# Thread: This is 3 phase service, right?

1. ## This is 3 phase service, right?

I don't do much commercial so I wanted to make sure, plus I didn't see any 3 phase equipment in the property. You should be able to put a volt meter across any two combinations of these hot legs and get 208 volts, correct?

2. ## Re: This is 3 phase service, right?

(Title) "Re: This is 3 phase service, right?"

Correct.

Originally Posted by Matthew Barnicle
You should be able to put a volt meter across any two combinations of these hot legs and get 208 volts, correct?
No. You 'may' get 208 volts from one phase to ground, but then you will not get 208 volts between each phase.

There are 3 phase systems which are delta connected or wye connected, and which have different voltages and different voltage combinations.

Last edited by Jerry Peck; 12-17-2007 at 09:34 AM. Reason: speelin'

3. ## Re: This is 3 phase service, right?

Originally Posted by Matthew Barnicle
I don't do much commercial so I wanted to make sure, plus I didn't see any 3 phase equipment in the property. You should be able to put a volt meter across any two combinations of these hot legs and get 208 volts, correct?
Phase to phase measurements should all be the same.

A-B=B-C=C-A

4. ## Re: This is 3 phase service, right?

Originally Posted by Michael Larson
Phase to phase measurements should all be the same.

A-B=B-C=C-A
Incorrect.

That's what I was saying - it depends on the 3 phase configuration.

What you are saying would only be true with a delta connected, corner grounded (not center tapped) 3 phase system, in which case *nothing* would be 208 volts. You might have a 240/240/240 3 phase system, but as soon as you center tap one of the 240 phases to get 120 for lighting you create a different voltage configuration.

5. ## Re: This is 3 phase service, right?

This building appears to have all standard 120/240V service panels, lighting, appliances, HVAC, etc. Is there anything I should be looking for regarding the 3 phase service? Should I be checking voltages anywhere or anything else?

6. ## Re: This is 3 phase service, right?

Originally Posted by Jerry Peck
Incorrect.

That's what I was saying - it depends on the 3 phase configuration.

What you are saying would only be true with a delta connected, corner grounded (not center tapped) 3 phase system, in which case *nothing* would be 208 volts. You might have a 240/240/240 3 phase system, but as soon as you center tap one of the 240 phases to get 120 for lighting you create a different voltage configuration.

If the voltage A-B, A-C, B-C are not all equal the connected 3-phase load would operate properly.
No argument with halving the voltage if a center tapped is used but that doesn't change the the voltage across each full winding of the WYE or Delta wound transformer.

The key to understanding three-phase is to understand the phasor diagram for the voltages or currents. In the diagram at the right, a, b and c represent the three lines, and o represents the neutral.

7. ## Re: This is 3 phase service, right?

Originally Posted by Michael Larson

If the voltage A-B, A-C, B-C are not all equal the connected 3-phase load would operate properly.
Let's try this: http://www.kilowattclassroom.com/Arc...WYEPhasors.pdf

Look at that diagram and ignore the "Corner Ground (if used)", and look at it with the "center-tapped lighting transformer (4-wire system)" ground.

Each phase has 240 volts, right? (Yes.)

The center tapped phase has 120 volts / 120 volts, which is 240 volts total, right? (Yes.)

8. ## Re: This is 3 phase service, right?

So, if I see 208 volts at the panels, let's say for an a/c condensing unit, then I need to make sure I use the ampacity for 208V on the dataplate. Are all household 220 V appliances designed to be able to run on 208V also?

9. ## Re: This is 3 phase service, right?

Originally Posted by Matthew Barnicle
So, if I see 208 volts at the panels, let's say for an a/c condensing unit, then I need to make sure I use the ampacity for 208V on the dataplate. Are all household 220 V appliances designed to be able to run on 208V also?
That is correct.

10. ## Re: This is 3 phase service, right?

Originally Posted by Jerry Peck
I'm sure you understand perfectly well from your point of view but let me ask you this:

On the original photo If I measured between any two poles on the disconnect I would expect to measure the same voltage between left and center, left and right and right and center.
Would you agree?

