Can I use 8 gauge wire with 12 gauge wire together on a circuit?

I am putting in a new bathroom where a kitchen used to be and I am considering putting in a whirlpool bathtub. I already have the wire that ran for the old oven and it ends up right where the new tub is going to be. My question is whether I can use this old oven wire (8 gauge) and wire nut it to some 12 gauge... show more I am putting in a new bathroom where a kitchen used to be and I am considering putting in a whirlpool bathtub. I already have the wire that ran for the old oven and it ends up right where the new tub is going to be. My question is whether I can use this old oven wire (8 gauge) and wire nut it to some 12 gauge wire so I don't have to run a whole new line for my bathtub. Any takers?
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