#12 Wire on 25A HVAC Circuit

Ran across a situation for an AC condenser with a 12-2 NM sheathed cable feeder on a 30A breaker. The breaker/wire combination wasn’t what rubbed me the wrong way as it met the listed nameplate max breaker size, and was within 175% of the RLA. But I did do a double take when I saw a listed “minimum circuit ampacity” of 25A on the nameplate (RLA x 125%) with the #12 Cu wire.

I would have expected #10 Cu wire with the listed 25A required circuit ampacity. But after digging a little deeper it seems that the usual small branch circuit limitations of 15A for #14Cu, 20A for #12Cu, and 30A for #10Cu (NEC 240.4.D) do not necessarily apply to AC circuits. Found that it seems you are allowed to use the full 25A rating for 12-2 NM cable (NEC Table 310.16) in certain cases (NEC 240.4.G) which includes HVAC equipment.

Does anyone know if thats considered poor field practice in general, even though it may be allowed by current construction standards, as it’s the first time I came across this.


Manytimes the ratings of the actual outside compressor unit will be like that in that you rate the minimum rating for the conductor and the max rating for the OCPD.

This factor has already been figured for the electrician when doing the installation and the NEC says in specific cases where the NEC allows it. Used to me an “oblist” on the 310-16 chart beside the 14AWG, 12AWG and 10AAWG…in fact I just did a seminar on that in Denver explaining all those little details for the HI’s…

Notice on the chart 310-16 the * beside the 14,12 and 10....at the bottom it refers you to 240.4(D)

Now....240.4(D) says unless specified in 240.(E) through (G)...then 14 AWG must be 15A, 12 AWG must be 20A and 10 AWG must be 30A.....now...

So 240.4(G) allows ampacity differences in many listed areas other than what is listed for 240.4(D)...which then in turn revert to the ACTUAL ampacity table of 310-16

Now the sizing of the Protection and Conductor size is marked on the equipment nameplate which has been already calculated by the manufacturer...important to know the MAX FUSE SIZE is the MAX for the protection and the MIN Circuit Ampacity is for the coductors...many get that confused...even some local inspectors...

BTW…if it says MAX FUSE SIZE…it has to have a fuse protection it somewhere…either at the unit disconnect or beside the panel…then it can be to a circuit breaker…BUT if it says FUSE it must be fused protected… If it says protected by “HACR” then it should be protected by a circuit breaker.

Ok…Now…the size of the conductors to the unit is based on the NEC 440.32 ( which should already be done on the nameplate…as shown above)

So…with no nameplate…you would use NEC 440.22 for the OCPD and 440.32 to size the conductors…and the 440.4(G) denotes this is a listed special allowance refered to in 240.4(D) and shown in 240.4(G) to refer you to 440 for A/C units…

At that point the verbage says to use 310-16…and then you are allowed to use the ratings of the charge…so here is an example nameplate


Translation - 40A OCPD
Translation - 25A Wire ( 12 AWG THHN rated at 25A under 60 degrees since it is under 100A…and we know that under 100A uses the 60 degree column)

I make things like this very simple in my seminars…lol…harder online.

Here is a good example and we will use it to learn from with the replies…

Question: What size THHN conductor and fuse protection are required for a 24A, 240V motor compressor.

A.) 10 AWG, 40A Fuse
B.) 10 AWG, 50A Fuse
C.) A or B
D.) none of these

Should be interesting…again use Art 440.22 and 440.32 to come up with the answer…

Ok…for those who dont want to learn and B*tch about code…SKIP IT…don’t waste my time…trying to teach something here…not something you QUOTE to someone as code…GESSSH>>>i get tired of having to put that in my posts…

… or 60a?

We are very lucky the manufacturer puts that label on. They get to do the engineering and give us an answer that will work.

I dont believe 60A will apply here Greg…Upsizing is not allowed in these cases…>>>>>BUT brings up a GREAT devils advocate type question if someone says 60A…great stuff…

Dang I get too excited over this crap…I really need a life other than electricity…

YOU got that right my brother…LOVE that nameplate rating…:slight_smile:

HOw will the upgraded new units effect us with new seer requirements . I understand they will cost more will the current draw be any different for the upgraded units ?
Roy Cooke sr …Royshomeinspection.com

Hey Roy,

I wanted an ANSWER fella…thehehe…BUT good question…

I think with anything new like that the nameplates will reflect it and they are good about doing that now days…its the old ones you have to worry more about.