PS. Been gone most of the day without computer access so I have not been able to keep the flow going smoothly on this post.

11. ## Re: This is 3 phase service, right?

Originally Posted by Michael Larson
On the original photo If I measured between any two poles on the disconnect I would expect to measure the same voltage between left and center, left and right and right and center.
Would you agree?
No.

It depends on where the other ends of those conductors are connected. Those conductors go back to the 3 phase transformer, and, depending on how the transformer is connected will be what you are asking.

That's what I was trying to explain.

12. ## Re: This is 3 phase service, right?

Jerry, I see no indication of a high leg on the wires to that disconnect so I think it's likely they are the same voltage phase to phase.
Of course metering it would clear all doubt.
If there are any connected 3-phase loads they would have to be equal.

I think we lack enough information to determine what's really going on in this particular photo. Thanks for the interchange.

13. ## Re: This is 3 phase service, right?

Originally Posted by Michael Larson
Of course metering it would clear all doubt.
To answer the original question, though ...

Originally Posted by Matthew Barnicle
You should be able to put a volt meter across any two combinations of these hot legs and get 208 volts, correct?
The answer is still ... No.

14. ## Re: This is 3 phase service, right?

Originally Posted by Jerry Peck
To answer the original question, though ...

The answer is still ... No.
And we still disagree as no information(wiring diagram) has been posted demonstrating your contention.

BTW-You still have not answer the question regarding any connected 3-phase loads.

15. ## Re: This is 3 phase service, right?

Originally Posted by Michael Larson
Of course metering it would clear all doubt.
To answer the original question, though ...

Originally Posted by Matthew Barnicle
You should be able to put a volt meter across any two combinations of these hot legs and get 208 volts, correct?
The answer is still ... No.

[quote=Michael Larson;27589]And we still disagree as no information(wiring diagram) has been posted demonstrating your contention.[/quote

Michael,

What voltages are available on three phase wiring systems? (Assuming a delta or wye and no center taps or grounded corners.)

When you answer that, you will see why you will not get 208 volt phase to phase to phase between all of the phases.

BTW-You still have not answer the question regarding any connected 3-phase loads.
Which question?

You can have 3-phase loads and single phase loads on a 3-phase system, and it all works with the 3-phase system connected the correct way.

I sure wish Corey would jump in here, he most likely can explain it better than I am.

16. ## Re: This is 3 phase service, right?

Originally Posted by Jerry Peck
Michael,

What voltages are available on three phase wiring systems? (Assuming a delta or wye and no center taps or grounded corners.)

When you answer that, you will see why you will not get 208 volt phase to phase to phase between all of the phases.
Commonly, 480 VAC or 208 VAC measured phase to phase.

I hope someone else will come along and explain this to both of us.

I have never seen a 3-phase motor or load that didn't require each phase to phase voltage to be equal each other.

17. ## Re: This is 3 phase service, right?

Originally Posted by Michael Larson
.

I hope someone else will come along and explain this to both of us. .
Michael,

I don't think Both of you need this explained.

Thanks,

18. ## Re: This is 3 phase service, right?

Originally Posted by Michael Larson
Commonly, 480 VAC or 208 VAC measured phase to phase.
Try going here: Three-phase electric power - Wikipedia, the free encyclopedia

"After numerous further conversions in the transmission and distribution network the power is finally transformed to the standard mains voltage (i.e. the "household" voltage). The power may already have been split into single phase at this point or it may still be three phase. Where the stepdown is 3 phase, the output of this transformer is usually star connected with the standard mains voltage (120 V in North America and 230 V in Europe and Australia) being the phase-neutral voltage. Another system commonly seen in North America is to have a delta connected secondary with a center 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 high leg) and neutral and 240 V between any two phases) to be made available from the same supply.

Then go here: Wye Systems

And here: Delta System

And here: Delta and Wye Circuit Equations

19. ## Re: This is 3 phase service, right?

Yes I understand that.

