208 volts is a weird voltage. Where did it come from?

208 volts is a weird voltage. Where did it come from?

written by Jim Dixon

Three phase power has several parts that could be classified as weird.

Three phase power is power that comes in on three separate lines. In each line, the voltage follows a sign wave pattern. Each wave is similar except they are out of phase. By that, we mean that the second wave begins at the point where the first wave is a third of the way through its cycle. Then, the sine wave for the third line begins at the point that the first wave is two-thirds through and the second wave is now a third of the way through its cycle. This is just like singing the song Row, Row, Row your Boat gently down the stream, in three rounds, with each part starting at a different point in time.

Each of the three power lines is hot. A load can be place between any two of the hot line OR between one of the lines and neutral ( a return path for the current that is at “0” volts.) If you place the load between two of the hot lines, one can be a positive voltage while the other is negative. This means that the voltage between them can be greater than the voltage from one of them to neutral (volts)

FIRST IMAGE IS 3 PHASE 4 WIRE in “Y”, SECOND IS IN DELTA

When we talk about the voltage between two hot lines, we call it Line-to-Line Voltage. When we talk about the voltage between one of the lines and neutral we call it Line-to-Neutral Voltage. The Line-to-Line voltage is always greater. And there is a constant ratio between the two. The Line-to-Line Voltage is always 1.732 times the Line-to-Neutral Voltage. (The 1.732 is the square root of 3). Unless we specify otherwise, when we say three phase we are talking about Line-to-Line Voltage.

Now to get back to our question. 208 volts is Line-to-Line Voltage. This three Phase voltage is frequently used in small businesses. Why? Well, what is the Line to-Neutral Voltage for this. It is 208 volts /1.732 which is 120 volts. The power is brought is as 208 volts 3 Phase and then the Line-to-Line Voltage will be 120 Volts. Each of the three Line-to-Neutral line will go to a part of the plant. When the lines were installed, the plant was divided into roughly 3 equal area from a power usage standpoint, with one line going to each area. This way, these areas have 120 volts to run their equipment from.