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Please could you explain the number of wires, connectors that go into a 4x4x1 1/2 square box that is only 14 gauge. I have only a switch (two wires) connected to that box as well. Do you count the connectors as one besides the three wires or so that might be connected to it? Also the switch itself? Or do you just count only the number of wires? Thank you

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I use a shorthand version of the full NEC treatment. Start with this table that says how much space for each wire size.

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  • Pick your largest ground wire and look it up and add it once. All other grounds are free. If you have four #14 grounds and one #8, that's 3.00 cubic inches.
  • Any cable clamps? Pick the largest wire clamped, ditto. Covers all clamps.
  • Any support brackets? Pick the largest wire associated with the supported item, look it up. it counts once.
  • Pigtails are free.
  • Other than that, every wire passing through entering and terminating in the box in a cap, splice or device, costs what the table says. e.g. if you have two 12/2 cables into the box, grounds are already accounted for leaving 4 wires, x 2.25 cubic inches = 9.00.
  • 2 wires nutted together don't count as one, sorry.
  • Any wires over 12" length, look it up, that per extra foot.
  • For each yoke, what's the largest wire going onto that yoke? Look it up and count it twice. So if you have a 10AWG wire in there for some reason, that yoke counts as 5.00 c.i.

There should be a stamping on the box saying how many cubic inches it is. Common box sizes have official sizes, and the sales literature for those boxes will also say.

What's a yoke? A switch, receptacle, GFCI etc. Yes I know they are dramatically different sizes. I don't write the rules.


So as an exercise. Box in EMT conduit. You have four #6 THHN wires (hot hot neutral ground) passing through the back of the box on their way to a subpanel. You also have #14 hot and neutral THHN coming in, going to a receptacle at this box. Then 14/2 Romex going out to some other location, with a clamp, and its ground goes to the steel box case. You have:

1 ground, largest is #6 so 5.00. 
3 #6 wires, so 15.00. Its ground was counted earlier. 
4 wires #14 (the Romex ground was counted earlier) so 8.00.
1 receptacle, largest wire involved is #14, x2 so 4.00. 
1 cable clamp, largest clamped is #14 so 2.00. 

You need 34 cubic inches total. It's a 120mm x 1.5" box, that's 29 cubic inches, so you get a raised receptacle cover that gives 5 more cubes and you're done.

  • So a raised receptacle cover would solve the problem. I didn't even think about that. I was on the verge of getting an extension ring (which would give me more unnecessary work). So for future reference Is it better to get something more than 5 cubic (in this case) to play it on the safe side or is it always best to be exact? Anyway Harper as usual thank you as usual. Will get that new cover. I have an open garage with no walls. i appreciate how you broke it down in your explanation. I was using the NEC fill capacity chart which confused me since i have ten 14 gage wires with the switch. – larry pinsky Jul 27 '17 at 23:57
  • Yes, definitely give yourself some margin if you can afford to. (sometimes you just can't.) A raised receptacle cover is more secure than a a box extension which can be knocked off. Never use the knockouts in a box extension, you'll regret it later. – Harper - Reinstate Monica Jul 28 '17 at 0:12
  • Didn't know that either about extensions. Appreciate your insight and tip. The other option was getting a 4x4x2 1/8. The knockouts are toward the back and i would have to either play with moving the box forward. Though it doesn't matter since I'm in the garage which doesn't have any walls. But again a simple solution that you provided me is perfect. There are four connections to the box. My home/garage is 40 yrs old. I was able to replace some of the older wires. There is no 12 gauge wire at all in the garage which is separate from the house. – larry pinsky Jul 28 '17 at 4:16
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As Tester101's answer that he referred you to states, go to Table 314.16(A) and you would see that a 4x4x1 ½ box can hold 10 #14 wires as counted according to the article. Read through Article 314 to see how to perform the count.

Do you count the connectors as one besides the three wires or so that might be connected to it?

Wire connectors don't count extra. Each wire that enters the box and makes a joint is counted as one wire. So, if you hae a joint with three wires that counts as 3. Each wire that enters the box and is terminated on a device counts as one. Each wire that passes through the box without making a joint counts as only one. All gound wires only count as one no matter how many ground wires you have. Wires that originate in the box and stay in the box, such as pigtails, do not count.

Also the switch itself?

Yoke mounted devices count as a 2 wire allowance for the largest conductor terminated to the device.

Or do you just count only the number of wires?

You then add up all the wires that are counted and all the items counted extra like yoke mounted devices and internal cable clamps. Multiply that by the cubic inch of the size of wire on Table 314.16(B). This is your total cubic inch fill required. You then select a box that will has that much volume.

A raised cover or mud ring (used to mount a device) also adds volume to the box. But it isn't much.

Most of the time electricians don't go through all this. We just use 4 square deep boxes and add an extension ring if we are crowded for space.

Good luck and stay safe!

  • Again thank you also for breaking it down and explaining it. Originally I was going to use an extension ring, but then thought of just getting a 4x4x2 1/8" box. Problem is that i would have to move the box backwards by a 1/4 or so. Im not sure that it wouldn't matter moving it since it is an open and separated from the house garage--no drywall. They used rigid conduit 40 years ago with 14 gauge wires that ran to it from the house. I however was able to replace many of the old wires if needed except i would rather had 12 gauge instead and for my age it isn't worth it to replace it now. – larry pinsky Jul 28 '17 at 4:31
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You need to go to NEC Table 314.16(B) which would make your answer 12 #14 in a 12x12x1 1/2 box with no devices. NEC 314.16(4) gives your device fill which by my understanding deduct two wires for the switch makes the conductor fill at 10. You do not have to include wire nuts or pigtails.

So there are all kinds of rules for box fill. which include the raise of a plaster ring and industrial cover. If you want to make it simple? calculate the total cu/in in the box and extenders then deduct the cu/in displacement of everything in the wire which can not exceed 40%.

  • The 40% fill is for conduit and does not directly apply to junction boxes. You can always edit your answer. – ArchonOSX Jul 27 '17 at 18:28

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