..

 

what do you mean usable? if it leads to inside the car, and the wire physically fits, then obviously it's usable...

 

^^^What he said.

What I do when running wire from engine bay to cabin is:
1. Drill a new hole and use grommets and a sealant like silicone. This is the best way because you can minimize the amount of wire being used in the engine bay.
2. Cut through a boot and run the wire through there. This is what I currently have done with my 1/0 gauge but I wasted wire running from the battery on the driver side over to a boot on the passenger side. I got a new amp ordered so I will be rewiring my car and using grommets this time.