Which wire do we use for a NEMA 6-50R plug?
A NEMA 6-50R is a straight-blade 250 volt, 50 amp receptacle: two hots and an equipment ground, no neutral. It is the standard welder outlet and a common hardwire-free EV charging receptacle. The wire depends on what feeds it and how it is run.
The default answer
On a 50 amp breaker with NM-B cable, run 6-2 with ground copper. NM-B is limited to the 60 degree column per NEC 334.80, and 6 AWG copper at 60 degrees is 55 amps, which covers a 50 amp circuit. 8 AWG NM-B tops out at 40 amps, so it does not qualify. In conduit with THHN/THWN at the 75 degree terminations typical of breakers and receptacles, 8 AWG copper is good for 50 amps, so a pipe run can legally be 8 AWG hots with a 10 AWG equipment ground per 250.122.
Welders get a duty cycle break
Article 630 lets welder circuits be sized from the nameplate rather than the breaker. 630.11 multiplies the rated primary current by a factor based on duty cycle, so a 40 percent duty cycle transformer welder drawing 40 amps calculates to far less than 40 amps of required ampacity. That is why manufacturers commonly spec a 50 amp receptacle on conductors that look undersized for a general purpose circuit. The overcurrent device can be up to 200 percent of rated primary current per 630.12. If the circuit will ever serve anything other than the welder, size it as a normal 50 amp circuit and skip the 630 allowances.
EV charging is the opposite case
EVSE is a continuous load per Article 625, so a charger drawing 40 amps needs a circuit at 125 percent, which is exactly the 50 amp breaker and 6 AWG copper NM or 8 AWG THHN setup. Do not use the welder allowances for an EV charger, and remember a 6-50R in a garage or outdoors now generally needs GFCI protection under NEC 2023 210.8, so plan on a 2-pole GFCI breaker.
Field notes
Use a receptacle with a proper strain-relieved box, torque the terminals to spec, and check the terminal temperature rating before leaning on the 75 degree column. Aluminum is workable at 4 AWG for a 50 amp run, with antioxidant and terminations listed for aluminum.