DIYbanter - View Single Post - What size wiring for Central A/C compressor?

The Daring Dufas[_8_]

On 3/4/2013 8:46 PM, wrote:
On Mon, 04 Mar 2013 16:55:43 -0600, The Daring Dufas
wrote:

If you'll look at NEC Table 310-16 you will see the amp rating for wire
size. It changes depending on the insulation on the wire. Every 40 amp
circuit I've ever installed in a home using 60°C NM/UF cable has been #8
copper. If I install a 40 amp circuit in conduit using 90°C THHN/THWN
insulated wire I'll use #10 copper. I don't usually run conduit in a
home. The last 4 ton AC condensing unit I installed used #14 THHN/THWN
copper in conduit. It was a 3ø unit that called for a 15 amp breaker. I
have to install what I know the inspector will pass and the inspectors
do look at the temperature rating of the wire and check if the circuit
breaker is HACR rated if it's hooked to an AC condensing unit. I don't
argue with the inspectors, you can if you wish. O_o

You have to look at the whole code, in context, you can't just cherry
pick one article and believe that is the way it works in every
situation.
When you are looking at an AC unit you start in 440 if you are sizing
the breaker, not using the label.

440.22 Application and Selection.
(A) Rating or Setting for Individual Motor-Compressor. The
motor-compressor branch-circuit short-circuit and ground-fault
protective device shall be capable of carrying the starting current of
the motor. A protective device having a rating or setting not
exceeding 175 percent of the motor-compressor rated-load current or
branch-circuit selection current, whichever is greater, shall be
permitted, provided that, where the protection specified is not
sufficient for the starting current of the motor, the rating or
setting shall be permitted to be increased but shall not exceed 225
percent of the motor rated-load current or branch-circuit selection
current, whichever is greater.

12 ga wire has an ampacity of 25a in the 60c column so you can start
with a 45a breaker and you might end up with a 60 if you use the round
up rule at 225%.
The label is actually rounding down to get to 40a.

The label is very unambiguous. It tells you can use 12ga wire and a
40a breaker. If an inspector is confused about that, he is
incompetent. I am not saying there are not incompetent inspectors out
there but at a certain point they need to be educated. That probably
does not happen on an active job but you can send a letter to the
state licensing authority asking for a ruling. Fix it for everyone,
not just that job.

What is your jurisdiction? I may actually know someone there.

It's Birmingham in Jefferson County Alabamastan. I noted the 25 amp
rating in the 60°C column too but I also read the notes at the bottom
of the chart and those jive with what has always been told to me. The
city code does differ a bit from NEC and that's what I've been told by
the inspectors I've dealt with. Now in your experience are you also
considering LRA of the compressor? Is that part of the 225% overload? O_o

TDD