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| Subject - Bundled Romex | |
| Electricman |
When I wire a single family dwelling to run my home runs to the panel I often drill 1 or 2 2" holes though the 1st floor floor joists the length of the home. Most of the feeds will go through these holes so my question is ,if I am reading article 310 correctly, do these home runs have to be derated, or is this just concerning raceways and cable assemblies? If this is a dumb question please feel free to tell me its just something thats always in the back of my mind when I am pulling these feeds.
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| iwire | NM is a cable assembly, so yes strictly speaking you need to derate these NMs where they are bundled. If you do the math you will find that 14 AWG is still good for 15 amps and 12 AWG is still good for 20 amps until you bundle more than 9 current carrying conductors together. So in a practical sense you are limited to 4 two wire NMs in one set of holes for more than 24". Get a smaller bit and make some more holes  |
| JimmyDee | I have the same interpretation as you do on this derating. I have always limited my runs to 2 per hole. I've had several electricians ask me why I do it that way and my response is "just to be on the safe side. I don't know of any inspector that is red tagging for the bundling of romex in this area. Jim |
| Ryan_J | quote: Come to my area...I have red tagged for it. Like Bob said though, it really is a moot point until there are more than 9 currnet carrying conductors. |
| David Hyatt | Electricman, this is almost the same principal as my topic on panel homeruns. And from the section you had me read (312.5(c)), I think this type derateing is not required. Through the pipe going to the panel you have many more than 9 conductors. With the holes 2" in joist they are bundled but not for more than 24", because they will have 16" space between each bundle. This is one of the hardest topics to interpret the intent of the code so far. Would like to here more about it from anyone. Especially someone who does residential everyday like myself. |
| iwire | David I agree this can be a little tough and some areas do not enforce it the same as others, here in my area a group of cables running through studs or joists is bundling. Take a look at an exception to the derating rules that I get to use on my MC jobs. quote: Notice part "(c)" are bundled, stacked, or supported on “bridle rings. Now the MC support requirements are every 6' so if I put a bridle ring (just a "J" hook) every 6' and the cables are left slack between them the NEC still considers this bundling. Now in real life you will almost never see the NMs in a dwelling unit get warm as it would be very unusual for all the cables in the bundle to be fully loaded which is what the NEC derarting is based on. As Ryan and I pointed out keep the current carrying conductor count to under 9 and it is not a code violation. |
| David Hyatt | Well how would you feed into the panel with more than 9. Outside panel? |
| iwire | 1)You can bundle as much as you want for 23.999" 2)We often use cable stacker's which space the cables apart. If we are not running MC right to a panel that means we are running pipe and wire to the panel. 99% of the time all the 20 amp circuits have to leave the panel as 10 AWG due to derating issues. And as Ryan knows  I live in a state with a much less restrictive derating table than the NECs. |
| David Hyatt | Yea, but what about a panel on a house , outside you have to bring romex wires through conduit. If you have 30 circuits, with nine in a conduit, it would take 4 conduits 2". That will not fit in any panel I've seen. So whats the option? This is the only way I have seen it done. |
| David Hyatt | OK, I called Tom Henry in Florida, spoke with his assistant. Turns out this is a very controversial subject. By code as stated previously, over 24", romex in a conduit sleeve, or bundled, more than 3 current carring conductors, derates do apply. In his opinion as well as mine this is something that should be changed. Nevertheless, it is code. I knew thats the way it read but couldn't comprehend it. Still can't. Don't see a better way of doing it. Never seen it enforced. Probably hard to enforce because in a residence the exact load can't be determined on general purpose anyway. Also conduit fill doesn't apply because it is not a complete raceway. My way of doing this is common practice around here so I think unless someone has a better idea I will continue doing it without concerning derates.
