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# How can 208 be single phase?

208 is single phase because there are only two conductors involved. There is only one way to look at them, one vs the other.

In a three phase system, there are three conductors (delta) or four conductors (wye).

Take the case of three conductors. Each phase is actually the relationship between two conductors, A-B, B-C, and C-A. Take any one away, and you are left with one phase, not two.

Take the case of four conductors. Each phase is the relationship between two conductors, just like before, except that one of those conductors is a common neutral. As a result, the three phases are A-N, B-N, and C-N. Take one away, and you have two phases, but the neutral is now unbalanced.

208 is a special case of a wye connection. Instead of neutral being the common secondary of three Transformers, each with 240 available with respect to neutral, neutral is the center tap of one transformer, and there are three hot legs, A, B, and C. A and B are 120 volts away from neutral, and they are 240 volts away from each other.

Stop and think. Considering only the A/B transformer, this is the same as 120/240 split phase as used in residential power in the US and Canada.

Now, add the third phase, C. C is 240 volts away from A, and also 240 volts away from B. You can connect a three phase delta load across A, B, and C, and it will work just fine. You simply cannot use neutral for anything because it is not in the center of the phase triangle.

Do the math (vector math, that is) and you discover that C is 208 volts away from neutral. (120 times tangent 60 = 208) So, if you connect a load between C and Neutral, you get 208 volts.

Interestingly, this 208 phase is actually not in phase with any of the three primary phases AB, BC, or CA. It is halfway between A and B.

To understand this, go back to Fourier Analysis. The sum of two sine waves of the same frequency is simply another sine wave, though with a different amplitude. The phase angle of those two sine waves does not matter - you still get one sine wave out, therefore you get one phase on 208.

Now, does that mean you could have 208 three phase? Certainly. You would need to center tap the other two secondaries, BC and CA, and load the center taps against the opposing high "or wild", as its called phase. Of course, two of those three center taps could not be grounded, so it is not cost effective to do so.

This configuration, three phase 240, with one phase 120/240 split, and 208, is known as quadraplex, and is a cost effective solution to providing three phase power at the same time as traditional 120/240 split phase power, to a small business. Study guides

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