I have a circuit in my basement that as 12/2 wiring from the main panel to a number of receptacles. At one receptacle, there's 14/2 wiring going off to a single 60w light switch. At the main panel, the circuit is on a 15A breaker. If I wanted to have the receptacles carry a load of up to 20A, could I change the breaker to a 20A in the main panel and place a 15A GFCI at the receptacle with the 14/2, and have the GFCI moderate the load for the light (ie. 15A GFCI would ensure the 14/2 never carries more than a 15A load)? Or am I stuck with a a maximum 15A for the entire circuit regardless because of the mixed wiring? Thanks.

 

No. A GFCI receptacle does nothing to protect against exceeding 15 amps, or 20 amps, or any amount of current. It's only job is to protect against a difference in current between the hot and neutral.

The only way for you to safely change the breaker to 20 amps is to replace that section of 14 gage cable, and any other section of 14 gage cable on the circuit.

 

Or you could split the circuit. Runa new 14/2 cable back to the panel and move all the 14 wire onto a the 15 amp circuit. Then with old circuit only having #12 wire you could install a new 20 amp breaker.
What needs to happen is only #12 wire or larger can be on a 20 amp cicuit. Only #14 wire or larger can be on a 15 amp circuit.

 

Please also keep in mind that the majority of receptacles found in a home are only rated for 15 amp circuits. 15 amp receptacles are not allowed to be installed on a 20 amp circuit. 20 amp receptacles can be identified by the T-slot neutral.

 

In the US it is perfectly legal and common practice to install 15 amp receptacles on 20 amp circuits.

 

Even I know enough to know this isn't a true statement.