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#67121 06/27/06 09:21 PM
Joined: Dec 2000
Posts: 4,289
The different voltages really don't pose much of a problem, Mike.

I haven't installed anything but 120/208 Volt or 277/480 Volt 3 phase services for at least 20 yrs., except for the odd one here and there that the PoCo didn't want to change out existing transformers because they served other existing services too.
(BTW, Canada uses 347/600V where we use 277/480) [Linked Image]

Single phase services have been 120/240V That's it....

As for telling a Delta 3 Phase 4 Wire system, the high leg should be tagged or marked, BUT DON'T BET ON IT and BE SURE TO VERIFY. This was a requirement since the later '50s or early '60s. In '71 the NEC added that the high leg could be orange or marked with orange for identification.
(San Francisco requires it to be marked in Violet)

What's confusing about that?? [Linked Image]

#67122 06/27/06 10:05 PM
Joined: Apr 2004
Posts: 349
Skingusmc - Ditto what Dnk said about no stupid questions, and back to your not stupid at all question for a moment. When you walk up to a panel or switchboard, either carefully check the nameplate voltage or take your own voltage readings (both if you can).

In a dual voltage system, if there is a factor of approx 1.73 between the voltages (i.e. 120/208 or 277/480), it's a Wye.

If there is a factor of 2 between the voltages (i.e. 120/240), it's a delta.

There are other wierd voltages for special applications (VFD's, etc), but the rules above apply. Hope this helps some.


There are 10 types of people. Those who know binary, and those who don't.
#67123 06/27/06 11:09 PM
Joined: May 2004
Posts: 116
All -

Let me also agree and state that there are no "stupid" questions (well, there was that one I asked when I was in Istanbul, but that's another story entirely)(grin).

I also knew I could get great info here, so thank you one and all.

As for the US being a PITA, it's a gift and talent!!!(grin)


#67124 06/28/06 02:21 AM
Joined: Oct 2000
Posts: 2,722
Broom Pusher and

Paul's reply is right on the money!!! [Linked Image]

Just for fun, I will add a few things:


[*] See what the Transformer looks like,

[*] See what the Service Feeders are,

[*] See what the kWh Meter on the Gear with the Service Disconnect is rated for,

[*] Ratings for the Service Equipment - 1 Phase, 3 Phase 3 Wire or 3 Phase 4 Wire,

[*] Customer-Owned Step-Down Transformers (SDS) - are there any?



[*] If 1 Phase, test L-L (Line to Line) with a Voltmeter that displays somewhat accurate values,

[*] If 3 Phase, test L-L between A-B, B-C, and A-C to see if you get an average reading for the 3 phase voltage,

[*] To verify a 208Y/120 3 Phase 4 Wire Wye System, vs. a 240/120V 3 Phase 4 Wire system, test L-G on each of the 3 lines,

[*] To verify Ungrounded 3 Phase 3 Wire Delta Systems, vs. Corner Grounded / Center Tap Grounded 3 Phase 3 Wire Delta Systems, test L-G on all 3 lines with a "Wiggy" type Voltmeter.


Lists in "A":

Visual check of the Transformer Network of Pole Mounted Transformers, to see if the service is Single Phase, or one of the various 3 Phase types.
For assistance on Identifying 3 Phase arrangements, see the Technical Reference section for Transformer Schematics.

Service Feeders- if overhead, you may be able to identify the system by number of conductors &/or size of conductors.
For example, 3 service feeders would indicate either a 1 phase 3 wire service, or a 3 phase 3 wire service.

4 Conductors of equal size would indicate a 4 wire Wye, whereas 4 conductors with one conductor being significantly smaller than the others (or two small ones, and the other two large), would likely indicate a 4 wire Delta service.

The kWh Meter will display ratings for Voltage and system type.
4 Wire kWh Meter are listed as "3 Stator" types.

The Service Equipment should have nameplate ratings for the Amperes and Voltage, along with "Neutral Voltage". This is not very accurate, but may uncover a 3 Phase system if only 1 Phase Panelboards are found.

If there are any SDS Transformers, the Primary (Input) Voltage may disclose what type of service is available.
If you see "Primary = 240V Delta, Secondary = 208Y/120V 4 wire Wye", the Service might be a 3 wire Delta (also could be a 4 wire Delta, so it's a crap shoot!).


Lists in "B":

Tests on 1 Phase 3 wire Panelboards to verify if the Service / Transformer is a 4 Wire Wye or a 4 Wire Delta:
Looking for a reading in the range of 208 VAC between each Ungrounded Conductor, for a Wye Connected Transformer; and in the range of 230 VAC for a Delta Connected Transformer.

Same as shown in "B1" above - trying to get the L-L System Voltage.

Verify Voltage on all 3 Lines to Equipment Ground - or Grounded Conductor ("Neutral").
If all 3 Voltages are equal (+/- 10%), then the system should be a Wye connection.
If one Line is vastly different than the other two, the system is most likely a Delta connection.*

To verify if your Delta system is Ungrounded or Corner Grounded, you need to use a Solenoid-Type Voltage tester, such as a "Wiggy" for these tests; as a High Impedance Input type Volt Meter (typical DVMs) will result in inaccurate readings.

Tests to determine if Transformer is setup as 3 Phase 3 Wire Corner Grounded Delta:

2 of the 3 Lines should have a "Solid" Voltage reading, from Line to Ground ("L-G"), and one should read "Zero Volts" from L-G, with all Voltage readings between the 3 Phase Conductors (A-B, B-C and A-C) being the same as the Voltage to Ground.

