Convert 220v cooktop feed to 110v sub-panel?
my kitchen originally had 2 220v 3-wire feeds (6 GA), one 30A for an oven and a 50a for a counter mounted cooktop. the oven was replaced with a standard kitchen range (cooktop + oven), and the counter cooktop removed. so i have a 50A 220v circuit sitting there doing nothing (breaker open at panel).
i want to install a 110v sub-panel for my dedicated theater room which is right below the kitchen.
since it's 3 wire (2 black insulated + bare ground), can i create a 110v sub-panel feed by replacing the 2-pole 50A breaker with a single pole 50A breaker at the main, use 1 insulated conductor for hot, the other insulated conductor for neutral and keep the bare wire for ground if i mark or label the neutral wire at both ends? say if i wrapped with with white tape? i know it will work, but would that be per code?
i do know i can't keep it a 220v at the sub, as i'd need 4-wire for that and can't connect the neutral and ground at the sub.