Subpanel wired with common neutral and ground
This is a 100A subpanel used for A/C, electric heat, and 2 other 20 A circuits. Service entrance cable was used and the bare aluminum is connected to the isolated neutral bus- and of course there is no ground. I know this has to be fixed, but running the proper new cable will be very difficult- But a single wire could be more easily managed.
My question- Is it permissible to run an insulated neutral wire outside and alongside the SE cable to the isolated bus and move the twisted uninsulated alum to a new ground bus properly bonded to the box?
If no- then how about a 10/2 with all 3 conductors twisted together to provide the missing neutral?