#10 wire on 40-amp breaker for A/C

[mmorgan]

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?

Thanks,


Mike

[Andrew Walters]

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."

[mmorgan]

Andrew,

Thanks for the info.



Mike

[Paul W. Abernathy]

Yeppers.....Got here late on this one.....Nicely Done Andrew......saved me from going into alot of additional detail...

Always refer to the Name Plate...... Saves you all the technical MATH...

[billy6139]

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.

[Larry D. Kage]

Originally Posted by billy6139

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.

Exceptions...the older I get the more I realize the variables on my absolutes.

[Greg Fretwell]

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.

[Michael W. Gault]

Originally Posted by Greg Fretwell

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.

Worth a repeat!

[Greg Fretwell]

I am surprised an adapter from medium base bulb socket to NEMA 1-15R even exists. It can be the biggest potential hazard I can think of, using "listed" equipment you can buy at the Piggly Wiggly.

[brian winkle]

Originally Posted by Greg Fretwell

I am surprised an adapter from medium base bulb socket to NEMA 1-15R even exists. It can be the biggest potential hazard I can think of, using "listed" equipment you can buy at the Piggly Wiggly.

Greg I think the biggest potential hazard is single pole 30 amp circuit breakers sold at home centers.

[Greg Fretwell]

Changing a breaker is not something a teenage girl is likely to do. Plugging her hair dryer into the sconce above her bed, using one of these adapters, is more likely.

[brian winkle]

ok how about a hair dryer plugged into a socket adapter screwed into a sconce wired with #14 connected to a 30 amp breaker installed by Pop.

[jthalmann]

Originally Posted by brian winkle

ok how about a hair dryer plugged into a socket adapter screwed into a sconce wired with #14 connected to a 30 amp breaker installed by Pop.

[Jeremiah Anderson]

ok how about a hair dryer plugged into a socket adapter screwed into a sconce wired with #14 connected to a 30 amp breaker installed by Pop.

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.

[Russel Ray]

Originally Posted by Greg Fretwell

Piggly Wiggly.

?????