How to calculate safe # of wires for box
I just finished reading the article titled, “More Wires Need Bigger Boxes“. It discusses how large of a box you need for receptacles and switches.
My question is how do you figure this out for boxes for light fixtures. Do I need to?
I have a box that has thre 14/2 cables entering the box, 15 amp circuit.
Cable 1: “hot cable” Cable 2: takes power downstream to next room
Cable 3: is attached to a light switch to turn the light at the box on and off.
The box is an octoganal box about 5 inches wide and long, approximate 1.5 inches deep. The box is located in the ceiling and on the second floor of the house. Therefore, the box is accessible in the attic and has one sided paper-faced fiberglass insulation on top of it.
The light fixture at the box is a simple dome light that I have 2 of those spiral “13 watt” flourescent bulbs.
With 3 cables in the box (plus 2 pigtails), it’s awfully cramped in the box. I know heat can be a concern but is it a concern here? How do I figure out the min size for the box?
Replies
Thang,
If the box isn't stamped you use Table 314.16(A) of the NEC to determine the volume.A 4" round/octagon box 1 1/2" deep contains 15.5 cubic inches.Your 6 jointed #14's add up to 12 cu.in.Add 2 cu.in. for the jointed equipment grounding conductors and you're just under the limit.If the box has internal cable clamps you have to add 2 more cu.in. and another 2 cu.in.if the box has a fixture stud,and then you'd be over the limit.
The compact fluorescents you have in the fixture now would yield the least amount of heat to the wiring in the box,but you have to consider if someone else may have been burning 100 watt incandescents in the past.Is it new thermoplastic Romex or old cloth NM cable?
FWITW I bet box fill is the most common violation of the NEC in house wiring.
Barry
Edited 4/2/2004 5:32 pm ET by IBEW Barry
Barry,
It's a brand new fixture attached to an old metal octagonal box. The cabling is all relatively new within the last 10 years. The previous owners had the house rewired (went from aluminum to copper). It appears to be insulated Romex;just like the new 14/2 cable that I'm putting in.
The old light fixture was one of those simple screw in bulb type deals. The new one is decorative flush mount dome light with insulation on the ceiling side and what appears to be a foil heat shield on the bulb side.
I have to say that my house does appear to have relatively small boxes. All the boxes in rooms that I haven't gotten to fixing up are all these small metal boxes. As I fix up each room, I've been cutting the old boxes out and replacing them with the larger (and deeper) blue plastic boxes. Most of the receptacle and switch boxes don't have a ton of cables. It's the light fixture boxes that tend to have tons of wiring in them.
What's a fixture stud? Is that the mounting bracket that the fixture screws into?
So do you only count "objects", i.e. wires, brackets, etc. that are actually inside the box? It sounds like I wouldn't count the light fixture itself because it's not actually inside the box. Is that correct?
Thanks for the help,
Thang
Thang,
A fixture stud is usually the metal bar that attaches to the metal box with two 8/32 screws,it has a hole in the center for a 3/8" threaded nipple that the fixture mounts to.Because the rod or nipple partially extends into the box it somewhat reduces the ability of the box to shed heat.Yes,only devices in the box count.A regular Romex connector that clamps onto the cable outside the box doesn't count,but the clamps that come integral to the interior of the box do.No,the light fixture itself doesn't count.
If your wiring is that new don't sweat it if your box fills are toward the upper limit.I really dislike plastic electrical boxes though.An electrical fault in a PVC box can yield really toxic smoke.In my house it's metal all the way.
Barry
thang,
Good question! I've explained box fill to dozens of Habitat volunteers, and many owner-builders, and I've never had anyone ask about it with regard to light fixtures.
So here it is--you count the conductors of the building wiring and any internal cable clamps just like you do to calculate box fill for switches and receptacle outlets.
The fixture wires--the small gage wires going from the fixture to the building wires--don't count in the calculations. The fixture wires in a residential fixture are usually 18 gage and don't take up much space, so they aren't accounted for in the box fill calculation. It's like a pigtail wire (if a wire starts and ends in the box, it's not counted against the volume). So not every wire has to be included in the calculation.
Too many wires in a box is a real common code violation, like IBEWBarry said. End of the world? No. A potential problem? Yes. Worth ripping out a box to put in a bigger volume box? That's up to you, but if the wires are newer (with thinner and tougher insulation than used at the time the box fill calculations were developed), being one conducto equivalent over the allowed quota isn't a big deal as far as I'm concerned.
Yes, with overcrowded boxes, there's the issue of heat buildup and degredation of the insulation. But if the wire has 75C or 90C insulation (NM-B with 90C insulataion has been required since '84), that's not the issue it would be if the insulation is rubber or old TW plastic.
When the box starts to get real crowded, like a couple of conductor equivalents over the allowed, then you should really consider replacing the box with a bigger one.
Cliff
In my experience, having gone through the code sections on box fill and used those computations in some real-life examples, the rules in the code are fairly pragmatic ones: If you exceed the code box fill by very much you simply won't be able to cram it all into the box.
I prefer to be well under the upper limit if at all possible. For anything other than a simple one-romex spur, use the biggest box that will fit, and for really complex nests try to split the wire between several boxes somehow.
You can put a lot of wires in a deep, 4-in. square box with a plaster ring.Arguing with a Breaktimer is like mud-wrestling a pig -- Sooner or later you find out the pig loves it.
Take the price of a 4 11/16 square box and plaster ring, and subtract the price of the cheapest little octagon box. Divide that number by the hourly rate your electrlcian charges. Multiply by 60 to get minutes. If it takes longer than that to cram the wires into the little box, then it's more cost efficient as well as safer to use the big box.
Take that first number again and multiply by all the boxes in the house. The number is at most somewhere in the low hundreds. Unless you're doing a bazillion tract houses, it doesn't amount to much.
-- J.S.