IDENTIFY SPA LIGHT - To make sure you are purchasing the correct replacement light fixture you may have to look at the light information on the backside of the old light fixture. This will involve removing the old light fixture out of the niche in the Spa wall. Generally this information includes the model #, the watts, and the volts. The manufacturer's name is often pressed into the lens. For information on removing a light fixture see How To Identify a Replacement Pool Bulb / Light. You may want to drain the spa water into the pool to get to the light fixture more easily.
TEST NEW LIGHT - When you receive your light fixture, test it to make sure it works. You don't want to go through the effort of replacing this light just to find out it doesn't work either. Get an extension cord and before plugging it into the wall, push the white and black leads into the slots of the plug. Then plug the extension cord into the wall for 2-3 seconds to see the bulb light up. ONLY LEAVE THE LIGHT ON FOR 2-3 SECONDS. These lights are designed to only operate in the pool and will burn out quickly if not cooled by the pool.
EMPTY THE SPA WATER into the pool. Most pool systems have diverter valves to move water from the spa into the pool. If it is too much trouble to dump the Spa water, you can remove the light fixture with a snorkel and mask.
DISCONNECT LIGHT CABLE - Unscrew the three cable wires from their terminals in the junction box. Normally these wires are black (load), white (neutral) and green (ground). Note where they are so you know where the new cable is attached. I sometimes take a picture of the setup before I take it apart. One other note. These lights will be powered by 12V or 120V. The 12V systems will have a transformer between this box and the house power box. This is not an issue here since it's on the other side of the junction box.
FASTEN FISH TAPE TO SPA CABLE - You will be using the existing old cable to help pull the new cable through the conduit between the power box and the niche in the Spa wall. Pull about 18" of the old cable out of the conduit at the junction box and strip about 1' of the casing off the end of the old cable. Double over the black and white wires to form a loop to connect a 1/4" flat fish tape onto. Tape the whole length with electricians tape (6" of doubled over black and white wire, the connection of the black and white wire and the fish tape, and 6" of the green wire and fish tape). Make sure there are no loose wire ends that could get snared in the conduit. String about 50' of fish tape straight out from the junction box so that it will play into the conduit easier.
PULL OUT OLD CABLE- At the spa, pull out the old cable from the conduit running to the house. As you pull this cable out, you will be pulling the attached fish tape into the conduit at the junction box end.
PULL IN NEW CABLE - At the junction box, pull on the fish tape to pull the new cable into the conduit from the Spa. When the new cable is through, continue pulling on the new cable until there is only 3-4 extra feet on the Spa side (from the light to the nitch). This wire will be coiled behind the new light so that when you need to replace it in the future, you will have enough cable to lay the light on the Spa floor.
ATTACH NEW CABLE TO JUNCTION BOX - Remove the fish tape from the new cable. Cut off excess wire and attach the three wires onto their respective terminals as noted above.
CLOSE THE JUNCTION BOX
RETEST LIGHT - Apply power for 2-3 SECONDS ONLY to confirm that the light is operational.
INSTALL LIGHT FIXTURE - With the power off, coil 3-4 feet of excess wire into niche and place the light fixture into the niche. Secure the light fixture with the pilot screw.
FILL THE SPA
TEST THE LIGHT after the Spas is filled to make sure it is still operational. You can leave it on as long as you want now.