My main electrical panel is full.

I found one breaker that controlled seemingly nothing.

Turns out it was one of the two hots in a multi-wire branch circuit -- it was the red wire. I located the red wire capped and unused in a utility room ceiling. The rest of the wire (black, white, green) is in use. Basically a 12/3 wire goes from the panel--with the red wire on one breaker and the black on a second breaker--to a junction box in the utility room -- at the junction box, the red wire is capped and the black, white, and ground move on to feed an upstairs bedroom.

Basically I want to abandon the red wire only. I see no future use for it and I want to free up that space in my circuit panel.

What I'm wondering is if I can cap the extra red wire in the main circuit panel or if I need to draw it out of the top of the circuit panel. The rest of the 12/3 wire is still in use.

Thoughts?