Why 277v, and not 220?

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StevieP007

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I friend of mine asked me this the other, and I felt like I should've kown this,being a licensed electrician, but it's just one of those things that I never asked or just took for granted;

When testing across 2 ungrounded wires of a 3-phase 277/480 volt panel, the meter reads obvoiusly 480 volts. Why does it read only 277volts when going from 1 leg to ground or neutral, and not 240? Or 554 volts when going across 2 277volt legs? I'm pretty sure it's because of either the way the secondary side of the xfmr is tapped, or the fact that it's a "y" secondary just about 99% of the time. I just don't understand the reasoning why this is.

Any explanation or linked picture would be greatly appreciated. thank you.

[ November 23, 2004, 10:25 AM: Message edited by: StevieP007 ]
 
Re: Why 277v, and not 220?

The mathematics are tricky to explain, since I do not know how much of the math you might already know. So let me try to explain by asking you to draw a picture.

Draw a circle with a radius of about 2.77 inches. From the center, draw three lines to the edge of the circle, and put an arrow head at the point each intersects the circle. The first should be horizontal to the right. The second should be at an angle of 120 degrees from the first. The third should be at an angle of 120 degrees from the second. If you drew it correctly, it should look like you cut a pie into three identical pieces.

Now, measure the distance between any two arrow heads. It should be about 4.8 inches.

The three phase voltages in a 480/277 volt system are at angles of 120 degrees from each other. The picture that I asked you to draw, with its three arrows, is one way of representing the three voltages. From any line (arrow point) to neutral (center of the circle), the voltage (distance) is 277 volts (2.77 inches). From any line (arrow point) to any other line (arrow point), the voltage (distance) is 480 volts (4.8 inches).

As to your question of why 277 plus 277 does not equal 554 volts, it is again a matter of the phase angles. You are not adding ?277 apples? plus another ?277 apples,? and expecting to get ?544 apples.? Rather, you are adding ?277 apples? plus ?277 oranges,? and getting ?480 pieces of fruit salad.? The mathematical way of saying this is that you are adding ?277 volts at an angle of zero degrees? (the ?apples?) to ?277 volts at an angle of 120 degrees? (the ?oranges?), and getting ?480 volts at an angle of 30 degrees? (the ?fruit salad?).

Does this help?
 
Re: Why 277v, and not 220?

I may yet go down that path. I am still young enough for a change in career: 51 today! :)
 
Re: Why 277v, and not 220?

Thank you so much Charlie,

"The light has dawned over Marblehead" a saying we have here in Massachusetts(meaning; Marblehead=local town, and/or my head=solid rock)

An excellent explanation, and I totally understand now. Thanks again, and a very HAPPY BIRTHDAY to you

Stevie P
 
Re: Why 277v, and not 220?

Happy Birthday Charlie B.

Roger
 
Re: Why 277v, and not 220?

Stevie, don't put yourself down. If you have never had it explained, how were you supposed to know? By the way, all of the voltage configurations work the same way. Draw them out and you can measure them or calculate them with trigonometry or Pythagorean's theorem. :D
 
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