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15 Amp Three Prong Plug 15 Amp Two Prong Plug

Extension Cords

Extension cords are commonly used for a variety of electrical devices. This page will show how to use the right type of cord, for different devices.

Be careful when using extension cords so they do not cause a trip hazard. Someone could get hurt or damage the receptacle or electrical device.

Extension Cord Amp Ratings

The amp rating of an extension cord should be the same or higher than the electrical device amp rating. If not, cords and electrical devices could be damaged.

Using an extension cord with a lower amp rating than listed on the electrical device could cause the wire, plug, or cord socket to overheat. Sockets have been known to melt under this condition.

High wattage yard equipment usually state the minimum cord wire diameter in the instructions.

Extension Cords with High Wattage Appliances

Certain high wattage appliances should not use an extension cord when possible (check the owner’s manual). Using an extension cord on these appliances could cause excessive heat and voltage drop, this could damage the appliance or motor.

Some high wattage appliances have a short cord to minimize voltage drop, and should be placed near a receptacle. Vacuum cleaners have a long cord and should not need an extension cord.

If an extension cord is absolutely needed, use one with the proper rating (derated by 125% when necessary) and as short as needed. You can find an extension cord specific for window air conditioners.

Below is a list of high wattage appliances that should not use an extension cord when possible.

Extension Cord Indoor and Outdoor Rating

Extension cords are rated for indoor or outdoor use. Indoor rated cords are usually 2-wire cords. Never use an indoor rated cord outdoors.

Extension cords rated for outdoor are usually more durable, have 3-wires (including ground), are longer, and use larger diameter wires.

2-wire and 3-wire Extension Cord

2-wire extension cords have no ground wire and no ground connection. They are commonly rated for indoors only, and intended for use on low wattages devices (table and floor lamps, etc). Never plug a grounded electrical device into a 2-wire extension cord. Also, do not use a 2-wire to 3-wire adapter.

A 3-wire extension cord has a ground wire and ground connections. 3-wire cords are usually rated for outdoors and carry a higher amp rating.

3-wire Plug

2-wire Plug

Extension Cord Derating

When an extension cord is used for an electrical device that may be operating for 3 or more hours, the device amperage needs to be derated by 1.25%.

For example, a pool pump motor is rated for 12.5 amps.

The derated amperage for this motor is 15.63 amps (12.5 * 1.25)

The extension cord must be rated for 15.63 amps or more. Choose the next higher rating, 20 amps.

Enter a number in either the amps or watts boxes.

120V Power Converter

Watts
equals
Amps

Derate Amps by 125%

Amps
derated
Amps
Extension Cord Amps Rating Table

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