DIYbanter - View Single Post - Using #14 wire for one light on 20A breaker

Doug Miller

In article , (Don Klipstein) wrote:
In article , Chris Friesen wrote:
Don Klipstein wrote:

Since resistance increases with temperature, actual ampacity does not
increase much with an increase in temperature rating. And ampacity
according to the code does not increase at all.

I'm going to have to disagree with that.

For simplicity, look at the "single conductor in free air" tables. (The
other tables show it too, just the absolute values are lower.)

With 60-90C rated insulation, a #14 wire is rated for 20A. With
110-125C rated insulation, that same size wire is rated for 40A. If you
go up to 200C insulation, you can put 45A through it.

Somehow, I suspect that upgrading from 110 or 125 C insulation to 200 C
insulation only improving ampacity from 40 to 45 amps supports my point!

Quite the opposite: it clearly contradicts your assertion that it "does not
increase at all". And, of course, the difference between 20A at 60 deg and 40A
at 125 deg makes that contradiction even more clear.

Meanwhile, also consider that wiring in a building is usually not single
conductor in free air, but 2 at least current-carrying conductors close to
each other and heating each other up with a sheath around them and the
environment outside the sheath usually not being "free air".

That was just an example -- and maybe not a real good one. But it does show
that (not to put too fine a point on it) you don't know what you're talking
about when you say that "ampacity according to the code does not increase at
all".

That's just not true.

See NEC Table 310.16 for abundant proof that the ampacity of a
conductor -- ANY conductor, ANY size -- absolutely DOES increase with
increasing temperature rating of the insulation.

Add to this the fact that extra high temperature rating wire is used
more where ambient temperature is higher, and I see good reason for AWG 14
romex to be only allowed by code to be used in circuits up to 15 amps
regardless of temperature rating.

That's a completely separate issue from the ampacity of the wire, which Code
specifies as being, for example, 25A with THHN insulation. In a separate
article, Code limits the overcurrent protection for 14 and 12 ga conductors to
15 and 20 amps, respectively, despite their having a higher ampacity.

--
Regards,
Doug Miller (alphageek at milmac dot com)

It's time to throw all their damned tea in the harbor again.