Hey, I'm an apprentice that's been in the trade for about a year and 5 months now. There's one thing I simply just don't understand, and that is panels. I know how to install/wire them up and all, but.... if I had a question like, hmm 3phase 4wire pannel that is 277/480, is there a high leg 208 or not.. Or is this a delta or Y pannel. I just don't understand this. The 277/480 is just one example, I basically don't get any of it, even if it is a 240/480 or 120/240 etc... When I will have high legs and when I won't, and if a meter reads 120v on all 3 legs of a 3phase, then what panel is it. Just throwing out examples. But was hoping you guys would be able to help me finally understand all this, thank you very much for your time! - Jason
This must be an extraordinarily stupid question o_0
Edit: But yes, I'm in school. We just got done with a test last class, and on that test was this question. Our teacher I guess just expected that we all should have known this, because we never went over it. But I defiantly need some help knowing what is what.
A question we had on the test is all three phases read 120v, what system is this? Can't remember but it might have said if it was a delta or a Y.
I thought that Delta systems have voltage drop so if you put your leads on 2 of the phases, you would read 208. But on a Y system, you will just read 240v. Idk...'
Edit:Another question would be... If you have a 120/208 3phase 4w system, what will the phases read? ABC
In a professional installation, it is likely labeled. Otherwise experience and knowing that maybe I should measure will hopefully keep you out of trouble. I don't believe panels are made specifically for high leg systems.
Yes I'm looking right now, I sort of better understand now. If you have a delta panel with 120 volts coming in on all 3 phases, from Neutral to B phase you get 208, but Neutral to A/C phase is 120v, And from B phase to C phase its 240, and lets say A to C phase is 240. Or A to B phase is 240. This is correct right?
And for Y panels, you will get 208 if you go from phase to phase no matter what. And from Phase to neutral it'll just be 120v. Correct?
Large commercial buildings.
277 to ground on each phase, 480 between any two.
480 3 phase powers AC and machinery
277 is for lighting
480 to 120/208 transformers would be installed next to the 277/480 panels to provide the 120 power.
The purpose of the high voltage systems is, you can put more fixtures and bigger AC units on smaller wires because the voltage is higher.
High voltage breakers are physically larger than the lower voltage ones. High voltage disconnects and fuses are bigger also.
3 phase 120/208
Newer (post 1970), smaller commercial buildings
120 to ground on each phase, 208 between any two.
208 3 phase powers AC and machinery
208 single phase will also be in use
120 (generally) is for lighting and receptacles.
3 phase 120/240
Old (pre 1970) commercial buildings and some old residential
120 to ground on two phases and 208 on the high leg.
240 between any two
240 3 phase powered the AC in the old resi applications.
240 3 phase powers the AC and machinery in commercial buildings
240 single phase circuits will also be in use.
120 is for lighting and receptacles.
One thing to remember is, the 120/240 (delta) will have a high leg. 120/208 (wye) will not.
Obvious signs of a high leg panel will be a smaller feeder on one leg, a bunch of empty spaces every third slot and, the most obvious, the 200+ volts to ground reading on your meter.
I left out the common 120/240 single phase but I think you can figure that one out.
And, don't ask me how any of this chit works because I have no idea. :jester:
Here is your assignment: look up diagrams for the following transformer configurations
1)Single Phase 120/240 volt
2)Three Phase 208Y/120 volt
3)Three Phase 480Y/277 volt
4)Three Phase 240/120 volt aka "Center-tapped Delta", "Delta High Leg", etc.
5)Three Phase 240 volt aka "Ungrounded Delta"
6)Three Phase 240 volt aka "Corner Grounded Delta"
And for the recent Canadian explosion,
7)Three Phase 600Y/347 volt aka "480 is for sissies, eh."
Your high leg will be present on a 120/240 volt 3 phase, 4 wire delta system. The reason for this is length of travel to the B phase, which is why you will read 208 volts from phase to ground.
If you get through all that, start looking at 6 phase Y and Delta connections, Scott-T connections, and zig zag connections. Transformers are just plain awesome.
btw, i belive you left out 2 phase, it now is relatively obselete but here it still exists. didn't it come from a scott connection?:whistling2::whistling2:
wow, got some extraordinarily good replies on this thread after I thought it died yesterday. I see some really good posts, going to go back and check it all out now, thanks a bunch!
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