I'm looking at Art. 440 parts III and IV and I don't see where it says you can use #12 for over 20 amps. Am I not seeing it? I don't even understand how the OP can get away with putting the #12 romex on a 30 amp breaker.
Is it 440.31 ?
The provisions of these articles shall not apply to integral
conductors of motors, to motor controllers and the like,
or to conductors that form an integral part of approved
equipment.
I thought nm-b had to always use the 60 degree column
I'm not going to get into the NM usage as we all should know NM is not allowed in a wet location and yes it is required to be sized at the 60?c column of table 310.16.
What I would like to discuss is what we protect conductors from and what methods are allowed by the NEC.
First there are two things we protect conductors from and these are two very separate and different requirements that we use to protect conductors.
First one is overcurrent.
Overcurrent is long term and can be quite a few minutes if just over the rating of the conductor, most are used to over current being provided at the supply end of the circuit by the breaker or fuses that supply a conductor, this is always true when you have a circuit supplying an unknown amount of load such as receptacles or branch circuits in a house or other, since we can't control what people might plug in or connect to this type of branch circuit we make sure the conductors are protected at the supply end.
But there are cases where the overcurrent protection is allowed to be at the load end and is just as safe as long as more loads are not likely to be added, the three most common times we will see this is: taps as we see in 240.21: HVAC as in 440: and motors: 430 Also if we look at table 240.4(G) we will see other places where the NEC allows end of circuit over current protection.
Now the second thing we protect conductors from is short circuit and ground fault.
Short circuit and ground fault protection is always at the supply end of a circuit and always has to be, a conductor can be protected from SCGF by OCPD many times the conductors rating because the available fault current in most circuits will always be in the instantaneous region of the trip curve of most all breakers and fuses with instantaneous trip, usually within 5 cycles,
the only time this can be a problem is if the circuit is too long and the resistance of the circuit limits the fault current to a level out of the IT of the breaker curve this is why we must pay attention to how long a circuit is run when we chose the conductor rating and the SCGF protection for that conductor.
Now if we look are the requirements in 240.4(D) we will see that it allows us to use 240.4(E) or (G) for small conductors instead of the normal 15,20,30 amp ratings for small conductors (14-10awg) also 240.4(G) tells us to use the articles in table 240.4(G) for the applications listed in that table for conductor protection, most of these articles will also direct us to use table 310.16 for the conductor Ampacity instead of the Ampacity listed in 240.4(D) for small conductors.
With all that said, if we look at 440 for Air-conditioning and refrigerating equipment, part III will tell us what we must do for SCGF protection (440.21-) you will find out that in most cases that this is the max circuit protection that we find on the label on the equipment.
For overload protection we will have to look at part IV (440.31-) we will see that like above we will chose the conductors based upon the minimum circuit size on the label, again this is the end of the circuit overload protection I mention above, the overload is provided by the overload protection built inside of the compressor.
As far as the 125% this is always provided in the minimum circuit size because the manufacture has already included it by UL requirements.
Heres the commentary from the 2011 NEC hand book on this:
Branch circuits for listed air-conditioning and refrigeration
equipment that have a nameplate marked with the branch circuit
conductor size and branch-circuit short-circuit protective
device size are not required to have the branch-circuit
conductors sized in accordance with 440.33. The testing
laboratory standard includes the 25 percent increase for the
largest motor or compressor in the group plus the other non motor
or non compressor load; therefore, the actual nameplate
full-load amperes for the complete assembly can be
used to size the branch-circuit conductors.
So if the air-conditioning equipment has a label that gives you the minimum circuit size you use this for sizing the conductors, and the max is to size the OCPD for SCGF.
It's a very common saying for AC equipment you wire for the minimum and breaker for the max.
Also 440.31 directs us to article 310 for the sizing of the conductors and we would use table 310.16 for this in most applications, again as 240.4(D) stated that this article will over ride the small conductor requirements of 240.4(D) as 440 is in the list of 240.4(G)
I hope this helps with a little better understanding of why we see smaller conductors being protected by larger OCPD's they are still being protected, just not in a way we are use to, in this case they are protected by the overloads built in each compressor and fan motor in the air-condition equipment, if it is listed then go by the label on it.
