Hi RC, If your bulb is a screw in style bulb and the bulb measures roughly 5 inches to 6 inches wide then this would be the correct replacement for a 12V 100 watt bulb. If you have a smaller 100 watt spa light which has a bulb that measures roughly 2-1/2 inches across then this would not be the correct light. A few other measurements which may help.... normally this bulb goes into a pool light fixture where the face ring of the pool light fixture measures roughly 10 inches across. If you have a spa light fixture where the face ring measures roughly 5 inches across then this would not be the correct bulb. I hope that helps!
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