What is the difference between a two- and three-wire pump?

To the end-user, both two- and three-wire pumps do the same job. Well pumps are induction motors that require a start-and-run capacitor. A two-wire pump includes these components as part of the pump motor. A three-wire pump has a start/run capacitor which is separate from the pump and located in a box (usually next to the house lateral in the basement or crawl space). One advantage to the three-wire pump is the ability to convert it to a constant pressure system, although some manufactures are now building constant pressure systems for two wire pumps.