The label of every piece of HVAC equipment is marked with a three letter acronym (TLA) and a number. That TLA is MCA or Minimum Circuit Ampacity. The MCA is a single number used to combine all the significant electrical loads in piece of equipment into a single number. All that is needed at that point is to reference the NEC and one can determine the minimum conductor size for the branch circuit supplying the equipment.

To calculate the MCA for the equipment you must first determine the full load amperage(FLA) of all electrical loads in the equipment over 1.0 amps. Compressors do not have FLA ratings, but instead have RLA ratings. You will use that instead of FLA for compressors. Once you have the FLA data you need to organize the data into groups, such that, each group contains all of the loads that simultaneously operate in one particular mode of operation. For most pieces of HVAC equipment this will be two groups; a cooling mode group and a heating mode group. The MCA is then calculated with the following formulas in each mode of operation. The largest of the MCAs is selected and that is the MCA for the equipment. MCA is usually listed with one digit to the right of the decimal.

MCA formulas:

MCA=1.25(Load 1+Load 3) + (Load 2) ;if Load 3 is under 50 kW.

MCA=1.25(Load 1) + (Load 3) + (Load 2);if Load 3 is 50 kW or more.

Load 1 is the largest motor load.

Load 2 is the sum of all other non-electric heater loads.

Load 3 is the electric heater load.