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Old 01-23-2009, 09:23 PM   #1
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Default 120/240 Volt 3-Phase Delta High Leg

Ok I have a question about a high leg delta transformer. I know that phases A and C to neutral you get 120 and that phase B to neutral you get 208. I also know that between any two phases you get 240. My question is why don't you get 328 volts from phase B to either phase A or C? I believe it has to do with the was the sine waves are off from each other but am not totally sure. Does any one have a good explaination? Or maybe a diagram of the sine waves?

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Old 01-23-2009, 10:41 PM   #2
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Old 08-05-2009, 11:15 AM   #3
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Default 208 / three phase

I am an electronic technician by trade, not an electrician, but have been forced into performing electrician type work from time to time. I am always reluctant to more than the most basic work - I know the difference, and my limitations.

Anyway, 208V has always been a bit of a mystery to me, but your diagram using the triangle and pythagorean theory finally made it very clear to me.

My only question now, is why do we use 208V in the first place?

Thanks,

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Old 08-05-2009, 11:27 AM   #4
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Quote:
Originally Posted by Goober Pat View Post
I am an electronic technician by trade, not an electrician, but have been forced into performing electrician type work from time to time. I am always reluctant to more than the most basic work - I know the difference, and my limitations.

Anyway, 208V has always been a bit of a mystery to me, but your diagram using the triangle and pythagorean theory finally made it very clear to me.

My only question now, is why do we use 208V in the first place?

Thanks,

Goober Pat
The 208 V in the delta setup is just an artifact. We don't use it. In a three phase wye connected system where we want single phase 120 V, the phase to phase voltage turns out to be 208, and we use this just like we would 240 V in a single phase system, or all three phases for large loads.
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Old 08-05-2009, 05:26 PM   #5
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I must say in my 30+ years in the trade I have never seen a transformer like that.
Why would one use this?
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Old 08-05-2009, 05:47 PM   #6
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I must say in my 30+ years in the trade I have never seen a transformer like that.
Why would one use this?
It was used here alot for heavy commercial and industrial buildings with lots of motors. There is less stress on a delta connected transformer from heavy motor starts than on a wye. And, of course, there is alot of single phase 240 V equipment as well as 240 V three phase stuff. And you need 120 V for general power and lighting, so the 120/240 V delta is pretty good for that.

Of course, the 208 V high leg isn't used for anything normally.
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Old 08-05-2009, 09:02 PM   #7
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Or when the POCO wanted to save money by supplying a three phase service with two pots.
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Old 08-05-2009, 09:28 PM   #8
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In which case, that is called an "open delta" service.
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Old 08-05-2009, 09:35 PM   #9
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Quote:
why don't you get 328 volts from phase B to either phase A or C?
Because, that's the way it is.

In my line of work, I don't need to know everything. My brain isn't big enough so I limit my knowledge to "need to know"
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Old 08-05-2009, 09:41 PM   #10
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Of which has 57% capacity of a three transformer bank if the transformers are of the same KVA rating. Most 240 delta banks I've seen have a large pot with a smaller " kicker" pot.
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Old 08-06-2009, 12:10 AM   #11
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Power companies are slowly doing away with these systems.
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Old 08-06-2009, 08:07 PM   #12
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Too bad that POCOs are not connecting deltas as much anymore.

A closed delta (3 transformers) will tend to balance voltage. This is why 480 to 120/208 transformers are almost always delta on the 480 side. Also why most larger motors are delta connected.

If there were 3 transformers on a pole connected wye on the primary, and delta on the secondary, and you opened one of the cut-outs, you'd still have 3 phase power available on the secondary. If you completely remove one of the transformers, you'd still have 3 phase at the secondary.

Voltage balance is very critical to 3 phase motors, and POCOs must maintain it within certain tolerances. Since they are getting away from delta secondaries, most of them are getting away from wye-wye padmounts in favor of delta-wye.

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Old 06-21-2010, 05:08 PM   #13
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what color is used to identify the high leg for this system?
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Old 06-21-2010, 05:10 PM   #14
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usually orange tape
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Old 06-21-2010, 10:42 PM   #15
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what color is used to identify the high leg for this system?
Purple in our neck of the woods
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Old 06-22-2010, 10:23 AM   #16
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Orange in the NorthWest
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Old 06-22-2010, 04:52 PM   #17
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NEC dictates that it must be orange.
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Old 06-22-2010, 06:13 PM   #18
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NEC dictates that it must be orange.
Or be identified by other effective means. (See 110.15)

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Old 06-23-2010, 03:49 AM   #19
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My only question now, is why do we use 208V in the first place?
I don't think it was chosen. It's just a consequence of choosing desired "primary use" voltage.

208Y/120v is used where a lot of 120v plug-in loads are expected. Many motors can run on 208v as well to take advantage of 208v. Apartment complex is a good example. Each unit definitely needs 120v outlets, yet three phase is available for large motors needed for elevators.

EU countries used to have nominal voltages ranging from 220 to 240v for plug-in loads. Now, they simply changed the nominal to 230 and places that used to be 220 go by -10% +6% tolerance and places that were 240 go by -6% +10% tolerance. I believe Mexico and Brazil chose 220v as the "main" voltage, so they have 220Y/127v. They have the same outlets we do, but nominal is 127v.

277v is also a "secondary" voltage. 480v was chosen for 480Y/277v to power three phase machines and single phase 277v is pretty much only used for lighting.

Over at Canadiana, they use 600Y/347v and put things together using square indented screws.
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Old 06-23-2010, 01:31 PM   #20
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Or be identified by other effective means. (See 110.15)

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