#12 Wire on 25A HVAC Circuit

[Greg Fretwell]

Although it will not affect this carefully selected example it should be noted most AC units use 75c terminals so the THHN could use the 75 column.

The other issue not addressed by the NEC is voltage drop.
If you use the smallest conductor the NEC allows you may end up with low voltage at the unit.
The other issue not addressed directly 440, but is in the bottom 310.16 table, is derating. If this runs through the attic you may be taking ampacity at 71% or even 58% of the top table. I can't imagine anyone who actually needs an A/C not derating at least 82% (attic at 46c/114f). I doubt there is one HVAC installer in 100 who goes beyond reading the first number he sees in 310.16. Perhaps simply using 240.4(D) is safer in the first place

[Paul W. Abernathy]

great points....lol......man I did not want it to get THAT complicated for the guys......BUT you opened that door fella...lol

I agree......also I dont see many A/C units that when sized to the nameplate which is usually already factored in....that they need to increase to 225% because of unit OCPD trips.....

In the end......THANK GOD for Nameplates.....

[Robert J. OConnor]

Paul ... Actually, AC equipment manufacturers will list maximum fuse/breaker sizes (based on the NEC limits), and some will also list minimum fuse/breaker sizes (based on start-up load). Attached are some examples. So to further complicate things (can't resist ... lol) the "required" fuse/breaker could be anywhere between the minimum and maximum ...

Can ya tell it's a slow week for me ... lol. Glad I could get the brain cells firing ... would ya expect anything less ...

Greg ... Good point about voltage drop, where using the absolute minimum wire size could get you in trouble on longer wire runs.

However, the units with a significant draw where temperature derating could be an issue are the condenser units with compressor motors, typically located on grade outside with wires run through cooler basements or crawl spaces. I have heard of feeders run to an attic sub-panel for both units, but thats pretty rare (at least around my neck of the woods). But temperature derating is a good thing to keep in mind.

JMO & 2-nickels ...

[Paul W. Abernathy]

oh dear....we are getting into HYPOTHETICALS now....did I spell that right...?

Lets just be GLAD for nameplate ratings and listings.

HOWEVER...if you notice the example you posted in PDF......the MAX breaker is 175% and rounded DOWN in all cases....

Example.....12.9A x 1.75 = 22.57....and a 20A OCPD is listed.....

This is all correct as previously stated...the NEC allows to go up to 225% if the OCPD does not allow the initial startup surge......but I have never seen any A/C unit not be fine with the 175% level....

[Greg Fretwell]

Basements are nonexistant around here (they are called indoor pools). Houses are usually built on slabs or on a stemwall with the "box" backfilled with dirt so "up and over" is usually the path of least resistance.
Add that to attics that cruise in the 140-150f range and you see why I like to point out derating.

[Paul W. Abernathy]

Originally Posted by Greg Fretwell

Basements are nonexistant around here (they are called indoor pools). Houses are usually built on slabs or on a stemwall with the "box" backfilled with dirt so "up and over" is usually the path of least resistance.
Add that to attics that cruise in the 140-150f range and you see why I like to point out derating.

lol.......WHAT....all you guys in FLORIDA have the potential for indoor pools....now that sounds EXCLUSIVE...lol.....

lol...as you stated greg...how many people ACTUALLY derate the NM-B running through their attic to the outside AC unit....

Regardless....you all made my BASIC example....no longer BASIC...lol

[brepanshek]

how do I attach a pic to quick reply????

[Paul W. Abernathy]

I dont think you can Brian.....you have to go into advanced I believe

[Greg Fretwell]

I can always get people to whip out the calculator when I start this derating thing. It is never pretty.
I ran 6ga through my attic to pick up the air handler and condenser (Trane specs this as an option in the big manual). The A/H is the "big" load because of the toaster wire heater but the attic is cold then. In the summer it is overkill for the condenser but the derating works.

[Robert J. OConnor]

If ya look at the tables I posted above they list "Minimum Circuit Ampacity" for selecting the wires, which equals 125% of the Comp RLA plus the Fan Motor FLA. The Min Fuse/Breaker seems to be 175% of the Comp RLA rounded off, and the Max Fuse/Breaker seems to be 225% of the Comp RLA rounded down.

You probably shouldn't multiply the ""Minimum Circuit Ampacity" by the 175% or 225% factors, as that already has a 125% factor included for continuous load. Also NEC 440.22 requires using the "rated-load current" (or RLA) for figuring the max fuse/breaker sizes (they dont list a "branch circuit selection current"). Went through this with a manufacturer on a past project a while back where there was a question.

It can get pretty confusing for HI's (particularly when ya start adding in things like voltage drop and temperature derating), where it's better to just check the nameplate with what is installed and look for visible signs of defects.

Most HI's are aware that breakers for AC condensers can be larger that usual, but I thought it was worth pointing out here that the usual HI rule of thumb for copper wire of #14=15A, #12=20A, and #10=30A doesn't apply to AC equipment ... but in some cases pushing things to the limit can result in issues if everything isn't considered. And "when in doubt, bail out" ...

JMO & 2-nickels ...

[Robert J. OConnor]

P.S. Ran across a calc example for an AC condenser in Mike Holt's book "Understanding the National Electric Code" under Article 240.4.G with a nameplate listed Comp RLA of 18.0A and Min Circuit Ampacity of 23A

Min Circuit Ampacity is 23A (18A RLA x 1.25 = 22.5A, rounded up)
Wire Size is #12 THHN (rated 25A at 60C)
Max Fuse Size is 40A (18A RLA x 2.25 = 40.5A, rounded down)