Sizing an Air Conditioning Disconnect

A common thing that home inspectors look for when inspecting and air conditioning system is the maximum Overcurrent Protection Device (OCPD) that the exterior condenser’s data plate recommends. This can either be a circuit breaker or a fuse, and must be sized appropriately according to what that particular condenser manufacturer recommends. One of the basics when inspecting electrical wiring that home inspectors check is how big a wire is compared to it’s circuit breaker. If a wire is too small for the breaker, then you have a potential fire hazard. That National Electric Code 101. However, on HVAC circuits, this rule does not always apply.

On every condensing unit outside a home, there is a data tag located somewhere on the unit. A wealth of information can be found on that data tag (Go check yours out!). Two things (In addition to many others) that a home inspector should check for on that data tag are: Minimum and Maximum circuit ampacity. The minimum circuit ampacity tells us how big that wire supply power to the unit needs to be. For example, a 15 amp breaker typically requires a 14-gauge copper wire, 20 amps typically requires a 12-gauge copper wire, 30 amps typically requires a 10-gauge copper wire, and so on and so forth. If the data tag suggests a minimum circuit ampacity of 19.9 amps, then you know you are ok to have a 12-gauge copper wire. If the data tags says the minimum ampacity is 20.1 amps, then you need to bump up to a 10-gauge copper wire. The minimum circuit ampacity also tells us the minimum size the OCPD needs to be. It needs to be large enough to prevent nuisance tripping when the air conditioner first kicks on.

Now, here is the part that contradicts the National Electric code: the wire size does not have to match up with the circuit breaker size. The wire CAN be too small for that particular breaker, just so long as it is properly sized for the MINIMUM circuit ampacity. Air conditioning compressors contain thermal cutouts, which are designed to help prevent damage to the unit. These thermal cutouts would open up before damage to the wires occur. Now, it is possible for the breaker or fuse to be too big. The most common defect found on a home inspection is a circuit breaker or fuse that exceeds the recommendation on the data tag. This typically happens when an air conditioning condenser is replaced with a new one, and the HVAC technician fails to replace the OCPD as well, based on what the new unit recommends. I see it at least once or twice per week. Now, on to some more contradiction: the breaker at the electric panel CAN exceed the condenser’s data tag recommendation, as long as there are fuses that are properly sized at the exterior disconnect located at or near the condenser. Confusing right? That’s why it’s so often called out on inspections.

In short, manufacturer’s instructions and recommendations can always trump the National Electric Code, and HVAC is the most common place it is done.