I'm rewiring my entire house and I have a 125amp panel. My house is small, about 800sqft but I'm finishing my basement then the area will be 1,600sqft. I don't have any 20amp breakers in the panel and I want to install a central A/C. Can I replace any of those 15amp breakers for 20amp if I use #12 wire? Thanks

I want to install GFCI receptacles in my kitchen counter-top. Can I use 3 wire cable or it won't work? Thanks

When you say 3 wire, what do you mean? Black, white and ground?

Central air is usually a separate appliance that often needs more that #12 wire and 20 amp circuit.

20 amp breakers need min of #12 wire, cannot just replace any 15 amp breaker with a 20, unless the entire circuit is #12 wire.

Seems your asking two questions.

Add more detail to what your trying to do.

Ask all of your questions in one thread.

I merged your two questions into one thread.

When I say 3 wire, I mean black, red, and white (and ground).

And if I replace the entire circuit with a #12 wire, can I replace the 15amp breaker with a 20amp breaker?

Thanks...

Can I do the same with the Dishwasher. I only have 15amp breakers in the Panel and I want to install a dishwasher. Can I replace the breaker for a 20amp if I replace the entire wire to #12, so the circuit would be 20amp/#12wire? Or is it not that simple.

Kitchen counter recpts are required to be on a 20 amp circuit. With a gfi if near a sink. Residential dishwashers may be on 15 amp circuit if its adequate. Likewise a dedicated line to a frig, over the stove microwave, garbage disposer, hot water dispenser etc. #14 wire is easier to work with. Any circuit with #12 may have a 20 amp breaker.

Some locations require ALL kitchen recpticles to be GFI.
Is the 3 wires for 2 circuits?

ALL kitchen counter receptacles must be GFI protected.

Thanks you for your answers. I still have the same question, can I replace a 15amp breaker with a 20amp breaker changing all the circuit to #12 wire or would the circuit be overload?

If circuits are wired with # 12, it can safely be put on a 20 amp breaker.
2 recepticles per circuit max, and a minimum of 2 circuits, Frige on its own.
Why 12-3, 2 circuits sharing a neutral? Still need answered, thanks

If you change the wire to number 12 it can have a 20 amp breaker. As to if it will be overloaded that is a function of what the load is. 20 amps at 120 volts is 2400 watts. Loads in excess of this will trip the breaker. As a practical matter keep the load to 80% of this to prevent nuisance tripping. Though for short times the breaker will handle the full load with no problem.If you have a continuous load, on for three hours or more, code requires that the load be no more 80% of the breaker capacity. As to the wire itself- the code has large safety margins built into residential requirements. The "ampacity" of #12 is 25 amps but limited by a footnote to 20. This is national code, most localities use it, but check local code.

If you are re-wiring anyway I would not consider anything as a "replacement". You are ripping the wiring out anyway, you just have to put it back in to code.
20A circuits are code for kitchen receptacles.

Remember, the "2 receptacles per circuit" in a kitchen that Strat mentioned is a local code to him. This is not a wide spread thing.