, although sometimes the "Greenie" brand nuts are helpful because of the hole in the top. The metal boxes need to be grounded with a green screw and a pigtail. I know that Ideal sells premade ground screw/pigtail/wirenut assemblies in packages of 10 for about $5.

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