Hey EEs, Electricians etc, have a weird one for you

Factual Questions
1

So today I had to set up for a gig at the Giant Convention Center, audio and lighting for about 5000 people. Daisy chained 2 Portable Power Distros, Via pass throughs, tied into their service with Cam Locs, 200 amp 3 phase. The cams they gave me were tied into an isolation transformer(dirty power). Volt meter says +/- 120v per leg, cyclic rate was a tad low about 59.2hz avg.

So the question

A leg to B leg 208v, B leg to C leg 208v, A leg to C leg 208v, used two different meters same result, what gives?

I am used to seeing 208v between, say, A and C but then you usually get 220v between the other two.

I have more info if you need it and truthfully it does not matter as none of the gear needs 220v but I have been scratching my head over this for a couple of hours and I cannot for the life of me come up with a reason for this other than the transformers wiring and why in the heck would you wire a transformer that way? This is all under no load BTW.

Thoughts anyone?

Thank You

Capt

2

Three phase voltage in the US is typically 208 phase to phase and 120 phase to ground.

3

Doh, I was having a brain fart you are correct Sir 120 x 1.732= 207.84. Sorry not enough sleep.

Capt

4

**Chefguy **has it.

What’s going on is that each leg is 120 degrees out of phase from the next, and you get a different result from subtracting any two legs than you get if you subtract zero (i.e., ground) from one leg.

Or, in trig identity form:
sin(x) - sin(x+120) = sqrt(3)*sin(x - 30)

That sqrt(3) provides the difference in amplitude. If one leg by itself is 120, then the difference between any two has amplitude 120*1.732 = 207.846.

5

This is why Rock and Roll should not start at the crack of dawn, it makes my brain hurt. Thanks for reminding me what I already knew. Doh:smack:

Capt

6

All that noise has caused dain bramage.

7

Ah That is It! Won’t my Mother be proud. :smiley:

Capt