You run 12/2 if you want to step up to 20 amps. 14/2 is only good for 15 amps.

If you really have to ask, you shouldn't be doing your own electrical work.

Just to piggy back on Ballenger's response.

A 12/2 AWG cable would be used for circuits where the higher amperage is needed. For example, I have an commercial grade laser printer in my home office. This printer places a 18 or 19 amp demand on the circuit in short bursts. If I use a 15 amp circuit, the lights in the room will dim every four seconds because of the demand put on the circuit by the printer, so I installed an independent 20 amp circuit for the printer.

Probably a better description would be a high end entertainment center. If the overall demand will cause a 15 amp circuit to trip, you would install a complete 20 amp circuit.

Also, if you install a 12/2 AWG cable you can use either a 20 amp breaker or a 15 amp breaker. However the inverse is not true. If you install a 14/2 AWG cable you can only use it as a 15 amp circuit.

If you mix the two cables (part 12/2 and part 14/2) the entire circuit cannot be larger than a 15 amp circuit.

Thanks all and I guess my ? Wasn't worded right because I was just try to make sense out of the old wiring in my garage that I am taping into rather then run more new than I have to.I have a 20 amp circuit with 12/2 wire to a juntion box and from there goes to 2 outlets and a light that's it.I am going to run 2 outlets and 6 lights off that with 14/2 and change the breaker to a 15 amp.I was just trying to figure out why they wired my garage that way back in 1950.

I would keep the 20 Amp breaker, and the current Number 12 wire, and run # 12 for the rest. The difference in cost is minimal between the 2. it may get cold and you may want to plug in a heater. Don't Skimp. How much will you save?

Hold on, in a garage you want the receptacles on a 20 amp circuit.

Under current code (2008) the receptacles are also required to be GFCI protected.

While you can install lights on a GFCI device it is not recommended to do that because if the receptacle trips and opens the circuit, there you stand/sit in the dark.

If you put the lights on the same circuit, make sure they are not connected to the load side of the GFCI receptacle. Leave them on the line side. I'd prefer them to be on their own 15 amp circuit using either 12/2 or 14/2 cable.