wiring question

I really thought one of the "code guys" would respond to you on this. Yes, there is a limit. It has to do with heat. Some inspectors are more concerned with this than others, but it is in the book. AFAIR (and I'm not a code man) the 3/4 would be OK for one wire, you'd need at least 7/8 for two. That's off the top of my head. Ask the local inspector - you don't want to have to redo it.

If you're using modern 90C wire (e.g. NM-B not plain NM), then you can usually run 9 current carrying wires in a raceway or bundled together before you have to derate the ampacity. You won't get more than 3 cables (6 current carrying wires) through a 3/4" hole. If you use 1" or larger holes, don't run more 4 cables through holes if the run is longer than 24 inches.

If you can, I'd avoid holes larger than 3/4 in 2x4 studs and rafters so you have over 1.25" between the hole edge and a face of the board. If its structural (like a rafter), that is another reason to keep the hole diameter down. If its an engineered truss member, don't drill it at all. Drill an additional hole at least 5 inches away from other holes if you need more in the same board.

-- Mark Kent, WA

According to Mark or Sue :

Actually "cable in joist hole" limits are more a matter of the damage you might do to the cable running multiple cables through a whole series of holes.

All that rubbing...

Heat dissipation is seldom an issue if you're only talking 1-3" worth of constriction.

Indeed, some electrical books _encourage_ single-cable through hole practises.

I don't think the code books specifically talk about this per-se (other than derating limits), but you could be subject to invocations of the "workmanship" rules...

Precise details of where the holes can be is usually building code, and I don't have a precise spelling of that out handy.

Perhaps more important, when you're doing long routings, is to do the best you can to avoid kinks - keep the holes _straight_ as much as possible.

Angling in spade bits results in frustrating experiences and cable damage.

Properly perpendicular holes drilled with auger bits are a joy to work with comparatively.

I tend to prefer a 7/8" auger. Clean, two cables still fit easily.