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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.