Paul's reply is right on the money!!!
Just for fun, I will add a few things:A: VISUALS;
[*] 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?
And:B: TESTING VOLTAGES;
[*] 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.
edited to correct type of Power Meter referred to
(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).]