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 = 125 x [motor rated current + heater current]. 

Things You Will Need

  • Air conditioner data tag/nameplate
  • Heat pump data tag/nameplate
  • Calculator

About the Author

Based in New York City, Alex Taylor has been writing culture, travel and science articles since 2004. Her articles have appeared in "Boston" magazine, “The Boston Globe” and "CITY" magazine. Taylor holds a Bachelor of Science in journalism and international relations from Boston University.

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