I had a 48' run from the electrical box to the 14-50 outlet.
You can run all the same color THHN if you ID them with colored electrical tape at the end, I happen to find 3 color wire for a good price.
You can possibly run 8 gauge wire as well in conduit, but you will be limited to 50 amps in the future.
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Ampacity Charts - Cerrowire
I went with flex and #6 because, I believe its better to use the 6 gauge wire in conduit since it will provide you with the most options in the future and it should only be about $35 more plus its a little safer. 6/4 ROMEX is also an option, but you will still be limited to 50 amps in the future, it usually costs more and builds up more heat.
Feel free to let me know if you have any other questions.
Parts list --
TayMac MX7200 Jumbo 2-Gang Weatherproof In-Use Cover, Extra Duty, Vertical, 55-in-1, 5-1/8 in. Deep, Gray,
https://www.amazon.com/gp/product/B07FWD3FKK/ref=ppx_yo_dt_b_asin_title_o06_s00?ie=UTF8&psc=1
Sigma Electric, Gray 14352 3/4-Inch 3 Hole 2-Gang Box
https://www.amazon.com/Sigma-Electr...+3+Hole+2-Gang+Box&qid=1570970632&s=hi&sr=1-1
50' EA THHN THWN 6 AWG Gauge Black White RED Stranded Copper Building Wire
https://www.amazon.com/gp/product/B07KQP6SR7/ref=ppx_yo_dt_b_asin_title_o07_s00?ie=UTF8&psc=1
Southwire 55082305 AL flex Fo7500050M Type Rwa Reduced Wall Flexible Conduit, 3/4 In X 50 Ft, Aluminum
https://www.amazon.com/gp/product/B002FYI6GY/ref=ppx_yo_dt_b_asin_title_o08_s00?ie=UTF8&psc=1
THE CIMPLE CO - Solid Copper Grounding Wire - Proudly Made in America - Ground Protection Satellite Dish Off-Air TV Signal - UV Jacketed Antenna Electrical Shock # 10 Gauge AWG THHN - Green 50 FT
https://www.amazon.com/gp/product/B01J2UOX8I/ref=ppx_yo_dt_b_asin_title_o09_s00?ie=UTF8&psc=1
You will need a breaker like this one (but it will need to match your electrical panel)
Siemens QF250A 50 Amp, 2 Pole, 120V, 10,000 AIC Ground Fault Circuit Interrupter
https://www.amazon.com/gp/product/B01H04XG6S/ref=ppx_yo_dt_b_asin_title_o09_s01?ie=UTF8&psc=1
Leviton 279-S00 50 Amp, 125/250V, Nema 14-50R, 3P, 4W, Flush Mounting Receptacle, Straight Blade, Industrial Grade, Grounding, Side Wired, Steel Strap, Black
https://www.amazon.com/gp/product/B01M9FC5AR/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1
I purchased this so I can use the outlet for other things as needed. (not required but worth it)
Epicord RV Cord RV Y Adapter Cord 50 Amp Male Plug(14-50P) to 2 Female Connector (50M152F)
https://www.amazon.com/gp/product/B07CQKJ8M1/ref=ppx_yo_dt_b_asin_title_o09_s00?ie=UTF8&psc=1