Phase voltage of 120/208 3 phase?

[mta145]

In the NEC exam preperation book on page 8 the square root example no. 2 states that the phase voltage of 120/208v is 208 divided by 1.732 which equals 120v. Can someone explain how the phase voltage of 120/240v equals 120 and not 138?

 

[ron]

Because 120/240 is not three phase.
A 240V residential style feeder is essentially 240V line to line with the 120V line to neutral voltage being a tap in the middle of the single transformer coil.

 

[infinity]

Think of it this way, you walk west 120' from a given starting point (X), now you go back and walk east from that same starting point. How far apart are the two end points?

120'.........................X..........................120'

0.........................................................240'


For a WYE system you would walk 120' and then walk on an angle 120 degrees away from the first line the resultant distance that you've walked in 208' (120' * 1.73)

 

[mta145]

In the NEC exam preperation book on page 8 the square root example no. 2 states that the phase voltage of 120/208v is 208 divided by 1.732 which equals 120v. Can someone explain how the phase voltage of 120/240v equals 120 and not 138?

what is the phase voltage of 120/240 3phase?

 

[erfitz1208]

high leg

high leg

120/240 3 phase is a high leg system. 120 A to neutral. 120 C to neutral.

208 B to neutral, B being the high leg (orange color coded)
240 A to B; 240 B to C ; 240 A to C

 

[realolman]

The squigglies are the three transformers... sorry about the crappy picture.

I also believe the 180 is not correct... I wanted to edit this before the 10 min deadline. SHould be 208 or .866 times any of the phases

 

[realolman]

Well I missed the deadline, so now my stupidity lives in cyber eternity... perhaps beyond the time I can keep it going during my lifetime.

Here's the edited drawing:

Hi leg = .866 x 240

 

[Greg Swartz]

Think of it this way, you walk west 120' from a given starting point (X), now you go back and walk east from that same starting point. How far apart are the two end points?

120'.........................X..........................120'

0.........................................................240'


For a WYE system you would walk 120' and then walk on an angle 120 degrees away from the first line the resultant distance that you've walked in 208' (120' * 1.73)

That's probably one of the best ways I have seen vector math explained.

Good job.

 

[infinity]

That's probably one of the best ways I have seen vector math explained.

Good job.

Thanks, just the way it was explained to me a few decades ago. :smile: