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What kind of hot tub was there before and what kind is there now?
If there's any wire sticking out (or even back at the box), you should be able to look at the jacket ans see what the size/rating is. He might be right on being too small. Does his price include supplying a GFCI? You should be able to estimate the electricals cost pretty closely by adding 25% to the materials needed, then labor x labor rate. Some guys include the time it takes to go get materials in their labor. I don't know what the wire run looks like or your local labor rates, it could be 1/2 day, or maybe a day job. Any electrician who's smart is going to shoot a little higher on the # of hours because these jobs always seem to take longer than they seem.A few more quotes will let you know if the guy is in the ballpark. Some guys will shoot a high a high number out there if they're really busy, or aren't that interested in a certain job.Good luck