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You might get away with the 40. It's possible that you may have nuissance tripping when you're running the spa at full speed- heater on and all pumps working. If you're having an electrician do your spa wiring, install that 50 amp breaker now. Way cheaper to have it done now vs making a return trip after all the big work is complete.If this is something you can do yourself, you can try the 40 and put in the 50 later if you have trouble. Never put in a breaker rated higher than the recommendation. Lower won't hurt except for the wear and tear from nuisance tripping, if any.If you question your electrician's opinion, why do you use him?
Go with the 50 but the other aspect to consider is the gauge of wire. Often, with a 40, they will use #8 wire but a 50 or 60 amp will require #6 wire.A smaller amp breaker will cause problems with overloading and tripping. You reqire an 80% leway for your breaker to function properly and it's why my understanding has always been that it's no problem to go larger than need be. A 40amp tub will run perfectly fine on a 60 amp breaker for example. I can't think of why it wouldn't Brewman?Steve
Not to be a jerk but if you don't know if the 40 will work when a 50 is called for by the manufacturer, maybe you shouldn't be doing the work?