I recently purchased a 240v Sundance spa. The wiring requires only three wires: red, black, and ground. Since I had space in my main panel, I placed the GFI there as follows: red to load hot, black to other load hot, pig tail white to neutral bus. The load neutral on the breaker was left vacant (as per Sundance's wiring instructions). I ran this out to the remote disconnect box and the spa will be wire into that disconnect box. My question is this: Since no neutral wire goes out to the remote disconnect or the spa, how does the GFI function?

The GFI's function has nothing to do with neutral.

It's all about what current leaves the gfi, has to come back thru it.

So normal current flow thru your system is between L1-L2.

Anything leaves L1 or L2 that doesn't come back thru the other side, will trip the gfi. that is, if it leaves L1 and doesn't show up exactly the same thru L2, then it's obviously going through ground.

Which trips the GFI.

It is a current leakage detector, not a current neutral detector.

PS: Next time post in one forum only. Thanks.

Yeah sorry about that I a new user. Should have deleted the post i made to to wrong forum. Anyway thanks for the reply. I understand it completely. I was thinking in terms of a 110 GFI where the current comes back on the neutral.