conductor size for 240volt air conditioning unit

[nmbme]

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,

[ January 24, 2005, 09:51 PM: Message edited by: nmbme ]

[drg]

See 240.4 (D) for your answer. NEC 2002

[aelectricalman]

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.

[electricman2]

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.

[charlie]

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.

[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? I realize from the disconnecting means you can use #12 to the unit

[peter d]

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?

What does the nameplate require for its minimum circuit ampacity?

If the MCA is 30 amps or less, then the 10/2 is fine.

[busman]

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

[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

[allenwayne]

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 .