I have a 12-2 wire connected to a 20amp breaker. This wire is currently not in use and capped off in a junction box in my attic. I'd like to bring this wire back into service to install two new outlets in two rooms below the attic. The outlets will be used for light/medium duty (plug-in light fixtures, a computer) and do not need to be 20amp receptacles. Can I add two new 14-2 lines into this junction box with the 12-2 wire and then use 15amp receptacles? Thanks for any advice.
Nope, you have to extend a 20A circuit with 12 AWG wire.
The logic that "these will be lightly used receptacles" doesn't buy you a thing, because the safety of the wiring can't depend on how you use it.
You are allowed to put NEMA 5-15 (common receptacles) on 20A circuits, due to a special exception.
Even in the industrial parts of the Code, in the byzantine Tap Rules, such an extension would required a hardwired load of known value, or a single receptacle. I'm sure you don't intend to install this:
So if you're installing the common duplex 15A recep, what keeps you from plugging two 9A loads into it?
No, you do not want to run a 20 amp circuit through 14/2 cable. Even with light usage it is clearly dangerous and against code. If someone later runs a heavier load on it it could cause serious problems. Why would you want to do that anyway? Wire it with 12 gauge per code so it'll be safe. There's no downside to running the proper cable.