Hello,
I am doing a remodel and have a open 20 amp breaker in my existing service. Can I run a temp line from the 20 amp breaker to my new box (which is not yet hot) into the 100 amp breaker. Just need to get some temp power up in the new space.
Thank you

Sure, the wiring will be protected by the 20 amp breaker.
You CANNOT have #12 off a 100 amp breaker, unless it is protected by a 20 Amp before.