Isn't UF for outdoors and THHN indoors? Also when you go to HomeDepot or Lowe's There big Hanging chart says "Outdoors use UF and with a 50 amp breaker use #8" ?

Ok,
Last question I hope. For my spa, Can I use THHN grade wire #6 copper(romex?)for outdoors if it is running through conduit ? And it is OK for a 50 amp breaker? Thanks

UF is Underground Feeder, like romex. I would use #6 for 50 amp breaker
#8 for 40 amp
#10 for 30 amp
#12 for 20 amp
#14 for 15 amps.
THHN is usually single conductors
I would Use 2 # 6's for the hots, 1 #8 for neutral, and #10 for ground, and in PVC
Use breaker at main Panel recommended for Spa. Also Needs GFI Protecton near Spa.