Tonage Rating of Air Conditioner Unit

When I’m checking and testing air conditioning units, I like to report the tonage of the unit I sometimes cannot figure that out. Any help would be appreciated? Jim…

Jim, what method do you use now? You can usually decipher it from the model number or you can purchase a $95 Preston’s Guide that will help in many cases.

Michael, thank’s for the response. I had forgot about Preston’s manual. Will probably not get it at this point because of the cost. But it would be a helpful tool.
Yes, I do try to figure the tonage out from the model#. For the most part that work’s, but I was just looking for another method. Tonage is listed on my report and for that reason I like to list it. If I’m not sure, I state it.

Usually in the model number there is a number that is a multiple of 12.
36 is 3 ton
42 is 3-1/2 ton
48 is 4 ton, etc


You can tell by the size of the liquid line (smaller and warm) and/or by the amperage rating.

Raymond -

Although I teach HVAC classes, I’ve never had anyone say they could tell the size of the HVAC unit accurately by the size of the freon line???

Expand on this please.

Boy, that would be a tough one. I have never heard of that either. In fact, all of our units between 1 and 5 tons have exactly the same size lines.

But then what do I know, Certified Mechanical Inspector is only one of my hats.


That is what I was taught. I will try and find the info. Are the liquid lines all one size?

There’s also the RLA # that would help on getting an approx size


I am curious about your statement, do you use O.D. Caliphers to determine the sze of the line? Anyhoo, I thought you guys up in Canada did’nt require A/C or do you require it because of all the hot air blowing out of the states? :twisted:

The numbers on the compressor and make will tell you the compressor capacity.

Scott, I don’t believe that the amperage helps at all as the amps draw varries very much due to the efficiency of the unit. Older units inherently have higher amperage draws than newer ones.



Name plate amperage rating can be an indication, not the actual running amps, as Gerry said.

1 HP = 1 Ton refrigeration work.

If you know what a 1 HP motor requires, I guess it would get you close.

There are different efficiency motors, so all will not be the same.

Ive heard/read too that the LRA and the RLA could be used to determine the tonnage but found it to be at best a swag, particularly with today’s newer equipment. I first saw this method in Norman Becker’s (page 219) book on home inspections. As previously stated the best method is by looking at the model number. This can be difficult / impossible at times if the data plate or tag is faded, missing or painted over.

Dan et al:

Upon looking at my info, I have got it backwards… so I stand corrected.

Here is what I was taught. Its the suction line not the liquid line size that can help in determining tonnage.

7/8 = 5 ton
5/8 - 3/4 = 3 ton
3/4 = 4 ton
5/8 = 2 ton

other info that may be an indicator…

Cable size Amps Tons
6 ga. 50 5
8 ga. 40 4
10 30 3
12 20 2

9 amps per ton

12,000 btu/per ton.

FLA 7-7.5 = tonnage
RLA 5.5-6.5 = tonnage

Thanks for catching my goof.

Using the refrigerant line sizes isn’t a good guide (although perhaps it was as some point in the past). Many manufacturers just use a 3/4" SL connection for smaller units and 7/8" SL connection for larger units. Also the power draw (RLA) is all over the place now with the various efficiency ratings out there.

I agree the model number is usually the best place to get the capacity from, and if you dont have a Preston’s guide you can always call the manufacturer.

JMO & 2-nickels … :wink:

No… but when all else fails its good to know there are other avenues to estimate age. :wink:

I gave my PRESTONS guide away as it was so unrealiable and many manufactures are missing .
There is no way I will ever buy another.
Roy Cooke sr.

I have never had a Preston’s guide as using the nameplate data and other clues is usually sufficient, or in a pinch you can call the manufacturer if it’s otherwise not clear. I have heard many say the Preston guide is very useful, and many say it’s worthless. My understanding is thats not too bad a reference, but ya have to keep it current (new model numbers every year) … I guess to each his own … :wink: