How to Calculate the Electrical Disconnect Switch Size

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Tip

A listed device is one that is listed with a testing laboratory such as Underwriters Laboratory.

Small appliances on branch circuit controlled by a circuit breaker may only require a simple switch, like a standard light switch. Be sure to ask your code department for local rules.

Warning

Be sure to check with your local building code enforcement department for code rules and building permits as required.

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.

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.

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.

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.

references & resources

Michael Logan

Michael Logan is a writer, editor and web page designer. His professional background includes electrical, computer and test engineering, real estate investment, network engineering and management, programming and remodeling company owner. Logan has been writing professionally since he was first published in "Test & Measurement World" in 1989.