Turn off the circuit breaker that supplies electricity to the shut-off box. If the shut-off box controls the inside air conditioning unit, then the correct circuit breaker has a "Furnace" or "Air Handler" tag. If the shut-off box operates the outdoor unit, then the proper circuit breaker uses an "Air Conditioner," "Heat Pump" or "Condenser" label. Sometimes the outside unit's circuit breaker mounts in the main service panel, which is located next to the electric meter.
Remove the insulating covering from the last 1/4 inch of the wires in the shut-off box with a utility knife. The shut-off box contains four insulated wires and two uninsulated wires.
Examine and understand the wire terminals in the shut-off box. The two shut-off box wire terminals with the "L1" and "L2" tags, usually located near the top of the shut-off box, hold the wires from the circuit breaker --- known as "line." The two shut-off box wire terminals with the "T1" and "T2" or "Load 1" and "Load 2" tags, usually located near the bottom of the shut-off box, hold the wires leading to the air conditioning unit --- known as "load."
Loosen the ground bus bar's screw with a flat-head screwdriver. The ground bus bar mounts to the shut-off box's case. Route the uninsulated wires away from the line and load wire terminals to the ground bus bar. Insert the ends of the uninsulated ground wires into a hole in the ground bus bar. Tighten the bus bar's screw.
Push the stripped ends of the line wires (the wires from the circuit breaker) into the shut-off box's "L1" and "L2" wire terminals. The wires and wire terminals are interchangeable. Tighten each wire terminal with the flat-head screwdriver.
Push the stripped ends of the load wires (the wires from the air-conditioning unit) into the shut-off box's load wire terminals. The wires and wire terminals are interchangeable. Tighten both wire terminals with the flat-head screwdriver.