If a 1 Phase 3 Wire Panelboard is encountered, the Voltage read between any one Ungrounded Conductor Bus and the Grounded Conductor's Bus (the Neutral Bus Kit), should be the same as the Voltage read between both of the two Ungrounded Conductors.


Tests to determine if Transformer is setup as 3 Phase 3 Wire Center Tap Grounded Delta:

Test L-G Voltage on all 3 Ungrounded Conductors, looking for one L-G test which shows a significantly higher Voltage than the other two.


Tests to determine if Transformer is setup as 3 Phase 3 Wire Ungrounded Delta:

Test L-G with the Wiggy should display a very short "Pulse" on the Solenoid, along with the Neon indicator Lamps displaying a very minute pulse, with an overall result of "Zero Volts" displayed.
FYI: these indications may not even occur at all!

After testing with the Wiggy and finding the above results, using a DVM (Digital Voltage Meter) set to the highest Voltage scale possible (at least 750 VAC), test from L-G to observe the various Voltage readings.
On a 480V Ungrounded Delta, values of 300 VAC to 500 VAC as measured from L-G at the Service Equipment are normal.

With the DVM reading a Voltage between L-G, have an assistant place the Wiggy's test probes between L-G on the same "Phase" as the DVM's test leads are on - and as close as possible to the DVM's leads.
This should result in the DVM's readings being dropped to near zero volt levels - possibly maintaining at a level of around 35 to 100 Volts maximum.
Remove the Wiggy's test lead, and the DVM will begin accumulating an increasing Voltage reading, until it peaks off once again.

BTW - if you are familiar with Capacitors, you will recognize the ways these Wiggy / DVM tests affect the Capacitive Charging effects on the system.

Good luck!


edited to correct type of Power Meter referred to [Linked Image]
(I played the Dumb-Dick card, and typed "KVA Meters", when meaning "kWh Meters"!!!)

[This message has been edited by Scott35 (edited 06-28-2006).]

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
#67125 06/28/06 04:55 AM
Joined: Aug 2001
Posts: 7,520
Having learned about the various U.S. systems from an outsider's perspective myself, I can see how it might appear that there are far more combinations than necessary compared to other parts of the world.

Examining the situation more closely, however, there's a lot of sense in choosing the voltage level, wye/delta arrangement etc. to match the need rather than having to force everything onto a "one size fits all" supply.

The 2:1 ratio of many of the voltages also makes it relatively easy to make motors and similar devices dual-rated for 120/240 or 240/480V use by series/parallel strapping of windings.

The high-leg 4-wire delta did seem a strange beast to me at first, but when you look at the reason it originated (to provide 120V for general loads from a basic 240V delta system), it actually makes perfect sense. Why go to all the trouble and expense of providing a separate transformer (PoCo- or customer-owned) for your 120V loads when you can get 120 by just grounding a center-tap on one side of the delta?

With regard to supply frequencies, it would be nice if the whole world could use the same standard, but I suspect we're way past the point of that being a practical aim, at least not without major disruption. North America (not just the U.S. but also Canada, Mexico, and some of the Caribbean) and Central America already have so much 60Hz equipment and Europe, Australasia etc. already have so much 50Hz that to change now would be a mammoth task.

In the U.K., we only standardized on 50Hz with the plans for the national grid in the 1930s. Prior to that, each generating area could use its own frequency: 50Hz, 40Hz, 25Hz, etc. Remember we've had similar discusiions here in the past about non-60Hz systems in the U.S. (e.g. the Los Angeles area changed from 50 to 60Hz in the 1930s, parts of upstate New York had 25Hz until the 1950s.).

Maybe, because we did it first, and other folks decided to be different from us?

A few months ago somebody in a telecom group asked (with a decided anti-American sentiment) why the U.S. "decided to be different" with its area code numbering and dialing plan. He thought the U.S. should have adopted the "worldwide" standard of dialing zero in front of an area code etc. Answer: The U.S. system was developed before the others, so if any accusations are to be made, the pertinent question would be "Why did the others not follow the existing U.S. standard?"

#67126 06/29/06 07:25 PM
Joined: Sep 2005
Posts: 202
WFO Offline
Good answer.
One point on the Kwh meter. With the newer solid state meters, many forms of metering are being combined so that one meter often works on wye and delta systems. To add to the confusion, they are often free ranging on voltage. So the nameplate may very well read, "120 to 480 volts).

#67127 06/29/06 07:33 PM
Joined: Dec 2000
Posts: 4,289
Remember not to believe everything you read.

[Linked Image]

Lots of guys do it wrong [Linked Image]

Testing, like Scott said, is the only sure way to know for sure.

#67128 06/29/06 10:47 PM
Joined: Jul 2002
Posts: 8,432
Likes: 3
Good Lord!!,
I said that??.
My apologies to all not only in the US but to all at ECN for having to read that sort of drivel.
That was ABSOLUTELY not the tack I was trying to take.
Each system to thier own I say.
Sorry guys. [Linked Image]

#67129 06/29/06 11:16 PM
Joined: Aug 2001
Posts: 599
I am surprised nobody mentioned the size of the transformers. If you look at the pole that feeds the building and the bank of transformers has one thats larger than the other two or two transformers with one larger than the other it's a 240/120 delta. Three pots of the same size...wye. Always test to be sure but different size transformer pots screams high leg.

#67130 06/30/06 12:53 AM
Joined: Jul 2004
Posts: 9,772
Likes: 14
If you see something that looks like 3p with 2 transformers that is usually open vee delta.

Greg Fretwell
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