When someone upgrades a unit since the SEER is changing the actual nameplate will change and you HIT a great point…people are only talking about the piping size change…and the cost of that…the better understand the COSTS of having the potential feeder changed as well to meet the new equipment.

Now I must confess…I have not done any RETRO’s yet for clients…but in new construction it wont really matter…we will size to the new nameplate but you are right…once an old model needs replacment…the new nameplate ratings will have to be met.

Typically a higher SEER will use less power so the wire should be OK but the O/C device may need to be replaced.

As for the 60a. 440.22(A) says “…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.”

Starting with 24a I get a max of 54, that rounds up to 60 since there is no 55. I would want to see the 45 or 50 trip before I believed the 225% was necessary.
That is one reason the label is so great. If the label says that machine should run on a 45 or 50 and it trips one … it is broke, fix it, don’t just put in a bigger fuse.

Thats a loaded question, cause it’s not known if 40A is sufficient for start-up, in which case a 50A fuse would be okay.

I would tend to agree with Paul that up-sizing the fuse above 225% would not apply, as that is a “maximum”. Also 240.4.B only allows “the next higher standard overcurrent device rating (above the ampacity of the conductors being protected) …” so I don’t think that could apply to AC units where the fuses/breakers are already well above the conductor ampacities at up to 175% [sometimes 225%] of the running load.

P.S. At least newer AC equipment almost always has “MAX FUSE OR HACR BREAKER” on the nameplate … so either a fuse or breaker could be used.

Paul … I generally recalled the minimum NEC requirements which would allow #12 Cu NM on an AC circuit with a 25A required ampacity, which is why I looked it up to confirm that. But an electrician I know thought that would be very poor practice cuting it so close given the breakers are often oversized for a given wire to begin with, and AC units often draw more than the nameplate rating after only a short time in-service.

Thus the reason I threw it out for discussion, and to see if anyone else thought that may be poor field practice (where sometimes what is considered good field practice differs from code “minimums”) … :wink:

Generally the newer 13+ Seer AC equipment does cost more, but will draw less current than older 10/12 Seer equipment. That means installing a 13+ Seer condenser as a replacement for say a 10 Seer condenser will also often require changing the fuse/breaker to one with a lower rating. That has happened on a number of AC equipment replacement jobs done by a family HVAC business I helped with in my spare time.

I think defects such as having a breaker that exceeds the max fuse/breaker listing on the condenser nameplate will become more common as older equipment is replaced.

JMO & 2-nickels … :wink:

OK…guys…no assumption of Start UP is needed to figure this example I gave…it is quite simple using the NEC…I will explain…You only need to know the 24A and voltage to figure this using the NEC…

Answer: a.) 10 AWG, 40A Fuse

Note: I said what is required not what is the MAX allowed…

Branch-Circuit Conductors ( Table 310.16 and 440.32 )
24A x 1.25 = 30A, 10 AWG, rated 30A at 60 Degrees C.( 110.14© and Table 310.16)

Branch-Circuit Protection ( 240.6(A) and 440.22(A)
24A x 1.75 = 42A, next size down = 40A

Why next size DOWN…well you already made the adjustment for the 175%…do you think the NEC will let you adjust again…lol

OK…here is the TRICK…the NEC says IF the 40A protection device is not capable of carrying the starting current, then the protection device can be sized up to 225% of the equipment load current rating ( 24A x 2.25= 54A, next size down 50A )

So why not answer © …is we said required…not the adjustment…you can only use 225% IF the 175% rated OCPD can’t handle the start up…sneaky…sorry…just wanted to use it as a learning experience for the other guys…:slight_smile:

Hmmm … lets think about this. It’s actually not known what the “required” fuse would be.

42A (175%) would be the “maximum” allowed (NEC 440.22 actually uses “not exceeding … shall be permitted”), which means using up to a 40A fuse … UNLESS the 40A fuse is not sufficient for the start-up load, in which case the “maximum” would be 54A (225%) and a 50A fuse would indeed be “required”.

P.S. Sorry Paul … couldn’t resist … :wink:

lol…man…I am die hard NEC…thehehe…it says 440 shall have exception in regards to the sizing…so you MUST size the OCPD to sizing of 175%…at which point ONLY then if the OCPD can’t handle the start up can you increase to 225%…

BZZZZZZZZZZ…the NEC lists 175% as the rating to determine the OCPD…so it is the REQUIRED size…the 225% is the MAX only IF…IF…IF…IF…IF…IF…theheheheh

Leave it to Education Committee Members to FRY members brains…thehehehe…lol…

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 :wink:

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…

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 … :slight_smile:

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

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 … :wink:

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…

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.