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| Ryan_J | If I'm off the record here, I only red-tag it if it is in an attic, floor joist or exterior wall...somewhere where insulation will add to the effect of the heating. |
| Electricman | I always seem to pick the controversal ones dont I? Well then how should I derate then?can anyone give me an example? Lets say I have 15 feeds in 1 hole, I always calculate my circuits around 80 % I know thats not necessary in residential but it helps me sleep at night plus I never have any complaints about dimming lights even when heat pumps and Ac units are being used. Ryan you seem to have some good insights how about this one? |
| Ryan_J | Hi Electriman, and thanks for the compliment. In a dwelling, you are typically going to have two current carrying conductors in each cable (with rare exception). I notice that a few people have talked about the actual load on the conductors, but in my opinion the load (whether connected or calculated) is irrelevent. Remember that overcurrent is any one of the following three: 1) Ground fault, 2) Short circuit, or 3) overload. It is overload that we are trying to address here, and when your circuit feeds receptacle outlets that can be no assurance against overload. So, back to your question. You have 15 cables bundled for greater than 24", which means that you have 30 currnet carrying conductors. This would require an ampacity adjustment factor of 45%, or .45. Now, type NM cable is a 90 degree cable, despite the limitations of 334.80, so doing the math I come up with: #12 AWG=30*.45=13.5 Amps. #14 AWG=25*.45=11.25 Amps...Neither of which could be used on a 15 amp breaker because of 240.4(B)(1). You have no option but to route your conductors in a different manner. Light at the end of the tunnel...Here is a diagram that discusses what you can do to a typical wood I-joist without damaging it.  As you can see, these are quite forgiving, so drill lots of holes...the house won't fall down. Or, use a SawZall and cut a rectangular slot and "fan" out your cables for your home runs. Courtesy link: http://electrical-contractor.net/cgi-bin/Ultimate.cgi |
| Dave Nix | Electricman, If you have 15 of 12/2 MN bundled for more than 24", you would have to derate them to 50%, which would be 15A. As long as your breakers are 15A, no problem. If they are 20A, you win a red tag! |
| David Hyatt | With all this being said, and sadly enough I agree now. No one has given a better was to run to a panel. Ryan being an inspector you must run into this all the time(not bundling but in conduit sleeve) what do you suggest to your contractors who do as I desribed above. |
| Ryan_J | Hi David. I just tell them to keep the conduits less than 24" long. It's too bad I didn't bring my camera to work last week, because I saw a great way of complying with the bundling provisions. Perhaps next time I see that electrician's work I'll post a picture of it. |
| SteveMc | OK Ryan, summarize this for us. 1)If I have a 2" conduit less than 24" long going into my panel how many 2 conductor cables can I put in it without derating? 2) If I'm running wire through the first floor ceiling joists (2" holes) how many 2 conductor cables can I put in each hole without derating? Assuming using NM cable. |
| iwire | When the distance is less than 24", cables or raceways, derating is not required. |
| Ryan_J | Hi Steve. If your pipe is less than 24", you may stick as many as you would like in it, as Bob stated. For your other question, typically you would only be allowed to bundle (4) 2 conductor cables for more than 24". |
| Electricman | Ryan,OK I get the whole jist of what the code is saying but in your calculations you say 2 current carrying conductors I thought it isnt neccesarry to count the grounded conductor in derating. Also wouldnt romex be considered an independent raceway or cable system so if they are installed through a round hole they would still be independent of one another. I like the info on the drillin and notchin(I have the same chart in my truck) my problem is 99% of the homes I do have real floor joists in them 2X12s so the method of notchin is a no no and most building inspectors do not want a whole bunch of small holes drilled they want 1 or 2 big ones 2". I know your answer is gonna be runner boards but talk about unsightly in a high end home so my delema goes on.Thanks for the info any thing else would be appreciated. Craig |
| Ryan_J | Hi Craig. In a two wire system, the grounded conductor will always carry current...the same amount as the ungrounded conductor. Because of this we must count it. We must also count the nuetral of a w-wire system when only three wires are used for a circuit (see my thread on nuetral current wye system calculations). We also must count the nuetral on a 4-wire wye system that serves non-linear to address the harmonic issues. As far as the cable system goes, you are correct, they are their own system, but they disipate heat to each other very well, and therefore 310.15(B)(2) applies. As Bob (Iwire) stated, there is a generous exception for MC cable bundlin, and I think that is there because a metal clad cable won't transfer as much heat between each other. |