Electrical Question

To save on long thick copper runs, sometimes 480v is distributed to local transformers which then drop it to the 120/208 needed. There is also less drop if you keep your low voltage transformers near to where the power is being used. Note how much smaller the primary feed (right) is as compared to the 120 volt output (left.)
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To save on long thick copper runs, sometimes 480v is distributed to local transformers which then drop it to the 120/208 needed. There is also less drop if you keep your low voltage transformers near to where the power is being used.
So is the 480V sent over one line and it's not sent as 3 lines of 120? If that is the case then does the transformer then split it up into multiple lines of 120 for the devices run off of it?
 
So is the 480V sent over one line and it's not sent as 3 lines of 120? If that is the case then does the transformer then split it up into multiple lines of 120 for the devices run off of it?
Just added the picture, which helps. Usually, the primary is "Delta" 480 volts, which only requires 3 wires (and ground.) It is 480 volts between any two of the three hots. The secondary (output) is "Wye" and uses 4 wires, 3 hots which are 208 volts between any two of the three, and a neutral. Any of the three hots will measure 120 volts to the neutral.
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In short 3 phase power is anything but simple!

277V lighting is common in commercial buildings because it's a 480V leg to ground so a transformer isn't needed. And don't get me started on "high leg", motors, or long distance transmission.
@RickR You of course are posting from the U.S. Here in Canada we commonly have 347 / 600 essentially most places you have 277 / 480. We both step down to 120 / 208 for our typical three-phase secondary distribution. @Dionysus @mikefellh Care to comment?
Toodleoo!
Ron Hebbard.
 
I often assume everyone knows what a transformer does. Sometimes, that is not the case so it is worth a 30 second review:
A transformer takes incoming electrical energy, converts it to magnetic energy which is then used to generate electrical energy. In doing so, the ratio of voltage to amperage can be "transformed." There can also electrical isolation between the primary and the secondary depending on the intent of the design. This makes it a very handy device!

EDIT: Great picture just posted by Rick above! A note of interest is that six wires are pictured leaving the plant. Often, large generators actually have two three-phase windings that are mechanically staggered (Basically 6 phase.) This is done to smooth out the rotation load of the turbine. (There is nothing that I know of that actually uses 6 phase power.) The mechanical load on the equipment increases near the peak of every waveform. A single phase home generator operating at 3,600 rpm has two points in it's rotation where the load is heaviest. A 3 phase generator at the same speed would have 6 load points. A 6 phase generator wound and operating at 1,800 rpm would have 24 load points. Wind and spin it at 900 rpm and you have 48 load points which is actually pretty smooth!
 
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Large motor variable speed controllers often generate 6- or 12- phase AC, I'm told.

Also, distribution voltages are usually 13k2 ish these days, and sometimes as much as double that. Sub-distribution is often 4300.
 
Large motor variable speed controllers often generate 6- or 12- phase AC, I'm told.

Also, distribution voltages are usually 13k2 ish these days, and sometimes as much as double that. Sub-distribution is often 4300.
@Jay Ashworth Two comments: Those same large VSR motor drives often create triplen (and more numerous) evil harmonics on neutrals.
Second point: In my area 13K8 and 27K6 are two of our commonest for installations such as college and university campuses along with decent sized theatres and arenas. I spent 6 or 8 months of my IBEW apprenticeship installing 13K8 and / or 27K6 lead and / or TEK (Sp?) jacketed overhead in literally miles of underground service tunnels on the campus of McMaster University in the 1960's. On the plus side, life in the tunnels was dry and cool in the summer and dry and warm in the winter regardless of the amount of snow and blustery wind above ground.
Toodleoo!
Ron Hebbard.
 
distribution voltages are usually 13k2 ish these days, and sometimes as much as double that. Sub-distribution is often 4300.
In this area (Eastern PA, PECO/PPL region), local distribution seems to all be 13k2. Many years ago, all local street polls were running 1200, with transformers taking it down to 120. Almost all of that is gone, although we still have a number of mini-substations that simply take 13k2 down to 1200, usually for older industrial parks whose equipment looks to be the old 1200. About 10 years ago there was a major revamp by PECO that trashed all the street-side transformers and replaced them with 7620 to 120-0-120 transformers (potential of each 13k2 leg to neutral.) So most of the street feeds were changed. Must say it was a good move as there is very little voltage sag in our area even in summer AC season.
 
In this area (Eastern PA, PECO/PPL region), local distribution seems to all be 13k2. Many years ago, all local street polls were running 1200, with transformers taking it down to 120. Almost all of that is gone, although we still have a number of mini-substations that simply take 13k2 down to 1200, usually for older industrial parks whose equipment looks to be the old 1200. About 10 years ago there was a major revamp by PECO that trashed all the street-side transformers and replaced them with 7620 to 120-0-120 transformers (potential of each 13k2 leg to neutral.) So most of the street feeds were changed. Must say it was a good move as there is very little voltage sag in our area even in summer AC season.
@JD While we're veering off to chattering about older systems, to this day the Sir Adam Beck Hydro Electric dam on the Canadian side of Niagara Falls continues to keep two 25 Hz. generators operating 24 / 7 for the benefit of a steel mill in Hamilton, Ontario and another large industrial manufacture in St. Catherines, Ontario along the same high-tension tower route.
Toodleoo!
Ron Hebbard.
 
Om my birthday in 1977 I got to watch as a transformer near the top of a telephone pole caught fire. About 5ft above was a Con-Ed (NY State) 3 phase feeder at 13kv as an insulated twisted bundle around a horizontal support wire. I pulled over in my car, parking directly across the street to watch as the fire melted the insulation and everything blew up. HUGE 5ft or more white/blue plasma ball. Now showered with flaming oil, cinders and other assorted debris, I moved away. Very quickly.
 
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Here in Cedar Rapids, you can get 34.5kV (couple of huge plants use it), 13k8, 13k2, 12k47, 4160, 480, 208, or 240 (and maybe even 25k, though I'm not certain). There's been industry here since 2-phase was common, so a lot of voltages all over.
 
@JD While we're veering off to chattering about older systems, to this day the Sir Adam Beck Hydro Electric dam on the Canadian side of Niagara Falls continues to keep two 25 Hz. generators operating 24 / 7 for the benefit of a steel mill in Hamilton, Ontario and another large industrial manufacture in St. Catherines, Ontario along the same high-tension tower route.
Toodleoo!
Ron Hebbard.
Interesting! Running 60Hz transmission lines along with 25Hz ones... Wonder if there were any strange Super-low EMF sub-harmonics effects... Any strange mysteries in the area or UFO sightings? ;)
 
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The Pole Transformers have multiple taps that allow the utility to best select the tap for the application. The multiple taps allow for accommodating line losses.
 
BIG house with three-phase AC! ;)
Or maybe Hot, Hot, Neutral, Phone line?
@JD Perhaps it's THE big house with that three phase electric chair that dims every lamp in the town alerting the townspeople to the precise moment of the execution. It's one of those film classics like the elevator doors which automatically open even when the car is not there and eager prospective passengers plunge to their deaths even though we ALL know this is an impossible invention of mystery writers as the motors which actuate the opening and closing of elevator shaft doors are mounted atop the cab thus if the cab's not there, neither are the motors to actuate the doors.
Back to our regularly scheduled post.
Toodleoo!
Ron Hebbard.
 

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