Ceiling lights wire guage
I think most answerers are thinking you are asking about the size of the wire to the light fixture. Is this what you mean or are you asking about the wiring inside the light fixture?
You should check your building code in your area even though 12/2 is the most common allowed wire for homes some juridictions require 10/2 which is a larger gage wire. If you do not use what is code than if it causes a fire your insurance may not pay due to you usse a wire that is not up to code
Most localities use the NEC (National Electric Code) as their source for wiring requirements. Smallest gauge allowed for ceiling lights is #14.
It may depend on whether you are connecting to an existing circuit or you are running an entirely new one from your panel. If you are connecting to existing circuitry, then you need to check the breaker size. If it is a 15 amp circuit, then #14 is acceptable. If it is a 20 amp, then you must use at least #12. It is very unlikely that you are required to use #10. NEC does not require it for a 20 amp circuit. If you are running a new circuit, then you have your choice. Just make sure that the breaker sizes and wire sizes match as described above. Contrary to the advice of some, it is just fine to run a 15 amp / #14 circuit for lights. The key is to avoid overloading the circuit. If you are just going to add a couple of fixtures, then the 15 amp will be fine. If you are adding 20 recessed can fixtures, you might consider the 20 amp circuit option. If you think that you are going to add more to the new circuit later, consider that when you decide which size to use. The difference comes in working with the wire. #14 is much easier to bend and work with. I certainly would not attempt to use #10 unless you actually do have some weird local codes that require it.
