For a 15 amp circuit, the appropriate wire size is generally 14 gauge copper wire. This wire size is rated for 15 amps and can safely carry up to 18 amps under certain conditions. It is important to note that wire size should always be selected in accordance with applicable codes and standards and with consideration of the specific conditions and requirements of the installation. While it is possible to use a 12 gauge wire for a 15 amp circuit, it is not necessary and can be more expensive. Using a 10 gauge wire for a 15 amp circuit is not recommended and could lead to safety issues. It is important to note that the wire size should be selected based on the amperage rating of the circuit breaker and the planned load on the circuit.