More than 3 NM cables through hole

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sw_ross

Senior Member
Location
NoDak
My current employer says he got dinged by the inspector for running more than 3 romex's through holes (regardless of the size of hole or situation).

Under my previous employer we used to drill 1 1/2" holes through the middle section of a laminated wood structural engineered I-beam floor joist's (the 1/2" thick OSB portion) for running romex's through, sometimes it was interconnects, sometimes it was home runs to the panel. We would try to limit the amount of cables through these holes to about 6-7, depending on the type and size of cable. That inspector never had issues.

The only thing I can find in the code book and in MH's books is 334.80, where if more than 3 cables are run through a hole that is to be sealed or caulked you have to derate the cable based on 310.15B3a.

Am I missing something?

Thanks,
Sky
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
You're not missing anything, this is a commonly misapplied section. If there is no fire caulk or draft stopping within the hole with the NM then there is no limit as to how many cables can go through the hole without derating. Other factors such as bundling for more than 24" may apply but since you specifically asking about the hole the answer is the number of cables is not limited.
 

sw_ross

Senior Member
Location
NoDak
Thanks, if you're running home runs to the panel (where gradually things start to get congested/concentrated) through these holes, would you consider it bundling for more than 24" even though the wires are going through a 1/2" of wood every 16"?
 

tom baker

First Chief Moderator
Staff member
No more than three cables thru a bored hole is an urban electrical myth. Maybe it started with NM cable that was bigger than NM-B, and three cables was all you could fit thru a hole in the middle of a 2x4
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Thanks, if you're running home runs to the panel (where gradually things start to get congested/concentrated) through these holes, would you consider it bundling for more than 24" even though the wires are going through a 1/2" of wood every 16"?


That is a million dollar question. Some inspectors see it as bundling while others do not. The code does not give us much help on this
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
334.80 states that where more than 2 nm cables are run thru a hole that is sealed with caulked then you must de-rate. NM cable is rated 90C so the ampacity is 30 amps. If you have 7-9 current carrying conductor's thru a hole ( 4- 2 wire nm cables) then you must de-rate 70%. 30 * .70 = 21 amps so you can still use 12/2 at 20 amps. Once your current carrying conductor's get above 9 then you are in trouble.
 

sw_ross

Senior Member
Location
NoDak
I definitely agree with the "no more than 3 romex's through a 7/8"-1" hole through a 2x4 partition wall" - due to potential damage done to the romex, as well as keeping the wire a safe distance from the nailing surface.
 

ceb58

Senior Member
Location
Raeford, NC
I definitely agree with the "no more than 3 romex's through a 7/8"-1" hole through a 2x4 partition wall" - due to potential damage done to the romex, as well as keeping the wire a safe distance from the nailing surface.

That's fine for a personal opinion and standard operating procedure but it has no validation for a code failure. I personally have no problem with installing 4- 14-2 or 12-2 through a 7/8 hole with a nail plate on the stud.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I definitely agree with the "no more than 3 romex's through a 7/8"-1" hole through a 2x4 partition wall" - due to potential damage done to the romex, as well as keeping the wire a safe distance from the nailing surface.

4 nm cablesncan be a little tight and you would not want to pull alot of cable thru the hole as it may burn the other cable it rubs against however your can install 4 cables as Ceb stated.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
If you're using Sim-pull you can really stuff the hole without damaging the cable because the jacket is pretty slippery.
 

kwired

Electron manager
Location
NE Nebraska
Were you guys using 30 year old cables? In recent years cable jackets have been getting smaller and if you limit yourself to only 14-2 you possibly get 6-8 cables in a 7/8 inch hole - physical size wise without much trouble - deration is a different issue though and has nothing to do with size of hole.

