How To Look At A House

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What is three phase electric service?

Saturday, June 30, 2018

Three phase is the way power is generated and transported over long distances by electric utilities. It is also the type of service to large commercial and industrial buildings. Three phase is converted to single phase (split-phase) by residential transformers, primarily because single phase is more suitable for residential power requirements for lighting and small appliances.

    The diagram above shows the wave pattern of each phase of one cycle of three phase power, with the vertical dimension being voltage and the horizontal representing time. Each phase is symmetrically 120º apart over a 360º cycle, and the elegance of this pattern is that it provides a continuously balanced flow of electricity, so that the sum of the three conductors is always zero. In other words, the voltage of one conductor at any point in time is balanced by the opposite voltage of the sum of the  other two conductors.

    If you took the black wave line in the diagram above and flipped it over the zero reference line below it, to create a mirrored opposing wave, the two lines would be a representation of single phase power. Although single phase has two hot wires and might seem like it should be called “two phase” power, the two wires are actually cycling in opposition over a single phase, as shown in the diagram below. The big difference between the two types of service is that single phase is a pulsating voltage. 

    Three phase power is extremely rare as a residential electrical service. Also, there are multiple variations of three phase service and understanding the difference between them gets very technical. But to learn how to tell what type of service a building has, see our blog post “What is a reliable way to tell if the electric service is 3 phase or single phase?”

   Visit our ELECTRIC PANELS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.

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