Conductor De-rating Due To Table 310.15(B)(2)(a) - Electrical - DIY Chatroom Home Improvement Forum

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01-18-2008, 04:08 PM   #1
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## Conductor de-rating due to Table 310.15(B)(2)(a)

Table 310.15(B)(2)(a) discusses percent de-rating for more than 3 current carrying conductors in raceway or cable. In 3- and 4-way switches, only two of the 3 or 4 connected wires can be carrying current when the load is switched on. Therein, can one (in the case of a 3-way) or two (for the 4-way) travellers be neglected for these calculations?

Seems like it would be legal but I wanted to make sure.

Thanks,
Jimmy

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01-18-2008, 05:00 PM   #2

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For 3 conductor cable (H-H-N-Grd) there would not be more than 3 conducters unless you bundle several together. So no deration would be necessary. It would take 10 current carrying conductors for #12 awg copper with 90C insulation before the ampacity would derate in conduit or cable bundling to a point where you would have to downsize the breaker. For 14 awg copper with 90C insulation 10 current carrying conductors would make the circuit unusable as a branch circuit in a house. Multiwires would only count the two hots as the neutral is not counted as a current carrier.

Last edited by Stubbie; 01-18-2008 at 05:02 PM.

 01-18-2008, 05:14 PM #3 Licensed Electrical Cont.     Join Date: Feb 2004 Location: NY State Posts: 7,736 Rewards Points: 1,822 This is completely a non-issue. With under ten conductors (sizes #10,#12, or #14) even if you derate the 70% from 310.15(B)(2)(a) you are still under the "standard" amperages in 240.4(D). __________________ Sometimes I feel like if I answer any more questions it is like someone trying to climb over a fence to jump off a bridge and me giving them a boost.
 01-18-2008, 07:06 PM #4 Power Gen/RS Engineer   Join Date: Jun 2006 Location: Oak Park, Illinois Posts: 785 Rewards Points: 566 Stub and Speedy- I think I finally understand. I have been wrongly assuming that a 12 awg. THHN copper wire is good for 20A (based on 240.4(D)) whereas according to table 310.16 (which applies to my case where I am running individual copper wires in thinwall), a 12 awg. THHN conductor is actually rated for 30A. This is why you indicate that you need to pull more than 10 wires in the same pipe before there would be any de-rating issue since 50% of 30A = 15A which would be acceptable for a 15A OCD. I feel pretty lame here I don't know how many times I've misinterpretted this point of the code but thanks for not berating me over my mistake! But, I'm still curious as to the travellers. If you are indeed counting current carrying wires, are you allowed to count 1/2 the travellers since only one will be carrying any current when the load is energized? If this is allowed, does the NEC address it specifically? Thanks again, BigDummy __________________ Well, now, there's what's right and what's right and never the twain shall meet.
 01-18-2008, 07:20 PM #5 UAW SKILLED TRADES     Join Date: Jan 2007 Location: Kansas Posts: 5,341 Rewards Points: 2,652 It's not an issue but the answer is no.... you would have to count any wire that is a current carrier whether switched power or not. also remember that not all deration starts with the 90 C column ampacities. Deration starts with the insulation rating of the wire. For example if i were using thhn/thwn, which is almost always dual rated, in a wet location I would derate using the 75C column because I am using the wire for the thwn rating of 75C insulation. If I had the thwn-2 rating then I would derate using the 90C column of 310.16.
01-18-2008, 07:31 PM   #6
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Quote:
 Originally Posted by Stubbie also remember that not all deration starts with the 90 C column ampacities. Deration starts with the insulation rating of the wire.
Thanks Stub-I chose the 90deg. column based on the fact that the insulation for the wiring that I am using is indeed THHN. Perhaps the one intelligent assumption I have made?!

Also, your explanation about using the 70 deg. column for the THWN is appreciated.

TTFN
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Last edited by BigJimmy; 01-18-2008 at 07:35 PM.

 01-19-2008, 01:16 AM #7 Union Electrician     Join Date: Nov 2007 Location: Chicago, USA Posts: 615 Rewards Points: 500 While I admire the fact that you bothered to look at the tables, let alone the derating factors, there is a slight error. If you look at 240.3 (d) you will see that your application does not include a higher ampacity than 20 A. If there has been a change, bear with me, I have an older version of the NEC. I'm in Chicago.
 01-19-2008, 02:18 AM #8 UAW SKILLED TRADES     Join Date: Jan 2007 Location: Kansas Posts: 5,341 Rewards Points: 2,652 NM-b 12 awg is derated at 30 amps 90 C if it were a situation where it was needed, this is not one of those. The fact that small conductors cannot have ocpd greater than specified in 240.3 d has nothing to do with the ampacity of nm-b for deration purposes. See NEC 334.80 where it states that "the ampacity of types NM, NMC, NMS cable shall be determined in accordiance with 310.15. The ampacity shall be in accordance with the 60 degree C conductor rating. The 90 degree C rating shall be used for derating purposes, provided the final de-rating ampacity does not exceed that for a 60 C conductor." For #12 awg NM-b that is 25 amps not a set 20 amps due to the ocpd rating. So if you are saying that you cannot use the 30 amps for derating of NM-b you are mistaken. But I'm not sure what point you are making maybe we are saying the same thing. The #12 awg would require 20 amp protection or less depending on the outcome of your deration calculation and considering the application. Last edited by Stubbie; 01-19-2008 at 02:34 AM.
01-19-2008, 10:09 AM   #9
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Quote:
 Originally Posted by goose134 While I admire the fact that you bothered to look at the tables, let alone the derating factors, there is a slight error. If you look at 240.3 (d) you will see that your application does not include a higher ampacity than 20 A. If there has been a change, bear with me, I have an older version of the NEC. I'm in Chicago.
Goose-

