"overwiring" like you mention is actually "allowing sufficient conductor area for minimal voltage drop" and is required, like the #10 on those 20 A circuits, when distance or load(large motor-starting) requires. I have installed Vac-Pak machines that required #6 for a 30 amp rla.
under-wiring is allowed also, on motors. You couldn't Over-wire if the terminal cannot handle the size of the wire that you install. They make pressure connectors, or other means for downsizing the conductor to fit into a terminal. Haircuts are not allowed.
Thank you all for your replies. I just didn't know if it was allowed for short runs but since it is I will plan accordingly. I only ask because I lived in a mobile home that only had 2 15 amp 14AWG circuits for receptacles, lighting, and the smoke detectors running the entire length of the house (1 on each side) and any time anything such as the refrigerator kicked on it dimmed the lights severely. So I am assuming it would also be OK to have an electrician wire an entire house with 20 amp branch circuits and #10 in rigid conduit to the boxes then 12AWG 6" pigtails to the actual receptacles (so the wire fits under the screws properly).
Originally posted by caosesvida if he is wiring a house in rigid, he must be a glutton for punishment.
True, but it'd be a way cool house wouldn't it! When I remodel I plan on doing a couple things rigid so I can "finish later" lol. But a whole house... wow.... wouldn't want to see the bill for that one.