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tiny fingers needed for this


Ken Meyer
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Here are some more pix from today's inspection. The first needs no explanation. The second is one of those "why did they do it that way?" ones, I pity the electrician who has to get in to those grounding and neutral bus bars. And yes, I did note the paint, the mixed grounds and neutrals, the excessive amount of romex, and the wires to nowhere on the left side with no wire nuts.

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It's orange tape.

110.15 High-Leg Marking

On a 4-wire, delta-connected system where the midpoint of one phase winding is grounded, only the conductor or busbar having the higher phase voltage to ground shall be durably and permanently marked by an outer finish that is orange in color or by other effective means. Such identification shall be placed at each point on the system where a connection is made if the grounded conductor is also present.

__________________

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Originally posted by Chad Fabry

It's orange tape.

110.15 High-Leg Marking

On a 4-wire, delta-connected system where the midpoint of one phase winding is grounded, only the conductor or busbar having the higher phase voltage to ground shall be durably and permanently marked by an outer finish that is orange in color or by other effective means. Such identification shall be placed at each point on the system where a connection is made if the grounded conductor is also present.

__________________

OK, there's a whoosh....... Chad, you're a mensch.

Can you explain the "midpoint" of one phase winding being grounding?

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Originally posted by Ken Meyer

. . . The second is one of those "why did they do it that way?" ones,. . .

Hi Ken,

As Neal pointed out, that's a three-phase system. More particularly, as Chad pointed out, it's a center-tapped, 4-wire delta system. With a system such as this one, you've got:

  • 240 volts between any two of the phase conductors.
  • At the top and bottom phase conductors, there'll be 120 volts between the phase conductor and the neutral
  • but at the middle phase conductor, there'll be 208 volts between the phase conductor and the neutral. This middle phase conductor is called the "high leg" or the "stinger." It's marked with orange tape.

Now, if you look carefully at the breaker arrangement, you'll see that the missing breakers are all on the high leg's bus. The only place where they're using that particular phase conductor is at the heat pump. This makes sense for an office building; there probably aren't many other motor loads.

In my report, I'd just tell him that he's got a 3-phase, center-tapped, 4-wire, delta system.

BTW, there are lots of other ways to configure 3-phase power. This is just one type. These systems are quite confusing.

- Jim Katen, Oregon

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Originally posted by kurt

. . . OK, there's a whoosh....... Chad, you're a mensch.

Can you explain the "midpoint" of one phase winding being grounding?

The "grounded at the mid-point" thing is no different than the single-phase systems you're used to.

Think about a regular, old single phase service to a house. At the transformer, one side of the secondary winding is connected to one leg and the other side of the winding is connected to the other leg. The neutral wire is connected to the midpoint of the winding between the two ends. Back at the house, you can measure 240 volts between the two phase conductors, but only 120-volts between one phase conductor and the neutral.

A center-tapped delta system has a similar configuration, but only between two of the phases. Make sense?

- Jim Katen, Oregon

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Originally posted by chicago

Jim are you saying that if I put a voltage tester on the center hot it will show the same as a 220 line.You mentioned between ground and neutral which confused me.On the outside hot's you only mentioned neutral.

I erred. Go back and read it again; I corrected the error. It should make sense now.

Is there a good on line diagram for this.?

I'm sure there are many. Try Google. I'm on my way out now. If no one's found a good drawing by the time I get back, I'll see what I can do.

- Jim Katen, Oregon

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Last sentence is where I get lost, but will look it up.

I was wondering if since the middle phase conductor has 208 with neutral will there be 328 between it and the top or bottom phase conductor?

(ok) went to wikipedia for a great animated 3 phase demo but still trying to grasp how 208 got to one conductor.http://en.wikipedia.org/wiki/Three-phase_electric_power

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Ha Ha ,the diagram at Wiki helps explain why there is a loss with two phases combined as they are not exact opposites in the tug of war.

(since there are three).

But if I take two hots and put them together they will short.

The hard part is understanding why they combine for 208 v with out shorting.

I assume it has something to do with the neutral being involved.

Now excuse me while I pop my head with a pin.

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From the Wiki site...Another system commonly seen in North America is to have a delta connected secondary with a centre tap on one of the windings supplying the ground and neutral. This allows for 240 V three phase as well as three different single phase voltages (120 V between two of the phases and the neutral, 208 V between the third phase (known as a wild leg) and neutral and 240 V between any two phases) to be made available from the same supply.

I like to think I have a fairly good grasp of residential wiring and I am now absolutely certain that it is only residential!

To paraphrase Kurt and Bob..."Whoooosh! Huh? Say What?" [:-dunce]

I do understand the bit about the tape being orange! [:-graduat

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Originally posted by chicago

Ha Ha ,the diagram at Wiki helps explain why there is a loss with two phases combined as they are not exact opposites in the tug of war.(since there are three).

I looked at the Wiki diagram and it's very confusing. Forget it. There's no loss with the two 120-volt phases.

But if I take two hots and put them together they will short.

The hard part is understanding why they combine for 208 v with out shorting.

I assume it has something to do with the neutral being involved.

Now excuse me while I pop my head with a pin.

It's got nothing to do with shorting. When we talk about 240-volts between two phases, we're talking about potential.

Here are some drawings that I made. Let's start with a simple 240/120 single phase residential service. Open the file called Single phase diag. Download Attachment: icon_adobe.gif Single Phase Diag.pdf

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Does everyone understand this? It's what we look at every day.

Now look at the file called center tapped delta.

Download Attachment: icon_adobe.gif Center tapped delta.pdf

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Does this make sense?

- Jim Katen, Oregon

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Originally posted by chicago

Ha Ha ,the diagram at Wiki helps explain why there is a loss with two phases combined as they are not exact opposites in the tug of war.

(since there are three).

But if I take two hots and put them together they will short.

The hard part is understanding why they combine for 208 v with out shorting.

I assume it has something to do with the neutral being involved.

Now excuse me while I pop my head with a pin.

The wiki diagram you're looking at is three phase WYE (three phases measuring 208 V RMS phase to phase and 120 V RMS phase to neutral.)

Katen is describing three phase DELTA, which measures 240 V phase to phase and 120 phase to neutral IF the windings are cnter tapped.

The difference between the WYE and DELTA phase to phase voltages are a result of the different phase angles with the different configurations.

Google delta and wye power or something similar, and the concepts should be explained somewhere out there....

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Originally posted by Ken Meyer

I'm getting everything except where the 208V comes in

I'm just getting dizzy. Jeez, I always feel like a noob whenever anyone starts really getting technical about electricity. [:-shake]

I gotta stop reading this thread or my wife's liable to come home and find me huddling under a blanket in the closet sucking my thumb. [:-scared]

OT - OF!!!

M.

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When you measure across a DELTA 240 leg, you're essentially measuring across two 120V secondarys that are 180 degrees out of phase. 120v plus 120v = 240v

When you measure across a the phases ofd a WYE secondary, you are measuring across two secondaries that are only 120 (i think)degrees out of phase (vice 180) , hence the nominal 208VAC vice 240.

....We really don't need to know this, do we.....

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I will need to absorb some of this and go back to it on google in a few days before I can go" a ha "I get it, but I do understand the triangle as the delta .

Generator has rotation with three hits .These are 120 degrees apart since a circle is 360.

Equal distance is a must for the phases to work in unison.If I get "a ha" I will post here again with my caveman comments.

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Originally posted by Ken Meyer

I'm getting everything except where the 208V comes in, and what the term "phase angle" means. I'll do some more web research later when I have time.

I can explain the 208v but you won't like it. It's the square root of three divided by two and multiplied by 240 volts.

The phase angle is referring to the amount that the three phases are out of phase with each other. (120 degrees)

There's a very clear description of the basics of three-phase power here:

http://www.radioelectronicschool.net/fi ... 3phase.pdf

- Jim Katen, Oregon

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