conductor size for 240volt air conditioning unit

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nmbme

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I am curious as to what size conductor should be used for a 240 volt , 2 1/2 ton a/c unit, in a 1 & 2 family dwelling. I am being told, that 10/2 romex( nm-b) is of sufficient size. I tend to disagree,seeing how the "B" indicates that it is rated for 90 degrees C.. but you would still have to use the 60 degree table for the proper ampacity. yes or no ?
I am also being told that 10/2 romex(nm-b)can fused up to 40 amps for the same application, :confused:

[ January 24, 2005, 09:51 PM: Message edited by: nmbme ]
 
Re: conductor size for 240volt air conditioning unit

See 240.4 (D) for your answer. NEC 2002
 
Re: conductor size for 240volt air conditioning unit

A 2.5 ton uses about 4400 VA which is about 18-19 Amps for a 240 circuit. You will want to use a 30 Amp circuit. As for the 3 ton (4700VA), you should use a 30 AMPs as well. You will not need a 40 amp cirucit for a 3 ton unit.
 
Re: conductor size for 240volt air conditioning unit

The nameplate will tell you the MCA (minimum circuit ampacity). It will also state the maximum size of the fuse or circuit breaker. Many times this will be above the ampacity of the conductors. It is for ground fault and short circuit protection.
 
Re: conductor size for 240volt air conditioning unit

The concept that you need to grasp is that for motors and HVAC equipment, the overload devices for the circuit are in the unit itself, not in the overcurrent device that is protecting the circuit. The device that is protecting the circuit is providing short circuit and ground fault protection, not overload protection.

It is possible to have a #12 Cu. NM protected by a 40 ampere short circuit and ground fault protective device. :D
 
Re: conductor size for 240volt air conditioning unit

O.K. , I guess what I should be asking is , will #10/2 romex be of sufficient size from the O.C.P.D., rated at 35 amps, to the a/c disconnecting means? I realize from the disconnecting means you can use #12 to the unit
 
Re: conductor size for 240volt air conditioning unit

Originally posted by nmbme: O.K. , I guess what I should be asking is , will #10/2 romex be of sufficient size from the O.C.P.D., rated at 35 amps, to the a/c disconnecting means?
Click to expand...
What does the nameplate require for its minimum circuit ampacity?

If the MCA is 30 amps or less, then the 10/2 is fine.
 
Re: conductor size for 240volt air conditioning unit

Also remember that if the nameplate does not say "max size fuse or circuit breaker", but says "max fuse size" then a fuse must be used.

Mark
 
Re: conductor size for 240volt air conditioning unit

I fiquired there would be more answers to this , considering the fact that were only talking about an a/c unit. (which could burn a house down if the wire size is not rated for it. I'll say again, what if the min.size amps required is 40 amps , can you still use 10/2 ? how about if the min. was 35 amps ? lol
 
Re: conductor size for 240volt air conditioning unit

I have always used this rule of thumb, wire for min. and fuse/bkr. for max. If the min. ampacity is 35 then # 8 nm is the min.sized conductor allowed.The required ocpd is whatever the nameplate lists as the max.fuse/bkr. size .The conductors that feed the unit must meet ampacity of the min. listed ampacity,NMBME if the min. listed ampacity is 40 then # 8 would be the smallest conductor allowed same for 35 . assuming this is residental and NM is the wiring method .
 
Re: conductor size for 240volt air conditioning unit

What everyone is trying to tell you is, "Ya gotta reed thu nameplate on thu AC." UH OH, I think I just broke the spell checker. :D
 
Re: conductor size for 240volt air conditioning unit

This is an un-scientific way to do this but the "rule of thumb" that is most commonly used for sizing feeds to AC units is "10 amps per ton". If you're any fraction of tons over that, go to the next size up. If you're not sure about the size wire to run - go to the next size up and install the lower size breaker. It's a lot easier to replace the breaker than it is to replace the wire. Why spend time on wondering what the proper minimum size is when it's much easier to bump the wire size up one notch and be done with the job.

BTW, you can't always expect 100 responses to your questions. A lot has to do with the way you present your question, the way you come across, how many subscribers are interested in your topic and how many are willing to respond. I've posted many questions here that I thought would warrant a larger response and got far fewer than expected. So, don't get discouraged and keep posting.
 
Re: conductor size for 240volt air conditioning unit

Originally posted by nmbme:
I fiquired there would be more answers to this , considering the fact that were only talking about an a/c unit. (which could burn a house down if the wire size is not rated for it. I'll say again, what if the min.size amps required is 40 amps , can you still use 10/2 ? how about if the min. was 35 amps ? lol
Click to expand...
nmbme, as many have pointed out it is all about the name plate on the unit.

You have never stated in clear terms what this units label requires for the MCA (minimum circuit ampacity) The MCA is not the size of the breaker used at the panel.

Here is an example of nameplate info from a HVAC unit.

Trane Condensing Unit

Unit Model No. TTA120A4

Tons 10

Unit Operating Voltage Range 414-506

Minimum Circuit Ampacity 23.3

Maximum Fuse Size or Maximum Circuit Breaker 35
Click to expand...
These specifications came from a Trane pdf that can be downloaded here

If I choose to use the 35 amp maximum breaker and run the circuit using NM would I be required to run 8/2 or could I safely choose 12/2?


I could safely use the 12/2 to run the 23.3 amp unit with a 35 amp breaker.

[ January 30, 2005, 07:02 AM: Message edited by: iwire ]
 
Re: conductor size for 240volt air conditioning unit

Really??? That sounds jacked up! :)

I tried wading through 440, couldn't glean a lot out of it. Wasn't in the right mood at the time. :D

Where do I find the references that make this installation legal?
 
Re: conductor size for 240volt air conditioning unit

I would guess that by using the 12-2 cable and reindentifing the grounded conductor as a ungrounded conductor this would be able to carry 40 amps on the 240 volts circuit that does not require a neutral.
My thoughts on what he is saying.
 
Re: conductor size for 240volt air conditioning unit

quote
I could safely use the 12/2 to run the 23.3 amp unit with a 35 amp breaker.
I see where you got the 35 amp breaker, but can you explain how the #12 can carry 23.3 amps?

Thanks

Jim T
 
Re: conductor size for 240volt air conditioning unit

Jim,
The ampacity of #12 at 60?C is 25 amps per Table 310.16. 240.4(G) applies to this installation. 240.4(D) does not.
Don
 
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