Refeed existing sub panel
I have a sub panel in my attic that feeds the upstairs lights and receptacles. That sub panel is fed with a piece of 10-2 BX. They used a jumper to make the other bus hot (there are no 240V loads up there, so it works fine). Is that in any way legal?
I'd like to refeed it, my plan was to change out the 30A single pole breaker with a 40A two pole breaker and run a piece of 8-3 romex up there, should be pretty straight forward.
40 amps should be more than enough since 30 amps has been fine and there is no possibility to expand the upstairs or anything like that. The AC unit in the attic is fed from the main panel.
My questions, is 40A too much?
Is using romex the best option, or should I use a different type of cable for some reason?