But, you keep trying to tell me that when measuring from phase to phase in all three combinations that I will not get the same voltage readings.

As long as the probes are measuring across the entire phase winding it will measure the same for each winding in either a WYE or DELTA configuration.

Perhaps we are unintentionally looking at things from a different perspectives but I fail to understand what that might be.

20. ## Re: This is 3 phase service, right?

Originally Posted by Michael Larson
But, you keep trying to tell me that when measuring from phase to phase in all three combinations that I will not get the same voltage readings.
I see where the problem is.

That IS NOT what I keep telling you.

I keep telling you that you will not get *208 volts* that way. And I've kept repeating *208 volts* ...

*208 volts* is the "high leg" 1.732 multiplier of the 120 volts of the center tapped phase (for example, in the delta example I kept giving, because it is a common example).

Just like 277 volts is not phase to phase on a wye system, 480 volts is, 277 volts is phase to ground (480 volts is the 1.732 mulitpler from 277 volts).

Last edited by Jerry Peck; 12-17-2007 at 05:12 PM.

21. ## Re: This is 3 phase service, right?

Jerry,

Below is from one of the links you posted. 120 and 277 are only available when measured from phase to ground(neutral). Otherwise its 208 or 480 measured phase to phase.
-----------------------------------------------
Wye Systems

The transformers supplying the plant loads can be connected on the load (secondary) side either in delta or in wye configuration. Either of these configurations can serve balanced three-phase three-wire loads equally well but they differ greatly in the way they accommodate single-phase loads. The other difference is the way the system can be grounded.
Standardized system voltages are based on integer multiples of 120 Volts and are historically related to the use of delta systems. That is why voltages that have the 1.732 as a multiplier (208 V; 4,160 V; 12.47 kV) are wye connections. A wye-connected secondary requires a primary winding connected in delta or supplied from a four-wire system with a firm neutral.
The most common three-phase wye system is the four-wire system shown here. The fourth wire is connected to the common point where all three transformer phase windings are connected together. NEC Section 250-5 requires that the system be grounded for AC systems of 50 Volts to 1,000 volts where the neutral is carried through as the fourth wire as in a 480Y/277-Volt system.
This wye system provides the three lines to serve three-phase loads at the system line voltage. It provides three other line-to-neutral circuits to serve the single-phase loads at 277 Volts, a voltage which is 57.7% of the line voltage. Common arrangements are 480Y/277Volts and 208Y/120-Volts. Single-phase loads can be connected line-to-line as well, if adequately insulated at both ends and protected by two-pole, common-trip circuit breakers.
A single-phase load connected in a line-to-neutral configuration only loads the transformer phase that it is connected across. A single-phase load connected line-to-line loads two transformers with a derating penalty of about 14%. (If these loads are balanced line-to-line across all three phases, this penalty cancels out.) Single-phase loads are almost always served line-to-neutral instead of line-to-line.
The common neutral connection provides a convenient place to apply ground-fault-current detection or protection. The grounding conductor at the neutral carries current only if current returns outside the normal four wires

22. ## Re: This is 3 phase service, right?

Too do a visiual inspection, on a commercial building.
1. Go to the electric meter. (Put your hands around the meter to block the sun) in the center of the meter,there should be two small lights, that indicates that it is a three phase service.
2.In picture 27k there is a rotation arrow, that gives you the three phase rotation A,B,C. (3 phases)
3. On an overhead service there should be more than 3 connections going into the service head.
4.On overhead service follow the electric wire to the utility pole, there should be more than 1 transformer. 2&3 transformers indicates 3 phases.

For safety, if you have visiauly inspected, and you want to do any testing on three phases. If, you have the slidest dought, please get an electricion.
3 phase panel is like a sleeping giant you don't want to wake him up.

23. ## Re: This is 3 phase service, right?

Originally Posted by Matthew Barnicle
I don't do much commercial so I wanted to make sure, plus I didn't see any 3 phase equipment in the property. You should be able to put a volt meter across any two combinations of these hot legs and get 208 volts, correct?

