Folks,
Please help with my understanding of the electrical world terminology. My knowledgeable brother in law, says there is no such thing as, or it’s incorrect to refer to, a two phase 240V service. Rather it should be referred to two single phase lines. Calling a service a three phase 208V service is correct however.
Did the old 2 phase 240 service reserve the terms and require some other naming?
What’s the correct terminology to use?
Comments? Insights?
Guy
As the other posters noted, indeed it is single-phase power.
The misunderstanding typically comes from people who think about the wires that deliver the power, instead of the coils in the generation equipment that produce the power. Typically, in any generator, you'll have three sets of coils - a/k/a the phases. At each junction point of these coils you'll find the phase-conductors that carry the power from the generation equipment.
Because you need two wires, one for each end of the phase, some people refer to this as "two-phase" power. While you have two phase-conductors, it's only a single phase of the generation equipment. When you have three phase power, it's supplied on three phase-conductors.
At that point, transformers (in residential systems, either up on poles or sitting on concrete pads, typically) take the two phase conductors and step the voltage down to 240VAC +/- 12VAC. A connection is made to the transformer's center and that's connected to earth - hence the "split-phase" terminology.
From Wikipedia:
Those two open lines on the left are the distribution phase conductors.
So now you see why some people would refer to it as "two-phase".
For 208V120Y service, it is indeed three-phase. In this case, one end of each of the phases are connected to a common point ("neutral"), and the other end of each phase is offered as a hot conductor. This is known as "wye" service, because of the shape of the phase connections. You get 120V from the neutral to any "hot" phase-conductor, and 208V between any two "hot" phase-conductors.
(Note there are different three-phase configurations, like "delta" power where the phases are connected end-to-end, but that's outside the scope of this.)