Turn off the power to the circuit you are working on at the home's distribution panel. Confirm the power is off by touching a non-contact voltage detector to the wire's insulation.
Remove the side cover on the hot tub and open the panel for the sentry control system. You may need to remove two screws to open the panel.
Loosen the three electrical terminals with a screwdriver. Strip 3/8 inches of insulation off the three wires coming from the distribution panel.
Slide the green or bare wire in the ground terminal and tighten it.
Insert the black wire into one of the hot terminals marked "TB1" and tighten it. Insert the red wire into the other hot terminal and tighten it.