MCA Calculator

Minimum Circuit Ampacity (MCA) is the minimum current-carrying capacity required for the branch circuit conductors serving a piece of HVAC equipment. Full load amps (FLA) is the current a motor actually draws at rated load; MCA is the larger number the wire has to be sized for. Per NEC Article 440, MCA is calculated as 125% of the largest motor's full-load amperage (FLA) plus the sum of all other connected loads: MCA = (Largest Motor FLA × 1.25) + Other Loads. The 125% factor on the largest motor accounts for the continuous duty rating of motor circuits, ensuring conductors do not overheat during sustained operation. MCA determines the minimum wire gauge for the circuit—the selected conductor must have an ampacity rating equal to or greater than the MCA value from NEC Table 310.16. Equipment manufacturers list MCA on the unit nameplate because it depends on the specific combination of motors and loads inside the equipment. Electricians and engineers use this value to specify conductor size on electrical drawings and submittals.