I run into this often. #10 wire on a 40-amp breaker which powers the A/C condenser unit. By code, 40-amp breakers should have #8 wire or larger. I’ve been told that when its for an A/C that #10 on a 40 is OK since you only need it for starting current of the compressor.
Is this allowed by code? Is it simply accepted in practice? Or is it not accepted?
Look at the data plate on the condenser. It will state the max and minimum breaker size per the manufacturer. As long as the wiring size covers the minimum required breaker size, you are ok.
The data plate will probably say something like this:
Max overcurrent protection: 40 amps
Min circuit ampacity: 28.5 amps
Wiring for these motor circuits are sized differently.
“The manufacturer has already calculated the conductor size to be based on the total of all of the motor loads in the combination-load equipment times 125 percent. It is not necessary to do these calculations again. For this type of equipment, the installer and the inspector only have to install and verify that the branch-circuit conductors supplying the equipment have an ampacity equal to or greater than the minimum circuit ampacity marked on the nameplate of the equipment.”
Seems to me, if you had a fault in the circuit(a slight short) the # 10 wire could draw 125% of 40 amps without tripping the breaker causing the conductor to overheat and possible threat of fire.I was always under the impression you could never put a conductor on less rated ampacity than the overprotection device.
You have to remember there are two levels of protection of a circuit. You have short circuit protection and overload protection. In most cases both are taken care of by the branch circuit O/C device (fuse/breaker for you home gamers)
In the case of a larger motor based appliance you can have overLOAD protection in the motor starter or thermal protection in the motor. This is what keeps the wire from burning up. Short circuit protection is going to be at a much higher level, typically 175% of full load amps, in the case of a motor. They are recognizing that a short circuit will draw a huge amount of current, compared to the safe normal load.
Another example of this principle is an 18ga lamp cord or fixture wire. Since the load is limited by the light sockets or other equipment installed, you can’t burn up the cord with an overload but an 18ga in limited length has the ampacity to operate the 15/20a O/C device if you have a short.
Of course you can still overload this circuit if you put in a medium base to receptacle adapter or hook up a 1440w heater on a 18-16ga extension cord, but nothing is idiot proof.
And who said inspectors cannont think outside of the box. Seriously though, that is the kind of stuff we can run into in our line of work–what seperates the good from the best is ones ability to explain why this is dangerous because pop will assert that it has been that way for 5 years.
Note that there is always (often TOO)conservative safety built into everything; the actual enclosed continuous current rating for #10 wire is 33amps, and the free-air rating (not enclosed) is 55A, and those are at the wires maximum temperature rating- usually 60/75C (140/167F). Also note that most current ratings for wires (except magnet wires) are based on permissible voltage drop, not temperature rise, and voltage drop is determined out to distances of 1000 feet, which is not typical at all of residential or even industrial applications. Add to that the fact that the standard “rule” for breaker operation current is 80% or less…
In reality you can do what ever you want in your own home (or business or equipment) when nobody is watching, but obviously you need to know what you’re doing…and do so at your own discretion. You can do something at home like put two 20 amp breakers (40 amps total) on a #10 cable (30 amp max rule), and have absolutely no issues at all, especially if the cables are not enclosed in a conduit and just run in an attic or subfloor. This would allow you to simultaneously operate two power tools typically 15A max each (like a miter saw, etc), and still be under the 80% breaker limit of 16A each (for a total wire current of 30A). You would not have to worry about the 12A 80% limit of the 15A breakers that would be “required” on a 30A wire, which would cause problems with 15A tools on 15A breakers.
All of this information is freely available from various sources.