How much margin for safety is built in to the NEC?


Without consulting the code, it seems that everyone "just knows" that 14 gauge is used for 15 amps, 12 for 20 amps, etc. My question is - how much margin of safety is there in this? I'm assuming that the temperature of the wire at load was used to actually establish the code. Presumably, though, 14 gauge could carry 16 amps without creating a hazard. How about 17 amps? How about 18? ... (How many hairs does it take to make a beard?). In a pinch, if you were building something temporary - something you were going to tear down in a week or two - would you use the 14 gauge you have on hand for your table saw, or would you make a special trip to the store for 12? At what point does the heat create an actual fire hazard? I'm not considering over-fusing anything - I'm just curious. In Grandpa's day, it was standard homeowner practice to simply insert a bigger fuse when one kept blowing. No doubt the elders who wrote the various versions of the code were aware of this practice and took it into consideration when deciding on the numbers.
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well in the case of the table saw the added voltage drop of 14 gauge wire will cause poor performance. stalling on knots, slow cutting etc.
I went with 12 gauge 20 amp breakers for a temporary install at a fire damaged home once. Ran a main line up the stairwell with outlkets on every floor, and a main off switch at entry door to kill everything at nite.
My friend the owner freaked because he had rolls of 14 gauge lying around, and lots of used junk outlets........
I told him it wasnt worth the hassles...
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Mike Hartigan wrote:

14/12/10 gauge wiring in particular when used for "regular" outlets has quite a bit of safety margin.
For instance, your standard #14 romex has 90degree insulation. Normally this would allow you to put 18A through it (if we derate it as two conductors together), but its limited to a 15A breaker except for special circumstances. That's an extra 20% margin on top of the regular margin for wire amperage ratings.
Chris
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then again a 15 amp breaker will carry more than 15 amps for a short period of time.
I checked my neighbors outdoor christmal lights one time. he was a griswald, has sice quit:(
anyhow the breaker that tripped would trip about every 1/2 hour, it was running about 17 amps.
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Why not ask whatever agency prepares the NEC?
--
Don Phillipson
Carlsbad Springs
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wrote:

I suspect that all you'd get from them would be a form letter explaining that you should never stray outside the code. And justifiably, since 'officially' specifying how much headroom there is in these requirements could be (and likely would be) interpreted as tacit approval for exceeding them. Kind of like a posted speed limit - if a cop tells you verbally that they won't write you a ticket for exceeding the limit by less than 5 mph, that's one thing. If, on the other hand, he gives you a piece of paper or an email saying this, then the speed limit 'officially' becomes 5 mph above the posted limit.
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Mike Hartigan wrote:

I'd say at least double. Double is the usual standard when making things safe.
The Brooklyn Bridge, for example, used a factor of eight, but that was because of the loonies living in Brooklyn. Plus, I think it was a cost-plus job.
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says...

Not counting the semi/Toyota part, I could have written that, myself.

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I just assumed that it was a right of passage.
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This has got to be some kind of record! It was a full 8 hours 48 minutes before somebody gave the compulsory "You're an idiot for asking a question like that - Hire a pro.". Mike asked a pretty simple question. The closest you came to answering it was saying that posting something (that wouldn't answer his question, BTW) can't be done.
snipped-for-privacy@mindspring.com says...

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Jeremy Scott wrote: > This has got to be some kind of record! It was a full 8 hours 48 > minutes before somebody gave the compulsory "You're an idiot for > asking a question like that - Hire a pro.". Mike asked a pretty > simple question. The closest you came to answering it was saying > that posting something (that wouldn't answer his question, BTW) can't > be done. > Jeremy Your pretty quick with the accusations. What I said was I can't give you an understanding of the ampacity of electrical conductors in a single news group posting. And what I suggested is that he "either learn to do it safely yourself or higher me to do it for you." I also said that there are indeed some circumstances were 14 gage copper can carry more than fifteen amperes safely. I never called him an idiot and I only offered the option of a qualified electrician if he was unwilling to take the time to learn to do it correctly himself.
It amazes me that some people think that the rules are some mega conspiracy to rip them off. They represent the accumulated experience of the industry in over a hundred years of actual use and each item in them is there because someone else learned the hard way that that rule was needed to provide a reasonable level of safety. -- Tom Horne
"This alternating current stuff is just a fad. It is much too dangerous for general use." Thomas Alva Edison
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On Mon, 12 Mar 2007 07:00:31 -0500, Mike Hartigan

The NEC is just a guideline. If you follow it closely (and use a little common sense) you get more safety and convenience. Unfortunately, few electricians follow the NEC due to cost. "Do you want the job done for $350 or do you want me to follow the NEC and do the job for $550?"
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snipped-for-privacy@nobody.com says...

optional in any way. I was simply curious as to how the numbers were arrived at, knowing that overfusing is a fact of life (although it's probably less common with breakers). One must also consider that badly behaving appliances may occasionally draw more than indicated on the label or that the consumer may plug in one too many 250 watt lamps. Presumably, the breaker would prevent things from getting out of hand, but what about the '15 amp' breaker that trips at 17 amps? Will that create a real fire hazard with 14 gauge wire? Or was that possibility considered when the code was developed?
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Yes. The people who write the NEC are fully aware that there is a lot of potential for people to use the wrong wire and overfuse circuits, especially the more common 15 and 20 amp circuits. See (2002) NEC 240.4(D). A 15 amp breaker that trips after 1/2 hour of being loaded with 17 amps sounds about right, and yes there is some cushion built in for such. Knowing these things, the NEC requires #14 to be fused at 15 amps, #12 at 20 amps, etc., per 240.4(D). Call it idiot proofing if you want.
Much of the NEC is based on past experience. It's actually quite difficult to make a significant change to the NEC, and people who submit proposals are required to substantiate their claims, be it documentation of deaths, fires, or whatever. Then the CMP's (Code Making Panels) consider it. Then it's voted on (IF it makes it past the CMP's). Actually it's a bit more involved than that, but that's the general idea.
It's much easier (IMHO, also safer) to make a general statement (240.4(D)) than to size wire according to the many NEC rules that apply. See 210.3, which says that branch-circuit conductors rated at 15,20, 30, 40, and 50 amps must be protected at there ratings and 210.19(A), both of which take precedence over 240.4(B).
See Table 310.16. It says that a #14 can take 20 amps. BUT, if you look at the bottom of the page, one must consider ambient temperature. Is it safe to say that attics reach 105 degrees F? The correction factor is then 0.87. 20 x 0.87 = 17.4 amps. That means a 20 amp breaker can't be used in that case per 210.19(A).
There are also correction factors that reduce the ampacity even more when there are more than 3 current carrying conductors in a cable or raceway. One must also be able to determine what a current carrying conductor is per NEC 310.15(B)4.
Also see 334.80, which basically says that ampacities for Romex _must_ be taken from the 60 degree C column in Table 310.16.
Of course, my favorite, 210.2 says that "the provisions for branch circuits supplying equipment in Table 210.2 amend or supplement the provisions in Article 210 and shall apply to branch circuits referred to therein."
See what I'm getting at? Why wade through all that crap just to run a wire? What layman is going to understand all that? What layman _wants_ to understand all that? I can assure you that even some otherwise good electrician's can't sort it all out. How can a homeowner be expected too?
The NEC is not intended as an instruction manual. It's a manual for experienced people in the electric industry, who already understand most of the principles discussed, and provides MINIMUM requirements for electric installations.
The NEC sections that I have referenced can be viewed here: http://www.nfpa.org/itemDetail.asp?categoryID0&itemID!227&cookie%5Ftest=1
As Tom Horne said, one just can't answer your question in one post. It takes many years of study and research to understand the NEC. Even then one must constantly seek advice from industry professionals and books/journals to insure that we are interpreting and applying it correctly. Experience in the field helps a lot too, as one gets input from electrical inspectors and also becomes familiar with the industry standard methods of doing things.
If you want to try to understand the NEC and have $150 (US) to spare, the NEC HANDBOOK is a good place to start, as it explains much of the purpose behind the NEC and clarifies a lot of it. One must also study the definitions provided by the NEC, otherwise one just will not understand what is being said. Many of the words used by the NEC don't mean the same thing in layman's speak.
That's the tip of the iceburg. Hope this helps.
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If that's the kind of experience that you've had with electrician's, then I'd say it's your responsibility to send 'em on down the road when they make offers like that. Somebody like that, even if they charge you for an "installation to code", you're NOT going to get it. Agreed, there's plenty of residential hacks out there who couldn't make a code compliant installation even if they wanted too.
As an electrician, I find that it's the opposite. It's the customers (some, not all) who try to get the electrician to cut corners. I simply tell them to call someone else. I gotta sleep at night. I'm not going to purposely do something that will jeopardize someones life and the ONLY way that I know how to do that is to install electric equipment according to code. I've been fired from jobs for refusing to do work to code, lost work, and will continue to refuse to not do work to code. How would you feel if a 4 year old child was electrocuted or killed in a fire because some asshole didn't have the intestinal fortitute to stand up to some clown who was too cheap to pay for a proper installation? When I am confronted by people who _truly_ cannot afford to pay for what needs to be done right, I just go ahead and do it right, then charge them what they can afford to pay. Now you know why I jump on people in this NG who post dangerous or bogus electrical info. I figure folks are going to DIY regardless, so one may as well try to help them get the right info rather than keeping them in the dark.
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Typo, make that:
"I've been fired from jobs for refusing to do work that is NOT to code, lost work, and will continue to refuse to not do work that is NOT to code."
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snipped-for-privacy@aol.com says...

I automatically made the correction the second time I re-read that sentence.
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