I have a 20 amp circuit going directly from the panel to the kitchen with 20 amp receptacles and subsequent 12 gauge wire. There are only 4 outlets on that circuit (one being a GFI). I am installing under-cabinet lights and lights above the sink, well within the total amperage and volts allowed for the 20 amp circuit. I will use the 12 gauge wire to power the 15amp single pole switches and go to the lights with 14 gauge wire. Can I connect the 14 gauge neutral to the 12 gauge already in the box or should I just run 12 gauge to the lights for the neutral and connect??.... thank you in advance for your input
Switches are not circuit breakers (overcurrent protection). They cannot protect wire and do not make it ok to use smaller wire past them.
If any 14AWG wire is used, you must downgrade the breaker to 15A, and downgrade the countertop receptacles to 15A. The other wire can remain 12AWG. This will mean it is not one of the two mandatory 20A circuits for countertop receptacles and you may need to add a circuit.
There is another reason not to put kitchen lights on receptacle circuits. An appliance trip will plunge the cook into darkness.
If you have any #14 wire in the circuit, you have to put a 15 A breaker on it. So, to keep your 20 A breaker, you should use #12 wire everywhere, and run #12 to the switch and lights.