Periodically, we must return to the basics on short circuits, ground faults and overload protection for motors and air conditioning equipment, also known as HVAC systems. What many people may not understand about motors and air conditioning systems is that Article 430 is a base article for most motors. Article 440 covering air conditioning and refrigeration equipment provides special considerations necessary for circuits supplying hermetic refrigerant motor compressors. 

Article 430 is used for any motors and includes many requirements for air conditioning and refrigeration systems. Article 440 is used to supplement or amend the requirements in Article 430 whenever a hermetically sealed air conditioning system is installed. In other words, Article 440 can be viewed as an exception or a supplement to Article 430. 

Compressors and motors

Article 440 does not apply unless the system employs a hermetically sealed refrigeration compressor. Many of the same requirements for disconnecting means, controllers, conductor sizing and the overcurrent protective devices (OCPDs) in Article 430 are similar in Article 440 for air conditioners. Articles 430 and 440 apply to supplying multiple motors in equipment, which is a common issue.

For example, when figuring the size of conductors for a multiple motor installation or a motor and other loads, 430.24 applies as follows: The conductors supplying multiple motors or a single motor with other loads would be based on 125% of the full load current of the highest-­rated motor and the sum of the other motors at 100% of the full load current of any motors in the group, as well as 100% of any noncontinuous nonmotor load and 125% of any c­ontinuous nonmotor load. 

Full load current, based on 430.6(A)(1), requires using the full load currents located in Table 430.247 for DC motors, Table 430.248 for single-phase AC motors, Table 430.249 for two-phase motors and Table 430.250 for three-phase motors. 

Using the nameplate full load amperes (FLA) on most motors would not normally provide the maximum size for conductors, the proper size of disconnecting means or the appropriate size OCPDs for the motor. Motors may come with low, medium or high power factors for the same motor horsepower. Low power factor motors would have more power loss than higher power factor motors.

With air conditioning and refrigeration systems employing hermetically sealed compressors, the compressor motor is actually embedded in the coolant. Two definitions must be understood when dealing with air conditioning and refrigeration systems. 

Rated-load current is the current of a hermetic refrigerant motor-compressor resulting when it is operated at the rated load, rated voltage and rated frequency of the equipment it serves. 

Branch-circuit selection current is the value in amperes to be used instead of the rated-load current in determining the ratings of motor branch-circuit conductors, disconnecting means, controllers, and branch-circuit short-circuit and ground-fault protective devices wherever the running OCPD permits a sustained current greater than the specified percentage of the rated-­load current. The value of branch-circuit selection current will always be equal to or greater than the marked rated-load current. 

Sizing conductors

For sizing conductors for air conditioning units, 440.33 states that conductors supplying one or more motor-compressors, with or without additional motor loads, must have an ampacity not less than the sum of the following: the sum of the rated-­load or branch-circuit selection current (whichever is greater) of all compressor motors, the sum of all other motors and 25% of the highest motor-compressor or motor full load current of the group of motors. 

The branch-circuit selection current located on the air ­conditioning nameplate should be used because the compressor motor is embedded in the coolant, and the higher the pressure of the coolant during very hot days, the more the compressor motor will draw current due to the compressor’s high head pressure. 

The manufacturer and the listing laboratory will have tested and calculated the highest head pressure of the hermetically sealed motor compressor and will have posted the maximum size OCPD based on the high head pressure and the minimum size of conductor required for that motor-­compressor, the cooling motor and the inside blower motor in the unit.

Similar to regular motors, an air ­conditioning or refrigeration system can have a conductor much smaller than the OCPD rating. For example, a minimum ampacity on the nameplate could be 25A and with the maximum fuse or circuit breaker may be 40A (compressor motor is 17.6 RLA at 125% [22A] and the outside blower motor of 2.8 FLA at 100%). 

Any authority having jurisdiction should not have a problem with this installation; however, I have had some in the past that disagreed with having an OCPD larger than the conductor.

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