The most common commercial electrical services are either 120/208 -or- 277/480. Both are four-wire systems (3-phase legs & a neutral) Wye connected systems.

Testing:

480v: phase-to-phase = 480v / phase-to-neutral = 277v
208v: phase-to-phase = 208v / phase-to-neutral = 120v
Note: Today High Leg Delta connections are less used.

Some on site clues:
If the service voltage is 480v, then to obtain 120v for common receptacles will require an additional transformer. So, if you see interior transformers connected to service equipment which then feed receptacle branch circuit panels (look for single-pole circuit breakers) you most likely have a 480v service. Without the secondary interior transformer feeding a branch circuit panel you most likely have a 208v -or- 240v service.

It would be very rare to find a 480v system that lacks any step-down transformers as 120v is very common and necessary almost everywhere. In a 480/277V system most likely the fluorescent lighting will be 277v, remove a switch plate and see if 277v switches are being used. If you discover transformers most likely you will be able to determine from the nameplate the input & output voltages. You should also be able to read the voltage rating of any panel from its label.

24. ## Re: This is 3 phase service, right?

Originally Posted by Joseph Burkeson
The most common commercial electrical services are either 120/208 -or- 277/480. Both are four-wire systems (3-phase legs & a neutral) Wye connected systems.

Testing:

480v: phase-to-phase = 480v / phase-to-neutral = 277v
208v: phase-to-phase = 208v / phase-to-neutral = 120v
Note: Today High Leg Delta connections are less used.
"Note: Today High Leg Delta connections are less used." I guess I'm just "less used" and out-of-date. Thus, I stand corrected.

Joe,

Thank you for contributing to the technical boards.

25. ## Re: This is 3 phase service, right?

If you are going to take a voltage reading in the electric panel, recommend:
1. Notify someone that you are working in the electric panel.
2. Wear dry gloves (600v rubber gloves) and eye protection
3. Stand on dry ground, a wooden platform or rubber blanket
4. Check voltage with a Fluke tester (digital)

In a commercial building, industrial and small business they all require 3 phases. When testing you are required to test the neutral to phase and phases to phase, for voltage.
All phase on the secondry side have names abc/xyz/ or boy (color code)

Type of voltage readings Phase to Neutral Phase to Phase
3-Phase n-a n-b n-c a-b b-c c-a
120-240-208 120 - 120 - 208* 240 - 240 - 240
120 - 208 120 - 120 - 120 208 - 208 - 208
120 - 240 120 - 120 - 120 240 - 240 - 240
277 - 480 277 - 277 - 277 480 - 480 - 480
Straight 480 480 - 0 - 480 480 - 480 - 480
B Phase grounded
All motor's are 3 phase, they operate on delta system. If a utility transformer is defective, the motor will operate on 57%, and not full power.
All industrial lighting, 3 phase operate on Y system. If a utility transformer is defective, or a breaker opens. A section of the building will be out.
Remember what ever readings you take they all should read the same across the board. If the readings are different, there is an unbalance load. Ex. on a 120-240 3-phase, the readings(phase to phase)are 230-250-240.
On a 120-240 3-phase the reading is 216-216-216 across the board, or 264-264-264 across the board.This is O.K. the utility company can supply a difference of 10% take or give of the 240v.
Another point of interest is the rotation arrow on the door. That also, tells the engineer the rotation of the motor (they can make the motor run clock-wise or counter-clock wise, by the change of two phases in the panel.
* Some customers will ask for a 208v line (high leg) the utility company will supply them with a 208v phase.
Ex. For lighting or a water heater ect. The 208v high leg will be on the right side of the panel.
I hope this can help us under stand electricity. And most of all be safe!

26. ## Re: This is 3 phase service, right?

3 Phase__________n/a---n/b--n/c_________ a/b--b/c--c/a
120-240-208*_____120--120--208*________240--240--240
120-208__________120--120--120_________208--208--208
120-240__________120--120--120_________240--240--240
277-480__________277--277--277_________480--480--480
Striaght 480 ______480---0---480_________480--480--480
B Phase grounded

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