can you use nm 8-3 to feed a 50 amp hot tub?
I think not. NM is not one of the approved wiring methods in 680 part II. I also could have problems with the ampacity, based on the nameplate rating of the spa and mfg instructions. 8ga is only good for 40a in the 60c column.
Greg- The question is not stating whether it is an indoor or outdoor spa. 680.42 and 680.43 come into play here.
Are you asking if you can run NM outside to a hot tub?
Romex (type NM) is allowed in the interior of a one family dwelling or the interior of a structure associated with a one family dwelling...
But read the instructions with the tub and labels at the control box behind the skirt of the tub. Some require a full size EGC to be run with the circuit conductors...
shortcircuit
Shorties right read your Manufacture's manual most spa manufacture's will put min. wire size and I've found more times tahn not it is a #6 copper, but check your manual. If customer doesn' know call dealer they bought it from.
If NM is in fact appropriate 8 AWG NM can not be used above 40 amps.
no i am not asking if i can run the nm outside. just wondering if i can calculate hot tub as a motor to use smaller wire.
any thoughts against using 6-3 aluminum?
A spa is a combination load with a significant part that is a "toaster wire" heater. The answer is in the manufacturer's instructions unless you want to start reading nameplate ratings and calculating.
You are still stuck in the 60c column with Romex.
any disadvantage to using alum. vs copper?
Yes, pretty much every spa I have ever wired has specifically stated to use copper conductors.
Also, the ground is required by code to be copper, so why would you even consider AL for the other conductors?