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On mine spot light terminal is marked 15A next to it. Fuse size is generally determined by size of wire in circuit, not size of load, but if you want to make sure you have enough capacity, divide the wattage of the lamp you will be using by 12 (volts). That will give you the amp load of the lamp. Multiply this number by 125, as you don't want to load the fuse more than 80%.
If you don't want to do the math, just use at least a #14 wire, and the 15A fuse will be plenty for the spot light load
Good luck, spot lights are fun