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  #1  
Old 2/15/06, 10:23 AM
mmorgan mmorgan is offline
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Default #10 wire on 40-amp breaker for A/C

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
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  #2  
Old 2/15/06, 10:42 AM
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Andrew Walters Andrew Walters is offline
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Default Re: #10 wire on 40-amp breaker for A/C

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."

Last edited by awalters; 2/15/06 at 10:45 AM..
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  #3  
Old 2/15/06, 10:52 AM
mmorgan mmorgan is offline
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Default Re: #10 wire on 40-amp breaker for A/C

Andrew,

Thanks for the info.



Mike
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  #4  
Old 2/15/06, 2:50 PM
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Paul W. Abernathy, CMI Paul W. Abernathy, CMI is offline
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Default Re: #10 wire on 40-amp breaker for A/C

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...



Paul W. Abernathy
Manager, Electrical Codes & Standards
Encore Wire Corporation

“Promise me you'll always remember: You're braver than you believe, and stronger than you seem, and smarter than you think.” -RIP Dad, Love You!....10/10/2014

Disclaimer - All my posts on this forum are of my own personal opinion and in no way represent the opinions of Encore Wire, or other associations, organizations or affiliations.
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Old 3/22/06, 11:18 AM
billy6139 billy6139 is offline
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Red face Re: #10 wire on 40-amp breaker for A/C

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.
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Old 3/22/06, 12:49 PM
Larry Kage, CMI Larry Kage, CMI is offline
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Default Re: #10 wire on 40-amp breaker for A/C

Quote:
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.
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Old 3/22/06, 1:22 PM
Greg Fretwell Greg Fretwell is offline
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Default Re: #10 wire on 40-amp breaker for A/C

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.
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Old 3/22/06, 1:25 PM
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Michael W. Gault Michael W. Gault is offline
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Default Re: #10 wire on 40-amp breaker for A/C

Quote:
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!
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Old 3/22/06, 9:28 PM
Greg Fretwell Greg Fretwell is offline
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Default Re: #10 wire on 40-amp breaker for A/C

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.
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Old 3/22/06, 9:44 PM
brian winkle brian winkle is offline
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Default Re: #10 wire on 40-amp breaker for A/C

Quote:
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.
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Old 3/22/06, 10:09 PM
Greg Fretwell Greg Fretwell is offline
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Default Re: #10 wire on 40-amp breaker for A/C

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.
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Old 3/23/06, 9:26 AM
brian winkle brian winkle is offline
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Default Re: #10 wire on 40-amp breaker for A/C

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.
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  #13  
Old 3/23/06, 9:36 AM
James Thalmann James Thalmann is offline
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Default Re: #10 wire on 40-amp breaker for A/C

Quote:
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.

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Old 3/23/06, 11:08 AM
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Jeremiah Anderson Jeremiah Anderson is offline
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Default Re: #10 wire on 40-amp breaker for A/C

Quote:
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.
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Old 3/24/06, 5:40 AM
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Russel Ray Russel Ray is offline
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Default Re: #10 wire on 40-amp breaker for A/C

Quote:
Originally Posted by Greg Fretwell
Piggly Wiggly.
?????



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