A Power-T is required when building an install which has a single run longer than 200 lights. The signal gets weak after 200 lights, so to add additional lights to the run, you will need to inject power. This T will have a female and male 4-pin plug to connect directly in line with your light run. The third leg is a 3-pin female plug which connects to an additional power supply to inject the power into the light run. From this point on on, you can add an additional 200 lights to the system for a total of up to 400 lights controlled by a single controller.