On Saturday, January 25, 2014 7:07:01 AM UTC-8, wrote:
On Saturday, January 25, 2014 9:55:33 AM UTC-5, (PeteCresswell) wrote:
I've got an outlet box that is fed by #12 Romex backed by a 20-amp
breaker.
If I were to tap that box to feed a ceiling light, would #14 wire be
code-legal for box-to-ceiling-light?

No, unless you change the breaker to 15 amps.

That also should not be code compliant as someone could see the 12 guage wire coming off the breaker and change it back to 20amp.

Harry K