On Wednesday, March 11, 2015 at 2:43:59 PM UTC-4, firstname.lastname@example.org wrote:
A dead short will trip the existing 40A or whatever breaker for
the stove instantly. It's an overload that exceeds the wiring to the
outlet, but not the breaker, ie the range of 15A or 20A to 40A, that would
be the fire hazard from the wiring overheating.
If you're lucky. only the MW will be destroyed as smoke pours
There really aren't any electrocution issues. No more so than with
the stove that also uses a shared ground and neutral that is connected
to it's metal case. And that is still code compliant as long as it
was wired before the code changed.
And, of course, while presuming 14ga/15A outlet, any load above that is
in violation of Code, the actual breaker thermal trip level is generally
about 5-7.5% _above_ the nominal breaker rating at which point the
bimetallic strip opens so the actual load might be as much as 43A or so
before the breaker itself saw the overload.
But, mitigating that is that the actual current-carrying capacity of
wiring or the outlet is quite a lot greater than that for which it is
rated for Code application so there's quite a margin of safety before a
real fire danger is present.
I don't have the data at hand on what the temperature rise in a 14ga Cu
would be in open air at 43A but it'll be warm...will it be fire-setting
hot I don't know for certain.
See the previous comment above as well...it certainly should be fixed
but I don't believe it's an imminent hazard in the next 10 minutes.
On Wednesday, March 11, 2015 at 3:14:01 PM UTC-4, dpb wrote:
Copper 14g is 2.5 ohms per 1000 ft, or .0025 per ft. At 40 amps,
I2R is 1600 x .0025, or 4 watts per foot. You have two conductors,
so double that, 8W. 8W spread out over a foot of wire doesn't sound
like a lot of heat to me either. Like you say, warm, but I doubt
it would melt romex insulation. There is a lot of margin in the code.
I agree, especially now that he knows not to plug another big load
into that same receptacle with the microwave.
Thanks for the update--I couldn't recall well enough to venture w/o
looking it up and didn't want to take the time/effort at the time.
Well, let's see if can do a little more on the temperature rise given
the heat load...
Well, shoot! Can't find in a quick search any specifically applicable
heat transfer coefficients for romex so it'll have to wait until can
find more time...by which time I'll likely have lost interest! :)
I've lost interest...but, I did find one study that bundled 12(!!!)
various-sized cables from 14 to 10ga in a single bundle through a top
plate hole as constricted as possible yet get them through it.
They then powered each and every circuit to it's 80% rating by adjusting
a bunch of resistance heat loads and watched. It raised the temperature
to above the 90C (~200F) rating temperature after a period of several
hours from which they concluded this was a bad idea and Code should
prevent it! (DOH!)
OBTW, this was on an experimental structure of a west-facing wall in the
AZ desert at full summer sun...
Check out 334.80, It has been in the code for several cycles.
Where more than two NM cables containing two or more current-carrying
conductors are installed, without maintaining spacing between the
cables, through the same opening in wood framing that is to be fire-
or draft-stopped using thermal insulation, caulk, or sealing foam, the
allowable ampacity of each conductor shall be adjusted in accordance
with Table 310.15(B)(2)(a) and the provisions of 310.15(A)(2),
Exception, shall not apply.
On Wed, 11 Mar 2015 11:53:22 -0700 (PDT), trader_4
Even back when it was legal, there were still some restrictions.
It had to originate in the main panel, not a sub, and the grounded
conductor was required to be insulated if this was not SE cable.
That usually meant they ran 10 or 8 3 wire romex plus ground so the
ground wire was still there, just nod connected to the receptacle,
that got the red, black and white. Typically they used it to ground
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