How to Calculate Minimum Circuit Ampacity

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Things You'll Need

  • Name plate of electronic unit

  • Calculator (or pencil and paper)


It's best to use a calculator for the math to ensure accuracy of the MCA measurement. Double-check your work at least twice.


Electrical wiring of large units such as air conditioners and heaters should be done by an experienced electrician.

Minimum circuit ampacity measures electrical wire size to ensure safe wiring.
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When performing electrical wiring of units such as heaters, air conditioners and other products with motors, the correct size wire must be used to ensure that the current will not overheat the wire and cause a fire. Ampacity is the measurement of the maximum amps of current that can flow through the wire before it starts to deteriorate. A circuit is a chain of electronic components connected by conductive wire. Thus, minimum circuit ampacity (MCA) is the minimum wire size that can be used to safely wire electronic products. Calculating the MCA requires other electrical information from the components in the circuit.


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Step 1

Locate the name plate of the product to be wired. Examine the air conditioner, heater or other electronic unit and find the name plate. The name plate is a small piece of metal printed with the electrical information for the product.

Step 2

Find the full load amps (FLA) of the unit's largest motor. This information will be listed on the name plate of the air conditioner, heater or other unit. The FLA is the product's amperage (strength of electric current) rating of the motor when it is running at maximum horsepower and on its specified voltage.


Step 3

Find the FLA of all smaller motors or heaters. The nameplate will have the FLA listed for all smaller motor or heater components.

Step 4

Add the FLA of all the motors together. Use a calculator or pencil and paper to find the sum.

Step 5

Multiply the sum of the motor FLA ratings by 1.25. Use calculator or pencil and paper to find the product.