How to Calculate the Electrical Disconnect Switch Size

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The National Electric Code specifies that large, permanently-wired appliances such as air conditioners or whole house attic exhaust fans have a disconnecting means within sight of the appliance. The disconnecting means must be a listed device and must be rated to disconnect the same horsepower as the appliance it is installed for. A 3-horsepower motor requires a switch rated at 3-horsepower or more. Most disconnect switches are fused so any disconnect installed must accept the correct fuse size for the appliance.

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Step 1

Examine the appliance manufacturer nameplate on the appliance. Determine the horsepower rating of the appliance. If no horsepower rating is given, look for volt-amps, watts or kilowatts.

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Step 2

Convert from volt-amps, watts or kilowatts to horsepower. Divide watts or volt-amps by 745.7 to obtain horsepower. Divide kilowatts by 0.7457 to convert to horsepower.

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Step 3

Select a disconnect switch with the same or greater horsepower rating as the appliance. The disconnect switch must also have the correct fuse size. An appliance requiring a 30-amp double circuit breaker will need a disconnect rated to its horsepower and will accept two 30-amp fuses.

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