I have 6/3 with ground and need to add 20 feet can I use a junction box to continue for my double oven or do I need to go back to the load center box, also can I run my double oven and cooktop off the same wire with a 50 amp breaker

The oven and cooktop have to be on their own circuits.Wiring on same circuit could cause fire if breaker does not trip off.

U can add the 20 feet through a junction box, but box must be accessible. It cannot be buried in wall.

Chuck

A typical oven and cooktop are allowed to be supplied with one 50 Amp circuit. The #6-3 feeder can be split into two feeders, usually two #10-3 cables, one for each the oven and cooktop.

Use a 4-11/16 in square box and cover.