Use this calculator to help select the correct wire size.

You should never use a wire that is not suitable for the amperage that it will carry. The wire might possibly melt in a worse case scenario or overheat. Say for instance you need to use the correct gauge for a 50 amp wire but instead you use a gauge that is rated for only 40 amp. This will overload the wire and probably cause a fire.

Using a smaller than needed wire is always a bad thing but on the other hand, using a larger wire than needed is not really a bad idea.

•  Terms such as "Heavy Duty" and "Contractor Grade" are often used in product descriptions of electrical cords.
• The terms have no universal definition.
• Do not just buy an extension cord simply because the packaging states it's HEAVY DUTY and rated for INDOOR and OUTDOOR use.
• "Heavy Duty" is often used to describe 8/4 SEOOW Portable Cable but the term is just as likely to be used to describe 12/3 SJT Extension Cords. However there is a significant difference between those two cables. 

If you plan on using a maximum of 30 amps, you should use 10 gauge wire up to 100 ft. You should use 8 gauge wire for distances over 100 feet or when operating any continuous use items between 50 ft and 100 ft.

To safely use a maximum of 40 amps, you will need a wire gauge of 8. This will allow you to safely use items in your house such as an electric cooktop.

To use a maximum of 50 amps, you will need a wire gauge of 4. There are some electronic devices in our household that require only a maximum of 40 amps. However, there are some appliances that will need 50 amps, such as a washer or dryer. You can sometimes even use a wire size of 6 for 50 amps but the smaller the number on the gauge scale is the better. This is because the lower the number on the gauge scale is, the thicker and more durable the wire is. This is especially true if you plan on running the wire for more than 100 feet.