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How do I Calculate MCA?

The minimum circuit ampacity (MCA) calculation for air conditioners and heat pumps establishes the minimum size of the branch circuit conductors--control switches and wiring--you need to supply your A/C or heater unit. The MCA is usually published on the equipment's nameplate, but you can calculate it yourself after locating a few figures on your product.

Calculate MCA to find the required size of the branch circuit conductors that supply your A/C unit.

Step 1

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.

Step 2

Find your unit's heater current, which is also labeled either on the data tag/nameplate or the product catalog.

Step 3

Use the data you collected to fill in the following equation and determine the MCA:

MCA = 1.25 x [motor rated current + heater current]