#6 wire splice to #12 for dusk to dawn
 

Couldn't come up with a good title. But the previous homeowner had a garage in the back and later tore down. I found the underground cable which is 6 guage and wanted to install a dusk to dawn light without having to run new cable. At the light I spliced in to it with 12 gauge, back at the panel its setup for 240 on 50 amp breakers. Can I just remove the white/neutral and splice it with #12 and tie it in the neutral section? Should I just leave the #6 black on the 50 or should I splice it with #12 and run it in to a 20 amp breaker? Or is all this nuts? I wanted to avoid trenching new wire but if this is a big safety risk I will. Thanks in advanced, let me know if you need more info.

Why, yes you can.
Be just as careful at the other end mounting your light and sensor.

Was searching and found this http://www.diychatroom.com/f18/50-am...-12-3-a-62545/ , seems like I should do 12 on both ends?

Thanks Tarheel, didn't realize you had answered until I posted the 2nd time.

The suggestion is made to splice on a short length (a pigtail) of #12 gauge wire at the other end (in the panel) also, to remind a future owner or electrician that this is a 20 amp circuit.

A two conductor cable (black and white) can be for 240 volts or for 120 volts but not both. Even for 120 volts only you can put a subpanel at the garage location with 15 and 20 amp breakers for lights, etc., and keep the #6 line on the 50 amp breaker in the main panel.