Do you know what size your air conditioner is? In the world of building science, you'll hear a lot of talk about why oversized air conditioners are a bad idea. Briefly, they may not dehumidify as well, short-cycling wears them out quicker, and your home will probably be less comfortable if the air conditioner is too big. But to know if your AC is oversized, first you have to know what size it is. (Note: This article is about finding the size of your existing AC, not determining what size you need.) Look for the label The good news is that most HVAC manufacturers make it easy to determine the nominal capacity of your air conditioner. It's in the model number. Go outside and find the outdoor unit, that metal noisemaker hidden away on the side or the back of the house. It'll look something like the one you see above, although maybe not quite so decrepit as that one. Then find the lable that gives the data about your AC. It'll look like the image below. Up near the top of the label, you see the model number (M/N) and serial number (S/N).

The model number is where you can find the number you're looking for. Not all manufacturers do this, but most will give you a 2 or 3 digit section that tells you how many thousands of BTU/hour your air conditioner can move out of your home. The first section in the model number gives you info about the type and efficiency of the unit you're looking at. In the case of this Lennox model (which, by the way, is not from the outdoor unit shown at the top of this article), the 13HPX tells you it's a heat pump with an efficiency rating of 13 SEER. The digits you need Just past that string of 5 characters, though, is the part that tells you the nominal size: 048. That means the air conditioner—or heat pump in cooling mode in this case—has a nominal capacity of 48,000 BTU/hour. I say nominal because the actual capacity is almost certainly going to be different. The numbers you'll see on residential air conditioners and heat pumps are: The 3 digits in the model number tell you the nominal capacity in thousands of BTU/hr.

Since each 12,000 BTU/hr is equivalent to 1 ton of air conditioner capacity, it's easy to figure out how many tons of nominal capacity your AC has. Why Is Air Conditioner Capacity Measured in Tons? 3 Reasons Your 3 Ton Air Conditioner Isn't Really 3 Tons Why an Oversized Air Conditioner Is a Bad IdeaYour comment will not appear below until approved.Choose the right size air conditioner to cool your home. Buying the right size of air conditioner is in your best interest; only the right size of unit or system will efficiently and effectively keep you and your family comfortable. Air conditioners that are too big use more electricity and leave the air in your house with excess humidity. Air conditioners that are too small do not cool your home to a comfortable temperature. Choosing the right size air conditioner requires some basic math and just a few minutes of your time. Measure the square footage of the room that you'll air condition. Use a tape measure to measure the length and width of the room.

Write these measurements on your paper. If the room is not square or rectangular, divide the room into sections composed of squares and triangles and take those measurements. buy central ac unit onlineCalculate the square footage of the room. types of central air conditioner unitsFor a square or rectangular room, multiply the length by the width. how to recharge ac unit at homeIf your room is some other shape, find the square footage of the triangular sections by multiplying 0.5 x length x width and sum each of the square and triangular sections to determine the total square footage of the room. Determine the square footage of each room of your home if you are purchasing a central air conditioning system. Reviewing your property tax statement or contacting your county auditor's office is another way to find out the total square footage of your home.

Don't include areas, like the basement, that won't be cooled. Determine the cooling capacity needed for the room or your home. Cooling capacity for room air conditioners and central air conditioning systems is measured in British Thermal Units (BTUs) per hour. The United States Department of Energy Energy Star website offers these guidelines for BTUs per square footage: 100 up to 300 sq. feet: 5,000 to 7,000 BTU 300 up to 550 sq. feet: 8,000 to 12,000 BTU 550 up to 1,000 sq. feet: 14,000 to 18,000 BTU 1,000 up to 1,200 sq. feet: 21,000 to 24,000 BTU 1,500 up to 2,000 sq. feet: 30,000 BTU 2,000 up to 2,500 sq. feet: 34,000 BTU Things You Will Need Steel measuring tape Paper Pencil Tip References Energy Star: Properly Sized Room Air Conditioners Photo Credits Jupiterimages/Comstock/Getty Images Suggest a Correction AC Unit Maximum Breaker Re: AC Unit Maximum Breaker Is this new construction? Because it is only 5 amps I would look closely at the wire and insure that it is rated for the higher amperage under an inductive load.

It may be wrong IAW the Mfg. but an over current device is there to protect the wire on the branch not the appliance. If the wire to the unit is rated for the breaker size at least you won't have a fire hazzard to the unit. Also a concern is if the breaker is rated for HVAC use. Also consider that the condenser may have internal wiring matched to the Max Fuse/Breaker Rating indicated on the label, and an older oversized breaker may not trip if the compressor motor locks up ... which could complete fry the equipment ... IMO report it as a defect needing evaluation/repair and call it a day ... Consulting Engineer & Inspector LIU CW Post Adjunct Professor I am absolutely amazed sometimes by how much thought goes into doing things wrong ... every HVAC guy and electrician I speak to say it is alright to go to the next 5 amp size. They want to see that the wire size is consistent with the higher size. None of them will change the breaker when called out to confirm this.

