Which conductor size is required for a 20-amp residential branch circuit?

For a 20-amp residential branch circuit, the appropriate conductor size is 12 AWG. This size is commonly used in residential wiring because it is designed to handle the ampacity required safely for a 20-amp load. The National Electrical Code (NEC) specifies that a 12 AWG copper wire can carry up to 20 amps under normal conditions, which include typical temperature ratings and installation practices.

Choosing a conductor that is too small could lead to overheating and increase the risk of fire hazards, while a larger conductor size, such as 10 AWG, would not be necessary for a 20-amp circuit and could complicate installation without any additional benefit. Thus, 12 AWG is the standard and most efficient choice for this amperage application.