Im wireing a 50 amp breaker from my fuse panell and I have 4 wires running from the pannel. (red, black, white, bare ground.) I need to connect a plug for my welder(s), which has a three prong plug. Which wires do I hook up?

2 hots and the ground, cap the white.

What size are these wires? Were they used for a 50 amp circuit prior to this one?