Is a standard receptacle connected to the LOAD terminals of a GFCI protected up to 15 or 20 amps?
If it is protected up to 20 amps and I connect a standard 20A receptacle on the LOAD terminals with 12/2 wire. Then plug in an appliance with a current rating of 18 amps for example. Will the GFCI trip or will the 14/2 wire before the GFCI overheat and potentially cause a fire?
I don't understand the purpose of a 20A LOAD terminal on a 15A GFCI which is meant for 15A circuit.
First, a GFCI outlet (or the GFCI portion of a GFCI breaker) does not limit current. That's the job of the breaker.
The functional portion of the receptacle itself is rated 15A (doesn't have the crossbar on the neutral blade slot), but the passthrough is rated for 20A. This is because you can actually use it on a 20A circuit since any device requiring 20A should have a plug that prevents it from being used in a 15A receptacle.
Here's a 20A receptacle:
Then plug in an appliance with a current rating of 18 amps for example. Will the GFCI trip or will the 14/2 wire before the GFCI overheat and potentially cause a fire?
The 15A breaker protecting that circuit will trip, preventing anything from overheating or causing a fire. There is a 15A breaker installed, right? If there is #14 wire anywhere in the circuit, it must have a 15A breaker protecting it, even if there is #12 (or larger) wire in the circuit.
isherwood's answer does a good job explaining the 20A pass-through.
This allows one product (stocking unit) to be used on both 15A and 20A circuits, just like every other receptacle.
Due to a fluke in Code, you can put the common 15A receptacles on either a 15A or 20A circuit. That supports both the old installed base of 15A circuits, and also the new 20A-required circuits in kitchens, bathrooms and commercial, all with the same parts. UL supports this by requiring all 15A receptacles have 20A pass-through, meaning they are designed to be safe if used in a 20A circuit.
A GFCI is not a receptacle. However they make GFCI+Receptacle combo devices, which is what you're thinking of, and many use 15A style sockets. Those have to be 20A pass-through.
