I am in the process of installing outlets in my new woodworking shop. I want to know if code allows me to run multiple cables through a single hole in each stud or if I have to keep all the cables separate with separate holes? I've looked in a couple of books and couldn't find an answer. If I can run multiple cables through one hole, are there any restrictions on number of cables, size of hole, etc...?

Thanks,
Lowell
Kennesaw, GA

 

You should not run more than two cables through a single hole.

 

Code does not restrict the number of cables through a hole. As far as the electrical code is concerned, you can put a hundred through a hole if it does not cause unacceptable structural weakness.

However, there are considerations that would lead you to do what Bob suggests. One is ampacity derating, which doesn't usually come into play until the fifth cable through the same hole. Another is distance of the hole from the face of the stud. If you have a 2x4 stud, you can usually use at most a 7/8" hole before you have to put a metal plate on the face of the stud. A third consideration is potential damage to the cable sheathing, which suggest you should not cram wires into the hole. And a fourth consideration is the affect on the strength of the stud of a large hole.

So all-in-all, I'd shoot for two cables through a hole as Bob suggests. I certainly would not hesitate to put three in occasionally. For structural reasons, holes should be dead center and no larger than necessary to comfortably fit the cable or cables. If you drill mutiple holes, then the farther apart the better for structural reasons. Consider three inches to be a minimum hole separation for any load-bearing framing member.

 

Thanks Bob and John. John thanks for the detailed explanation, that really helps. One question about your statement of ampacity derating. Why does this happen? I understand that a magnetic field is generated by the flow of current. Is this somehow responsible for lowering the ampacity? Can you explain this in more detail? Not that I'm planning to run more than two or three through a hole but I'm curious to understand what is going on.

 

The number also depends on the cable size. I would say feeding 3 14/2 though the same 7/8" hole is fine. However, with 14/3 or 12/2, I would reduce the count to 2. Runs of larger gauge, things like 10/3, should have there own hole. I don't know if this is a code requirement, or only just highly recommended.

 

Lowell, ampacity derating is because of heat. As more an more cables are bundled together, they become hotter and hotter. So the code restricts the amount of current that each wire can carry as the number running together increases. The code addresses cables running together for more than 24 inches. So running a bunch of cables through one hole is no big deal, but when you run them through several studs in a row, it becomes a factor.

 

That makes sense. One clarification please, I was planning on running 2 to 3 cables, mostly 14/3, 12/3 and 10/3 down the length of one wall, around the corner and down most of the length of a second wall. I'll be doing this in both directions from the subpanel although the one direction will be up to the ceiling and across to the other side of the shop to avoid the double doors. As long as I stay with only 2-3 cables, can I go more than 24 inches and do as I plan or after 24 inches must I separate them? Your first response had me thinking I could and now your followup has me thinking I can't. With a Woodshop, one tends to have many outlets and I don't want to perforate my studs any more than necessary. Not to mention the effort of drilling all those holes. However, I did pull a permit and I'm going to have it inspected and approved so I want to do it right the first time.

 

I didn't use any numbers in my second post, since I was just addressing the theory and not the application.

Until you get over nine current-carrying conductors bundled together, and assuming the ambient temperature is not excessive, and assuming we're talking about #14, #12 or #10 wire, then there is no problem bundling.