Category: Heating & AC
AC Condensers, Understanding Over-current Protection
By John Chavez
Oct 26, 2007 - 4:34:51 PM

Over-current protection for AC condensers and motor circuits is different than the protection needed for circuits with simple resistive loads.

AC condensers are equipped with a fan to circulate the air through the coils that is powered by a hermetically sealed compressor motor. When the motor starts, it draws a larger amount of current than what is needed for the motor to run continuously.

The higher current load of the motor can easily exceed the rating of the breaker or fuse disconnect that is sized ONLY to protect the wiring or conductors for the unit. Although the higher starting load only lasts about 6 electrical cycles, or 1/10 of a second and this load is not enough to burn the conductor insulation or damage the wire, if the over-current device (disconnect) was only sized to protect the wire against the overloads, the disconnect device might trip and the condenser motor would not be able to start.

The condenser motor circuit gets around this potential problem by separating the function of the over-current protection device. To understand this better, it is important to know that typical disconnects will protect against “overloads,” which are currents that if allowed to run long enough could damage a circuit, and “ground faults and short circuits” which are   high currents that could potentially cause immediate damage.

AC condensers separate these two types of protection. The fuse or breaker disconnect only protects against “short circuits” and “ground faults.” The protection against overloads is actually built-in into the compressor itself. Here is where the “size of the conductor” rating comes into play where the wire size has to be rated to carry the running load of the equipment, while the disconnect device (fuses or breakers), must be large enough to allow the condenser motor to start.

So in short, the breaker or fuse disconnect is not rated to protect the wire or conductors against overloads. The wire is actually protected against overloads by the built-in thermal protection of the compressor.

It is not unusual to see number 10 wires connected to a 40 or 50 amp breaker if the conductor rating of the label is below 30 amps and the breaker rating is labeled at or below the 40 or 50 amp rating.

The NEC (National Electrical Code) (440-4b) says that the specs listed on the condenser label should not be exceeded.

The IRC (International Residential Code) says the following: “E3602.11 Branch Circuits for Air Conditioning and Heat Pump Equipment. The ampacity of the conductors supplying multi-motor and combination load equipment shall not be less than the minimum circuit ampacity marked on the equipment. The branch-circuit over-current device rating shall be the size and type marked on the appliance and shall be listed for the specific purpose.”   In other words, DO NOT change the conductor or disconnect breaker or fuses from what is written on the label.

It is also important to understand that the condenser label may list for fuses to be used as the disconnecting device. In this case, fuses should be used in a separate disconnect box if the main system is all breakers, typical on some older units. In other instances the label will call for a “HACR,” (Heating, Air Conditioner, and Refrigeration) type breaker which is the type of breaker that should be used. On some units, the label will not specify what type of breaker or disconnect to use and only list the maximum disconnect device rating. For these units, any type of disconnect device would work so long as it is properly rated to within the label specs.

It is also possible to have e pull disconnect at a box near the AC condenser and the main protection disconnect device at the main panel or sub panel. This is acceptable so long as the pull disconnect is properly rated for the loads of the unit.

AC Condenser Label. A JC Media Photo

On the condenser label in this article, the minimum ampacity of the conductor is 22 amps, which can be provided by a number 10 copper conductor, while the minimum protection device is listed as 30 amps and the maximum at 35 amps. Notice the asterisk or start mark by the minimum and maximum listings for the breaker sizes, which specify at the bottom of the label that a “HACR” type breaker must be used as the protection device if a breaker is used. However, the label also lists that a fuse within the minimum of 30 amps and maximum of 35 amps can also be used in place of a breaker.