Need clarification

The data plate of this fairly new Exterior Trane A/C unit shows a minimum circuit ampacity of 17 Amps and is provided with a 12 AWG Cu (rated for 20 Amp) conductor connected to a 30 Amp breaker (unit disconnect also has 30 amp fuses).

I figure that this is so the breaker doesn’t trip at start up each time. But the question I have is**…should the conductor be a 10 AWG Cu (for 30 amps) or is the 12 AWG CU considered okay since the start up surge is so short and unlikely to damage the 12 AWG Cu or create a fire hazard.**

My feeling is that the conductor is too small…but wanted to check it out before submitting my report (later tonight)




Nope, there are special exceptions for things like motor loads and HVAC loads. The breaker in this sort of installation is providing the short circuit and ground fault protection, and the thermal overloads in the compressor unit are providing the overload protection.

It is quite common to see a great disparity between the breaker ampacity and the wire gauge for AC compressor feeds. This is completely permissible.

“Wire to the min, breaker to the max”, are the limits of this rule when reading the dataplate.

Nice quote Marc, I hope you don’t mind considering it stolen :wink:

Very well put



Nope, not at all. That’s my memory aid too.

I just noticed that this Trane dataplate also notes “min fuse or breaker size”. That’s something you seldom see noted on a dataplate. I suspect they do this to assure trouble-free future operation and not to mitigate any particular hazard. Usually it’s just “min circuit ampacity”, which relates to the wire gauge, and “max fuse or breaker size”, which relates to the overcurrent protection size.

Agreed, it would be nice if all manufacturers spelled it out interms of wire guage and breaker sizing, we would see many fewer potential issues.

BTW, down here duplicating it in Spanish would be a huge plus!!




I’ve seen both min and max listed a lot but rarely have I seen them both listed at the same breaker size for both min and max. Figured this why there may be a need for 10 AWG.

Thanks for your answer…I liked the quote…easy to remember.

Gerry…are you claiming it as your quote originally?

Thanks again…


Same on the vapor barrier of attic insulation. I constantly see it installed upside down.

Hi Jeff, heck no, credit where it’s due, but I will use it, and I won’t forget where I got it either, I love simple ways of remembering comlex issues.



Congradulations guys this is the way a message board is suppp0se to work ask a question get an answer. I used to have a good friend and tool buddy that drove me crazy in a good sort of way you could ask him the time of day and he would tell you how to build a watch. Keep up the good work:D :smiley:

Sorry, I won’t let it happen again. :mrgreen:

Not every question has quite as straightforward of an answer as this one did.

Define “upside down” It is not the same in places where AC is your biggest season as it is where heating is the biggest season.

If you have the AC on more than the heat you probably want the vapor barrier on the top where the warm moist air usually is.

I agree Marc as you do provide straight answers.

Upside down…as in the flammable vapor barrier exposed to the attic space.

Pretty dry in San Jose California. Definately more furnace days than Air Cond days. Need A/C about 5-10 days per year. Furnace for 60-90 days/year.