Find your equipment's motor rated current (MRC) on the unit's data tag/nameplate or in the product catalog. Some manufacturers also call it the unit's full load amps (FLA).
It is usually determined using high-current tests by the manufacturer.
Find your unit's heater current, which is also labeled either on the data tag/nameplate or the product catalog.
Use the data you collected to fill in the following equation and determine the MCA: MCA = 125 x [motor rated current + heater current].