I have two questions (for now):

First, does the NEC limit the number of cables that can be run through a hole bored through a ceiling joist; i.e. something similar to the conduit fill limits, but based on the diameter of the hole and wire size? I saw a short thread on this forum that mentioned a limit of two, but there was no indication as to whether it was an NEC requirement as opposed to general good practice. I am running 14/2 and 12/2 Romex from a subpanel in a finished area and want to limit the number of holes I have to drill.

Second, I am trying to understand how canopy space in a light fixture works to expand the capacity of a box. I need to place a light fixture directly on-center in a stairway, which is where a joist is located. I was planning on using a 4" pancake box, since this is the end of a 14/2 run, but the light fixture has a hickey, and that would seem to take me over the box fill limit. However, as I understand it, the canopy allows me to exclude up to 4 wires and one ground from the fill calculation. Does this exclusion cover only wires in the literal sense, or does it also apply to other items that contribute to the fill calculation? If the latter, then the exclusion would seem to permit the installation, since the hot, neutral, hickey and ground would be excluded.

Thanks.

 

No, the NEC does not limit the number of cables through a bored hole. However, there are other reasons (e.g., ampacity derating, and the structural strength of the joist) why you may wish to limit it. In general, several spaced smaller holes is usually better than one larger hole for both ampacity derating and structural strength.

 

Thanks, John. My house has 2x10 floor/ceiling joists (it was built in the 1920s), so I think I have a fair amount of room from a structural perspective. Also, the length of the run together is only about 6-7 feet, and I am contemplating a maximum of four cables through each hole, so I wouldn't expect ampacity derating to be a problem.

 

As long as there are only two current-carrying conductors in each of those four cables, then ampacity derating won't be an issue. But if two or more of those cables have three current-carrying conductors, then you'll want to separate the cables.

 

just to add a little to what John wrote, even if you have 2 current carrying conductors in each of the cables you still have to have them spaced from each other (as opposed to bundled). however you are allowed to bring them together (bundle) to get through your hole provided you dont bundle for more than 24 inches.

 

Just to clarify hex's clarification of my post, I see no problems with bundling them without spacing for as long as you want as long as you have nine or fewer current carrying conductors in the bundle.

 

In relation to ampacity and derating, the circuits I am working on run from the main floor to the attic in order to feed bedrooms, etc. I am not clear on how (or whether) to apply a combination of bundling and ambient temperature derating factors. If, for example, a two-wire cable is run with another two-wire cable for 24 inches or more, and that cable goes through a 110 degree the attic later in the run (separate from other cables), do you apply both derating factors, such that the aggregate percentage derating is .696? Or do you apply the derating factors separately, with the lowest adjustment controlling?

Also, do you have any thoughts in relation to my question about the pancake box?

Thanks again.

 

Apply both derating factors, one after the other. My earlier comments about not more than 9 current-carrying conductors only apply if no temperature derating is also necessary. If you need temperature derating, then you may need to further limit the number of bundled wires.

 

My earlier comments about not more than 9 current-carrying conductors only apply if no temperature derating is also necessary.

After making my last post, I did a further internet check to inquire more about the application of temperature derating, since I am not sure how Table 6 is applied. After all, in most states you can have summer temperatures reach 100+ degrees fairly easily, in which case the attic temperature can easily reach 140+ degrees. Even with 90 degree cable, a .58 derating factor would mean that attic runs would be very inadvisable, and probably impossible for 14 gauge cable, if you look to the highest reasonably expected temperature during the course of a year. My assumption is that most jurisdictions apply an average temperature or simply do not apply temperature deratings for attic runs absent unusual circumstances.

 

Yes, that's probably true. I suggest you call your building department and ask them how they handle this. Then post back and tell us what they say.

 

John, maybe i mis-interpreted what i read in 310.15(b)(2)(a)[adjustment factors] not the table. what i get from that is why i posted what i wrote. however that is talking about cables in general, is there a specific section that talks about allowing romex to be bundled together up to 9 current carrying conductors?

 

Although derating is specified for 9 or fewer current-carrying conductors in the table, if you do the math (applying the derating to the 90-degree ampacities), you'll see that the breaker size you will choose after derating is the same as the breaker size you would have chosen before derating. That is, nine bundled #14 wires can still be protected by a 15-amp breaker, nine bundled #12 wires can still be protected by a 20-amp breaker, and nine bundled #10 wires can still be protected by a 30-amp breaker.

 

John thanks for the info i think i finally got it...using the 90 degree column if you derate 0.70 x 25 it gives you 17.5 amps correct? so you can still protect with a 15 amp breaker...but if henry does bundle and his ambient temp gets to the temps he wrote of then i beleive that would take him below the 15 amps. am i understanding correctly? 17.5 x .58 (140+ degrees) = 10.15 amps?
and if thats the case it is simply just easier to keep them spaced and just bundle when is necessary (eg., running through a bored hole)
btw im interested in learning this because i hardly work with romex and was not sure if you derate the same way. i also wanted to show henry how we use the derating tables. John let me know if im wrong. have been none to be wrong many many times. thanks again John

 

Hex,

Thanks for the help, and I understand how the derating tables work on paper. The point I am now most interested in is how the derating tables are applied in practice. In other words, do you derate based on the highest temperature that an attic can reasonably be expected to reach, a year-round average, or something in between?

I mentioned 140 degrees only because where I am, it sometimes reaches 100+ degrees in the summer, in which case some attics can get up that high. Of course, this situation is even more common in the southern states. If you derate based on the highest reasonably expected temperature, then you could never use 14 gauge wire in an attic run (25*.58=14.5) unless you go with a less than 15 amp breaker (and I'm not sure they make those). You also would be severely limited in your use of 12 gauge wire in an attic run, since you would be limited to a 15 amp breaker and could not bundle for more than 2 feet, which can present practical issues, particularly in a remodel.

Now, I am not saying that these results are wrong. Instead, my impression is that the ambient temperature derating table is applied differently, since it seems that licensed electricians regularly use 14 gauge wire in attic runs with the blessings of inspectors. The only chatter U have found on the web suggests that this issue is overlooked, if indeed it is an issue.

I sent an e-mail to my local inspectors office to see their position and will let you know when I receive a response.

 

Henry,

personally when i install anything wether it is in an attic or top of a comercial building i always take ambient temp into consideration thats what the nec dictates. the way i see it you do base your derating on the highest temp and size wire accordingly just like sizing your wire for the highest possible load in a circuit. as far as ampacity is concerned the nec doesnt let you load a 15 amp circuit to 14.5 amps anyway it only lets you use 12 of the 15 amps (80%) and 16 amps for a 20 amp circuit so your limited right from the start. as far as spacing is concerned it doesnt mean you have to have cables a foot away from each other, an inch spacing will do just fine. derating is always a complicated issue and is debated from all angles. do i think that ambient temp is overlooked? yes but if it were my house i would not hesitate to be as safe as i can, even if that means exceeding the minimum standards that the nec provides. too often licensed electrician and diyers have the mentality what is the minimum i can get away with instead of what will be the safest. the best advise if your set on 14 awg is what you are already doing talk to the ahj and see what he/she has to say. and post back so we can all see their response. i will also bring this up to some of my electrical inspector friends and get their take on it. hope we were able to help some way and good luck with your project.