I previously ran a 50a circuit for a spa and it had no place for a neutral at the spa end, so I did'nt run one. I put in a 50a GFI breaker (not using a neutral) and tested the GFI hot to ground on both legs at the spa and it worked, the GFI did trip fine. Was I wrong for not running a neutral? Even though it trips ok? I'm not sure why this works since GFI's use the nuetral and ground.

If the spa did not require a neutral then there are no line to neutral loads. The GFCI breaker is seeing equal balance between the phases and thinks life is rosy. When you connect a phase to ground current flows outside the ground fault circuitry, it sees an imbalance, and trips. No you do not need the neutral. Where would you terminate it?

Duh, It just hit me, that makes total sense.
Thanks,