The sub panel in my 40 year old house has all of the neutral and grounds on the same bus. I know this is incorrect but would like to figure out the best way to fix it. The current wire feeding the panel from the main is a 2 wire plus ground "romex" type insulated cable. It feeds the whole house other than the A/C compressor so this feed is probably 4-6 gauge (EDIT: actually looked at it today and it's #2) because it makes a 55' run from the main panel. The ground on this cable is non insulated like in a romex type cable.
To separate the neutrals and grounds I would assume I need to add another bus bar and then run another wire from the main panel to the sub. Would this new wire need to be the neutral since my current ground is non insulated? If so, I assume it would need to be the same gauge as my current hot feed?
Lastly, how important is it that I fix this? I also need to add GFCI's in bathrooms and the kitchen (maybe a GFCI breaker in the sub panel) and wasn't quite sure if this neutral ground bond issue at the sub panel would affect GFCI operation.