How many conductors (of the same size) are allowed per circuit breaker? I thought more then one as long as the load requirements were met. A friend told me he thought only one wire (circuit) per breaker; is this a NEC rule? :)

You could have several wires pigtailed and have 1 wire on breaker. And don't know of a limit, except wires in a wirenut, then you could go to a second and 3rd wirenut.
Only 1 wire wire can be on breaker.
If I am wrong, we will both know shortly.
Only time I see a need for this is If 1 receptacle in each room was on a back up circuit, each would be homerun from multiple connections, or Smoke Detectors.

Some breakers are only rated for 1 conductor, some are rated for 2 or more. It depends. Yes, it's an NEC rule. See NEC 110.14 for more information.

Basically, both you guys are correct!

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