My Hot Spring Grandee (old, but new to me) has a junction box adjacent
to the main black control box. I'm hooking it up for the first time,
and have a "spare" black wire on the spa. I'm not sure what to hook it
up to. It's caped off now, and nothing is powering up-no jets, no
light, no heater.

The spa uses both 230 and 110. The light is on 110.

The way it is wired now is:

Sub panel wires -- Spa side wires are...
---------------------------------------------------------
Red wire from subpanel to -- Red spa wire
Red wire from subpanel to -- Red spa wire
Blk wire from subpanel to -- Blue spa wire
Blk wire from subpanel to -- Blue spa wire
White wire from subpanel to-- White spa wire
Ground wire from subpanel to -- grounded to the spa junction box.

THE PROBLEM: --There is one black wire remaining on the spa, and
I do not know what it should connect to.

Anyone know where the black wire on the spa should be connected?