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Derating?? Wires on different circuits through same fire blocked hole.

Derating?? Wires on different circuits through same fire blocked hole.

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  #1  
Old 01-18-11, 12:07 PM
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Derating?? Wires on different circuits through same fire blocked hole.

I have a question about the 2008 NEC 334.80. I have installed a few new circuits for bathrooms. I ran dedicated 20 amp circuits to each bathroom, using 12 ga wire. I had to take some drywall down to do this. Near the breaker panel I found the spot where I ran my new wiring through the top plate foam sealed holes where the original wiring goes into the panel. Plenty of room in the holes to fit my new wiring along side some of the original wire as they are 1 1/4" holes. So now one of these holes has three 12-2 and one 14-2. The other hole now has two 14-2 and one 12-2. Each cable goes to it's own circuit breaker.

These cables are not bundled so I'm not worried about the derating issue there. I'm wondering if I have to derate the wiring because more than two cables are going through a fire blocked hole near the panel.

I noticed that the original electrican ran 4 cables, of various gauges, in one fire blocked 1 1/4" hole near the panel when the home was built in 1985. That particular hole has a 6-2, 12-3 and two 14-2. I don't think you could put anymore cables in there and it is foam sealed. So, I guess derating wasn't an issue in 1985?????

I can do the calculations if all the wires passing through the hole are of the same gauge. But, what about a situation where the gauges are mixed, such as 12 and 14 gauge?

Thanks.
 
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  #2  
Old 01-18-11, 12:21 PM
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Derating is only required when the conductors are bundled together for at least 24", so one hole through a firewall is not a problem. Furthermore, derating does not have a practical effect on 15A and 20A circuits until you get to 5 cables.
 
  #3  
Old 01-18-11, 01:00 PM
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ibpooks,

You might want to take a look at the 2008 NEC section 334.80. This section is new to the 2008 code. It is my understanding that, in all new construction work, there is to be no more than 2 cables per hole unless the cables are derated. And no-one wants to do that with 12/2 and 14/2. The 24" rule applies to raceway, but this rule applies to fire-stopped holes.
 
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Old 01-18-11, 01:27 PM
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Originally Posted by DIYMaster View Post
The 24" rule applies to raceway, but this rule applies to fire-stopped holes.
You're right about this part. A foam firestopped hole does eliminate the 24" requirement; however you can still install up to four #14 or #12 cables per hole. Article 334.80 simply refers back to the usual de-rating tables in 315.something(?) which means you start derating from the 90C ampacity column. This leaves plenty of derating room before the actual circuit ampacity is impaired beyond the imposed 60C limit for NM cable.
 
  #5  
Old 01-18-11, 03:48 PM
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I'm afraid you are wrong on this. The code clearly states this:

Where more than two NM cables containing two or more current carrying conductors are installed.......through the same opening in wood framing.......the allowable ampacity shall be adjusted......

Read it again...it does not say four four cables per hole but two......

I know because I have heard all the electricians in this area gripe about this, but with all said and done, I see them drilling a kazillion holes so that no more than two are in each hole. I install alarms systems but I see the electricians all the time, and see their work.
 
  #6  
Old 01-18-11, 05:28 PM
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I am with Ben on this. The derating will have no effect until you get to five cables. Don't forget you start with the 90 degree column for the derating. for example #12 is rated at 30amps x .7 = 21 amps.
 
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Old 01-18-11, 06:09 PM
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Ampactity adjustment factors in Table 310.15(B)(2)(a) apply where more than two NM cables with two or more current-carrying conductors are installed in thermal insulation without maintaininhg spacing between the two cables. The exception in 310.15(A)(2) is not applicable in either of these.
Also.....where two or more cables pass through wood framing without maintaining spacing and the wood framing is draft or fire-stopped using thermal insulation, sealing foam or caulking material.
From Analysis of Changes NEC-2008


I don't have a 2008 NEC book handy so I can't look up the references.
 

Last edited by CasualJoe; 01-18-11 at 06:11 PM. Reason: typo
  #8  
Old 01-18-11, 06:18 PM
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DIYMaster, I beleive you can run more than 2 cables through a hole but derating must be figured. As you wrote ( there is to be no more than 2 cables per hole unless the cables are derated). So what you said about only 2 wires allowed isn't correct. He shouldn't have any problems at all.

