"blueman" wrote in message
...
Can wiring to a light off a 20A circuit be done in 14ga wire?
What about wiring to an upstream 15A receptacle?
Or, does all wiring on a 20A circuit breaker need to be at least 12ga?

That is correct; with the slight exception if the #14 wire has a separate
15a breaker on it.
Some people will use #14 on a lighting switch leg, on the theory that it
couldn't possibly see more than a couple amps; but it is a code violation.