On Saturday, January 25, 2014 10:46:53 AM UTC-8, wrote:
On Sat, 25 Jan 2014 10:06:13 -0800 (PST), Harry K

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

What code is that? The NEC does not address what an unqualified person
might do later.

NEC code does not even address anything about WHO wires it. Ever hear about getting wiring inspected when selling a house? Violations don't pass no matter who screwed it up.

Harry K