When it comes to ampacity reasons and use of say 6 AWG or less the general rule that works 99% of the time is no more then 4 cables in a bundle IMO, but you do need to pay attention that you have no more then 2 current carrying conductors in any cable - which doesn't really happen all that often unless you have some 14-2-2 or 12-2-2. Otherwise a three wire cable with MWBC is two CCC, a three wire cable used for wiring three way switches is only a two CCC - one "traveler" carries current at a time and never both. But even if you have one three CCC cable in the bundle of four - you are still ok because you can have 9 CCC before you get into the 50% deratation level. Oh, and if you do derate there is no limit on how many can be run, just the deration level and size of the conductors changes as you get more cables. So if you want to put the home runs for more then 20 amp circuits (giving you over 40 CCC's with a deration factor of 35%) all in one bundle - you can run all of them in 6-2 NM cable and put them all through the same set of holes - probably not quite going to fit through a 7/8 hole though. Or you could save a few pennies and drill 10 sets of holes and pull four 12-2 in each hole.

Of course there is the reality that the load diversity is low enough that there is little chance of there being much troubles here, but we are not permitted to assume that will always be the case. 25 or so years ago we used to drill a line of 2 inch hole and make most home runs to the panel through that set of holes - not one of those homes has burned down because of a fire starting in that run of cables yet that I am aware of. Outside of HVAC or a pool pump or other loads that may run continuously for long periods of time - the load diversity in a dwelling is very low on average.

Holes that are sealed is a little bit different ball game though - I've seen demonstrations - though they did push things hard, it was real world possibilities and when the hole is sealed it traps more heat in that space. Local community college made a demonstration wall passed same cable multiple times through sealed holes (only to get multiple conductors to contribute heat in the holes but only needed to supply one load for the demonstration) and supplied a electric heat load that ran continuously and placed some temperature probes in both the sealed and unsealed holes and did have a pretty noticeable difference in recorded temps. (can't recall off hand what they were it has been a while since I saw this demonstration)
 

electricalist

Senior Member
Location
dallas tx
We tried to take all the factors that could be an issue such as how many wire under 1 staple ,,not putting a 2 in ma in the panel for the hrs and just put 2 12/2 per mc connector 2 12/2 per hole drilled and 2 12/2 per staple. I dont know whats the actual code but thats how I did it.
 

Steviechia2

Senior Member
Location
Massachusetts
In residential wiring there is typically not much load. I don't usually put a lot of wires in one hole but would not worry to much if i did.
If you bundled all the home runs together, and on a hot summer day having a party with all lights on, a/c blasting, and oven running
you might heat up a bit but i doubt it would heat up that much to damage insulation.
 

mwm1752

Senior Member
Location
Aspen, Colo
Part of the equation includes thermal insulation -- does your NM run come in contact with thermal insulation anywhere? I agree with Kwire in that a 2 wire and 3 wire NM has 2 current carrying conductors -- general cu nm cales of 14#,12#, & 10# can be derated by 80% and still be breakered 15a, 20a, & 30a. That said 4 cables in a hole should be acceptable.
 

kwired

Electron manager
Location
NE Nebraska
In residential wiring there is typically not much load. I don't usually put a lot of wires in one hole but would not worry to much if i did.
If you bundled all the home runs together, and on a hot summer day having a party with all lights on, a/c blasting, and oven running
you might heat up a bit but i doubt it would heat up that much to damage insulation.

The only conductors/cables I ever encounter that have some elevated temperature in a dwelling (outside of something failing) is for heating or cooling or some other continuous/near continuous load that is at/near capacity of the conductor.

You even get more heat around the breaker for such loads but all other breakers are at room temp.

Go to a commercial/industrial site and you find more of that kind of heating then you usually do in a dwelling.
 

tom baker

First Chief Moderator
Staff member
I definitely agree with the "no more than 3 romex's through a 7/8"-1" hole through a 2x4 partition wall" - due to potential damage done to the romex, as well as keeping the wire a safe distance from the nailing surface.

Right. Does anyone recall the old style NM that was much bigger than NM-B? With that larger NM it may not of been possible to get more than 3.
The change was 'back in the day' or mid 1980s.
 
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