I'm a little confused. I'm using the 2005 handbook and article 240.3 Other Articles does not have any subcategories. If you are referring to 240.4(d), then I guess I don't understand. I am using 12AWG. single Cu conductors, THHN in thinwall. The way I understand 240.4(d), this stipulates that while for instance the above conductor can be assumed to be rated for 30A (per 310.16 and I do believe that I am correct in using this table as opposed to one of the following tables), you cannot use a breaker in excess of 20A rating for the circuit. But for derating purposes, the initial un-derated rating is 30A. If I'm wrong, by all means, please explain.

BTW, which outfit are you working for in Chicago? Meade, AEC, Divane?

Thanks!
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01-19-2008, 12:00 PM   #10
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Quote:
 Originally Posted by Stubbie It's not an issue but the answer is no.... you would have to count any wire that is a current carrier whether switched power or not.
Something you may be interested in Stubbie if you read the proposal near the bottom.
http://forums.mikeholt.com/showthrea...ing+conductors

01-19-2008, 12:14 PM   #11
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Quote:
 Originally Posted by Cow Something you may be interested in Stubbie if you read the proposal near the bottom. http://forums.mikeholt.com/showthrea...ing+conductors
Cow-

BigJimmy here.

I find your link interesting. Near the bottom of the discussion, George Stolz, II is quoted as requesting that an exception #6 be added which is essentially what I am talking about. However, what I found more interesting was in the panel's statement: The proposed exception is not necessary. The present language of 310.15(B)(2) already permits what the submitter is proposing.

IMO, logic would dictate that not all travellers should be counted. On the other hand, as another poster put it "(paraphrased)the code does not speak of part-time current-carrying conductors." Certainly, all travellers are indeed current-carrying. But, based on the panel's statement, they are implying that it is perfectly acceptable (i.e. "...already permits...")to not include all the travellers when totalling the number of c/c conductors for consideration of derating.

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 01-19-2008, 03:02 PM #12 Union Electrician     Join Date: Nov 2007 Location: Chicago, USA Posts: 615 Rewards Points: 500 Big Jimmie, you have the right table and are, in fact reading it correctly. However, if you look at the asterix on #14,12 and 10 and find the corresponding footnote, it refers back to 240.3 Protection of Conductors. This section will tell you to use 310-15 unless permitted or required by (a) through (g). This is where (d) Small Conductors comes into play. This is actually the section that has most people glued to idea that certain size wire can't get used on other sized breakers. You can use a #12 in a 30 amp, but not in the condition you describe. Again if my numbers don't jive I apologize. I have an older version of NEC. I should remedy that... I work for Rex electric.
 01-19-2008, 03:52 PM #13 UAW SKILLED TRADES     Join Date: Jan 2007 Location: Kansas Posts: 5,341 Rewards Points: 2,652 Goose..... No problem I see where your going now. I wouldn't fret the older version I new what you were tallking to. 2008 would be the one you want to buy anyway. I'm still using 2005 with the proposed changes from mike holts cd. Cow.... Thanks for that link I wasn't aware of Georges proposal. I speak (over the internet) with him on occassion and he is a damn fine residential wireman. But someone is going to have to give me the interpretation in 310.15 b2 that suggests one traveler in 3 and 4 way circuits needs not be counted as a ccc. However the CMP for that section says it does so I will withdraw my contention that you must count all the travelers. I would agree with George though an exception or language change needs to be implemented because it is one hell of a stretch to read that in the present language. I will also agree that there really is no call to count both travelers. Stubbie
01-19-2008, 03:56 PM   #14
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Quote:
 Originally Posted by goose134 However, if you look at the asterix on #14,12 and 10 and find the corresponding footnote, it refers back to 240.3 Protection of Conductors. This section will tell you to use 310-15 unless permitted or required by (a) through (g). This is where (d) Small Conductors comes into play. This is actually the section that has most people glued to idea that certain size wire can't get used on other sized breakers. You can use a #12 in a 30 amp, but not in the condition you describe. Again if my numbers don't jive I apologize. I have an older version of NEC. I should remedy that...
I'm sorry, Goose. My entire discussion was based on the wiring of my home. I have certainly seen exceptions to 240.4(d) in my past life working in power generation facilities but these involved applications that were way different than branch circuit wiring. You wrote "...but not in the condition you describe." and that pretty much sums it up. Thanks.

Take care and good luck. BTW, your youngin' is a cutie!

Jimmy
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01-19-2008, 04:03 PM   #15
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Quote:
 Originally Posted by Stubbie However the CMP for that section says it does so I will withdraw my contention that you must count all the travelers. I would agree with George though an exception or language change needs to be implemented because it is one hell of a stretch to read that in the present language. I will also agree that there really is no call to count both travelers. Stubbie
I have to chime (ding).

We're all having this conversation about something that does not seem straightforward. From COW's link, it is apparent that the CMP doesn't believe that all the travellers need to be counted as ccc's but then they reject the notion of adding language to clarify it. And obviously, it is not clear!

One of the things that has always tweaked me about the NEC is that in some instances, the language seems to say a lot without simply making a nice, concise point. I don't mean to get people started debating this point but why make the code needlessly vague or grounds for interpretation?

Sheesh!

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