At ITA I remember them saying that the wire size can be smaller if matched to the minimum ampacity rating on the dataplate. Being an HVAC tech for a long time that is what would happen. The tech will say no problem. As long as the breaker is correct for the wire size 5 amps isn't a big deal. Start up amps can be in the 60-amp range. I've seen units under this same scenario (25 amp nameplate) that would cause nuisance trips when a 25-amp breaker was installed and when the 30-amp breaker was put in the trips stopped and there were never any problems. It is over fused if a breaker larger than the manufacturer's label calls for is installed. Here is a good explanation I picked up somewhere: A/C condensers contain a hermetically sealed compressor motor as well as a fan to circulate air across the coils. The rules for protection to motor circuits are different than for circuits with simple resistive loads. When a motor first starts , it draws a much higher amount of current than it does after it is running.

The high "inrush" current can exceed the rating of a breaker or fuse sized to protect the wire. The inrush current lasts typically only about 6 electrical cycles, or 1/10th of a second - less time than it would take to damage the wire or its insulation. However, if the overcurrent device is sized to protect the wire against overloads, the device might trip, and the machine would not be able to start. Motor circuits get around this problem by dividing the two separate functions of an overcurrent protection device. "Overloads" are currents that can damage a circuit if allowed to continue for a sufficient time, whereas "short circuits" and "ground faults" are high currents that can cause immediate damage. Large motors and air conditioners separate these overcurrent functions. The breaker or fuse ahead of the air conditioner only protects against short circuits and ground faults. Overload protection is built into the compressor itself. The wire to a condensing unit must be large enough to allow the equipment to start.

The device is therefore NOT sized to protect the wire against overloads. It is not uncommon to see a 50 Amp breaker on a #10 wire. The wire is protected against overloading by a separate thermal protection inside the compressor. How do we know what the motor requires for wire size and proper fuse/breaker protection? It's all on the mfg. label attached to the appliance. If the label calls for fuses, there MUST be a fuse in the disconnecting means, not a circuit breaker. If the label calls for a "HACR" type breaker (Heating, Air Conditioning and Refrigeration), then that is the type of breaker that must be used. If the label simply states "maximum sized over current protection" then any form of overcurrent protection may be used. The HACR designation is not always visible on a circuit breaker once it is installed inside a panel or disconnect. Note on the label taken from a Lennox A/C data plate:FUSE - 25 UL (CSA is a Canadian standard. We use UL in the U. S.)UL - Not allowed in U. S.

All this means is the MINIMUM wire size required is #12 (solid copper); the MAXIMUM rating of a fuse (if a fuse is used) must be 25 amps and the MAXIMUM rating of a circuit breaker (if used) must be 25 amps AND the breaker must be a HACR type.However, if the over current device is sized to protect the wire against overloads, the device might trip, and the machine would not be able to start. "it draws a much higher amount of current than it does after it is running." This is called an inductive load. "The high "inrush" current can exceed the rating of a breaker or fuse sized to protect the wire. " This is why HVAC breakers must be rated for HVAC use. The wire, breaker (to protect the wire), and type of breaker must be correct and is reflected in the "minimum circuit data". Michael MerinoMerino's Home Inspection & Education Inc." IF YOUR INSPECTOR IS NOT USING THERMAL IMAGING, YOU'RE NOT GETTING THE WHOLE PICTURE �Jeff Pope JPI Home Inspection Service

Santa Clarita Home Inspection Originally Posted by jpope I understand that the circuit is over-fused, but the appliance is connected to that circuit. If for some reason it were to draw more than 30 amps (outside of the inductive load) the breaker will not trip. I am confused by the differing opinions. I have been told by inspectors it is OK, and some say it is not. Some say that you can over-fuse the AC up to 40%. I do not seem to be able to find a definitive answer. I am thinking that if the max breaker is supposed to be 30 amp, then the breaker should be 30 amp not 50 amp, even if the conductor is the proper size. Professional Certified Master Inspector In Edwardsville, Glen Carbon, Godfrey, Maryville, Fairview Heights, O'Fallon and surrounding areas of Illinois Metro-East Need a home inspection in South Dakota? listing of South Dakota certified home inspectors. As a Professional Home Inspector, I support the privacy of my clients. Services provided in East MN and West WI