On Mar 13, 8:01?am, Mike Hartigan wrote:
Frankly, I never considered the NEC and its local variants to be
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?
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 he
http://www.nfpa.org/itemDetail.asp?c...kie% 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.