Electric Motors And Generators/Three-phase
Three Phase Alternating Current
In North America, three phase alternating current is generated and transmitted using three conductors to industry, commercial buildings and to a group of houses. Some older installations use 2400/4160Y three-phase systems, but most currently use 7200/12470Y. Near a group of houses is a step down transformer wired in a "Wye" or "Star" configuration. Each individual house receives one of these three phases. The neutral conductor comes from the middle tap of the "Wye." Other houses receive different phases which are balanced throughout the group. One 7200 volt line feeds a transformer to a house, which then creates two hot legs and one neutral 120/240.
In a house, the breaker panel provides connections between the two phases and the neutral conductor. Most of the household outlets are connected between one of the "hot" phases and the neutral with a safety breaker permitting a maximum of 15 amperes. For the larger loads, such as the oven and the clothes dryer, the breaker panel connects across the two hot phases to provide 240 volts, limited to a maximum of 40 amperes.