On Mar 3, 8:55*pm, wrote:
On Sun, 3 Mar 2013 17:22:54 -0800 (PST), Mikepier

wrote:
I'm running new electric in a friends basement and so far uncovered a
lot of bad wiring jobs, buried splice boxes, you name it. *So now I
noticed that the central A/C compressor is wired with 10 guage wire.
I'm pretty sure its suppose to be 8 guage. And also the *existing 10
guage wire is hooked up to a 40 A breaker, which of course is not
right, its suppose to be a 30A breaker.
So since everything is wide *open now, I was wondering should I run
new 8 guage wire to the A/C? The unit is a Lennox, I beleive a 3 ton
unit, and the plate says "Min circuit ampacity 24.4 amps" and it also
says " Max fuse or ckt bkr 40A".
Even if the unit has been running fine all these years, I'm wondering
what happens down the road if it needs to be replaced with a new unit
that needs more amperage. the existing 10 guage wire will not be
sufficient I'm thinking.

OK hold on to your hat.

That label says max fuse or breaker 40a ... that is what you have

It also says minimum circuit ampacity 24.4a so the wire can be 12 ga.

You are larger than you have to be with 10ga.

That is what happens in article 440. They know you need a big breaker
to get the compressor going but the actually running current is much
lower. If you look at 310.15 you will see #12 is actually rated at
25a,.
The common axiom that 14ga = 15a, 12ga = 20a comes from 240(4)(D)
where they build the 80% safety factor in. (it is actually the max
breaker size) It does not apply on dedicated motor circuits
Homeowners will keep plugging things in until the breaker trips, then
unplug the clock.

That label has it built in *already by the engineer. You will see that
compressor actually runs more like 16-18a on a hot day.

I understand what the article is saying, but at the very least,
shouldn't the 10 guage wire be on a 30A breaker and not 40A?