Proper wiring

[Spiderman]

A friend just bought a used hot tub so I went with him today to buy the sub-panel and wiring.  It's a Great Lakes Manitou with a 50amp sub-panel with one breaker.  I told him he could use 8/3G NM-B wire, but after we did a little research, I'm not sure I'm right.

On my Jetsetter, the manual said to use #8 AWG from the panel to the sub, which I did, but it seems that #8 copper is only rated up to 40amps.  I then called my brother the electrical engineer who said that I should be fine and that the ampicity ratings are very very conservative.  Keep in mind he's not an electrician.

So, did I wire my sub-panel with the wrong size wire?  And, I'm thinking my buddy should use 6/3G, just to be safe.

[Vanguard]

Try the Great Lakes website.  They have downloadable owner's manuals with the wiring schematics.  You'll need to know the year of the model.

http://www.greatlakeshomeandresort.com/manuals.html

Keep in mind, Hot Spring Spas are wired differently than most spas, so don't use that as a guide.

[Brewman]

Go with the 6 gauge.  And your buddy should check with the local electrical authority.
Regardless of what the spa manual says, local regulation will prevail.  I'd think that in most cases the spa manuals would be in agreement with NEC, though.
 The cable you mentioned may be okay for indoor wiring (in my area it isn't, but that may vary by locale).  The run from the outside shutoff to the spa should probably be 6 gauge THHN/THWN individual wires.  This is partly because the bundled cable has an uninsulated ground, where the NEC requires the ground to be insulated.  The cable wrapping doesn't count as insulation, nor does the paper wrapped around the ground wire.  This wire would then need to be in conduit- totally enclosed it's entire length.
That's another reason why the bundled cable might not be allowed outside- when you put that cable in conduit it could be subject to de-rating.  
 Before spending the bucks on wire, find out from the local inspectors what is required.  In most locations, a permit and inspections are required for this as well.  
The cost of going from 8 to 6 gauge is minor, and the peace of mind that you did the right thing is worth it to me.  

Did your Hot Springs spa have 2 breakers?  I thougt I read some posts on this forum that at least some HS spas use 2 smaller capacity breakers instead of one.  If that's the case it could explain why you can use the 8 gauge.  The general rule of thumb is that 8 gauge is good to 40 amps, 6 gauge is good to 60 amps, if the wire runs aren't too long.  

[Spiderman]

Did your Hot Springs spa have 2 breakers?  I thougt I read some posts on this forum that at least some HS spas use 2 smaller capacity breakers instead of one.  If that's the case it could explain why you can use the 8 gauge.  The general rule of thumb is that 8 gauge is good to 40 amps, 6 gauge is good to 60 amps, if the wire runs aren't too long.  

Yes, the HS subpanel has two breakers; one 20amp and one 30amp.  It seems 8/3g NM-B is 60 degree rated wire, which is only good for up to 40amps.  

My friend went with #6 just to be safe.

[Brewman]

Never hurts to upgrade to the next higher gauge when in doubt.  Costs a bit more, but sure doesn't hurt.  Especially if the spa is replaced in the future, and the new one requires the heavier wire.  It can be a real PITA replacing wire.