Jim
 
  #9  
Old 01-19-11, 03:51 AM
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Well, yes, of course, more than 2 can be run through a hole, but, as you and I have been stating, they must then be de-rated. So what happens when they are de-rated? They must be upsized, meaning that then #10 must be used instead of #12. Seems to me that it is better to limit the number to 2 or less in a hole, than to upsize all the cables so you can put them all in the same hole.
 
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Old 01-19-11, 09:34 AM
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I took the time to study up on derating. You are correct pcboss and ibpooks, but now I know why. It is because that even with the derating in effect, the ampacities of the conductors are still above that of the circuit breaker. The derating is applied to the 90 degC colum of table 310.16 so that helps a lot. In summary, for #12 AWG, with 4-6 conductors in a hole the ampacity is 24A, for 7-9 it is 21A, and for 10-20 it is 15A. For #14AWG, and 4-6 conductors on a hole the ampacity is 20A, for 7-9 ampacity is 17.5A, and for 10-20 ampacity is 12.5A.

Thanks for helping me to understand this.
 
  #11  
Old 01-19-11, 10:01 AM
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No problem. Though it is important to note that this nuance we've been discussing applies only to #14 and #12 cables in typical indoor conditions. Sizes #10 and larger (30A), hot attics or different wiring types are affected by derating with just two cables, so what we're talking about is not a "general case" for derating.

Also just because you can put 4 cables in a hole doesn't mean you should. I try to do no more than 2 or 3 in a hole just because the insulation really binds up when you try to pull more through and a ripped cable is a wasted cable.

I guess derating wasn't an issue in 1985?????
Historically derating was only applied in practice to conduit systems almost exclusively in commercial buildings. In the last few code cycles, they have clarified and expanded derating rules to include nonmetallic cables and wiring in residential buildings. It was discovered in the late 90s or early 00s that the newer building codes requiring tight sealing of buildings for energy efficiency and fire protection was also causing "hot spots" in cables where foam insulation was used to seal air flow. The foam keeps the building warm but also blocks convection cooling of the cables.
 
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Old 01-19-11, 10:16 AM
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To everyone who replied, thank you.

Well, it appears that derating, as outlined in the NEC 2008, is in need of some simplification. Otherwise, those who wont bother doing the math will simply drill more holes unnecessarily and that could cause structural framing problems.

I checked my copy of the 2008 NEC and I think I've figured out one of the areas where I wasn't sure about. I need to install two 14-3 and one 14-2 (total of 8 current carrying conductors) in a top plate in wood framing that will be foamed sealed. So, 334.80 is the governing section. Here is my calculation:

310.15(B)(2)(a)
7-9 ccc = 70 %

Table 310.16 90 degree column
14ga = 25 amps

25 amps x .7 = 17.50 over current protection (breaker rating)

This means to me that no upsizing of the wire is necessary and a 15 amp breaker is allowed under these circumstances. The bottom line is that I can put two 14-3 and one 14-2 in one hole in my wood framing and seal it with foam.

Now, to my other question about #12 and #14 in the same hole. How do you calculate for derating when wiring of different gauges are in the same hole. Hope someone can help me here.

Thanks again.
Rick
 
  #13  
Old 01-19-11, 11:20 AM
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It appears to me that you would do the same thing as you already did. Say you had two 14-3 and one 12-2. ccc = 8; then the derating factor is 70%. Ampacity of the 14/3 is 17.5A and the 12/2 ampacity is 21A. So a breaker of 15A is good on the 14/3 and 20A is good on the 12/2.

Correct?
 
  #14  
Old 01-19-11, 11:44 AM
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Originally Posted by Rickm15752 View Post
I need to install two 14-3 and one 14-2 (total of 8 current carrying conductors)
14-3 only counts as 2 CCC as only 2 of the three conductors are active at any given time, so you have 6 CCC through this particular hole.

Now, to my other question about #12 and #14 in the same hole. How do you calculate for derating when wiring of different gauges are in the same hole. Hope someone can help me here.
It's the same as if they were all the same gauge; you just apply the percentage derating factor to each conductor's 90C ampacity to get its derated ampacity.
 
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Old 01-19-11, 02:15 PM
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Actually, the 14-3 will be used to feed an outlet and to switch a string of lights. So, both hot wires could be in use at the same time. Similar to a bathroom light/fan combo. I didn't mention this in my original post. So, I think I have to count the 14-3 as 3 ccc.
 
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