Using 6/3 in place of 6/2 Question

I am running a 40 foot run of 6/3 wire for a 240v 50 amp max welder. Using a 60 amp breaker in my 200 amp box in my house. Entire run is indoors. My question is this:

I ran 6/3 to future proof my run if I decide to add a subpanel in the garage instead of just the outlet. Because of this I will have an unused wire in the romex jacket. What is the proper way to make the unused wire legal? (put in box or out of box, cap with wire nut, etc).

Also is there a recommendation on using certain color wires, white / black I am assuming is the standard. Also should I use the 8 gauge ground wire for the ground, or switch to the unused 6 gauge wires as the ground?

Thank you.

 

For a 240 volt welder use the black and the red and the bare or green. Cap the white in the box for future use.

 

6-3 NM B cable (aka romex) has a #10 grounding conductor.

 

Use the green or bare wire in the cable for the ground (equipment grounding conductor).

In the U.S. it is actually mandatory for EGCs to be bare or green, and neutrals (grounded conductors) to be white or light gray. (For wires #4 or larger, black wires with the correct color tape at both ends are permitted.)

For wire sizes 8 and larger the EGC may be thinner than the current carrying conductors hence the #10 EGC in the case above.

For futureproofing, I would run #4 gauge wires (#8 EGC). Using #6 wire, everything else (perhaps including an air conditioner?) fed by the subpanel can be using at most 10 amps @240 volts or 20 amps @120 volts before you use the welder.