For temporary power only
Flexible extension cords are designed as a convenient, quick solution for temporary wiring. Not all extension cords are created equal and must never take the place of permanent wiring.
When choosing your extension cord, take note of the gauge or size of the wire it is made of. A heavier gauge wire will have a smaller or lower number. For example, a 12 gauge wire would be larger, and can therefore power larger wattage appliances than a 14 gauge wire.
Purchase the correct gauge of cord to meet your needs. When you use a cord that is too light, for the load plugged in, the amount of extra electricity travelling down it overheats it making it a fire hazard.
The length of the cord and the weight of it both are factors in what the cord can supply. For example, a 12 gauge extension cord, up to 100 feet in length, can safely power a 12 amp circular saw. A 14 gauge cord can only be 60 feet long, and a 16 gauge cord can be no more than 35 feet long to provide sufficient power for the same saw.
If you are going to use an extension cord to power two or more appliances you must add the total wattage rating of all appliances to determine which gauge size you will need. This is an important tip for people planning to solve all their electrical needs with a power bar extension. Overloaded extension cords can get hot enough to burn.
Also remember that because electricity is lost as it travels down the cord, a longer cord cannot power as big of an appliance as a short cord. Always choose the minimum extension cord length that you need to do the job.