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The Different Types of Amperes For HVAC Motors & Contactors
The Different Types of Amperes For HVAC Motors & Contactors
In heating, ventilation, and air conditioning (HVAC) systems, electrical current behaves differently depending on whether it is starting a machine, running it normally, or passing through a switch.Â
Because of this, motors and contactors use entirely distinct sets of ampere metrics to ensure safe operation.Â
1. Amperage Types for AC Motors
AC motors (specifically the compressor and condenser fan motors) rely on specific current metrics stamped on their labels to indicate power draw under various mechanical loads:Â
- RLA (Rated Load Amperes) / Running Load Amps: The continuous current a compressor draws during normal, steady-state cooling operations. Technicians use RLA to evaluate system health and verify normal cooling pressure performance.Â
- FLA (Full Load Amperes): The maximum continuous current a fan motor is designed to draw under its heaviest mechanical load. While RLA is preferred for the compressor, FLA is exclusively used for standard induction motors like condenser and evaporator fans.
- LRA (Locked Rotor Amperes): The brief, massive surge of starting current pulled the exact moment the motor attempts to turn from a complete standstill. Because the rotor isn’t spinning yet to create back-EMF (Back Electromotive Force), LRA can be 5 to 6 times higher than normal running current.Â
- MCA (Minimum Circuit Ampacity):Â A calculated safety value printed on the outdoor unit. It dictates the minimum thickness required for the field electrical supply wiringÂ
2. Amperage Types for Contactors
A contactor is an heavy-duty electrical switch that uses an electromagnetic coil to snap internal contacts shut, powering up the compressor and fans. Its current capacity is rated based on the type of electrical load it is handling:Â
- Inductive Amps (AC-3 / FLA Rating): The rated current capacity when handling highly inductive motor loads. When a compressor kicks on, the contactor must withstand the brutal arc-flash of the LRA, and when it shuts off, it breaks the running current. The inductive FLA rating is always much lower than its resistive rating.
- Resistive Amps (AC-1 / Resistive / General Purpose Rating): The maximum current capacity when switching purely resistive loads that have no startup surges, such as the electric auxiliary heat strips found in heat pumps. Because there is very minimal arcing when switching resistive loads, a contactor can handle significantly higher amperes under this category.
- Ith (Thermal Current Rating): The absolute maximum current the physical metal contacts can carry continuously for 8 hours without overheating, assuming the switch never cycles open or closed.
