George;
To find out what type of incoming (PoCo supplied) Power system you have, check Voltage to Ground at the Service Equipment - or a Panelboard relatively close to the Service Equipment.
First, test Voltage With a "Wiggy":
<OL TYPE=A>
[*] Test between Line "A" and Metallic Enclosure of Panel / Gear,
[*] Do the same for Line "B",
[*] Do the same for Line "C"
</OL>
If any of these L-G tests give a solid reading, the system is Grounded at some point.
If the Wiggy acts like it's going to kick in, but quickly stops and shows nothing - on all 3 L-G tests, the system is Ungrounded.
Next, test Voltage with a DVM:
<OL TYPE=A>
[*] Test between Line "A" and the Metallic Equipment, as was done in the first tests,
[*] Do the same for Line "B",
[*] Do the same for Line "C"
</OL>
If the Wiggy tests showed a Grounded system, see if the DVM's readouts are somewhat equal on all 3 Lines.
If they are within 20-25 Volts of each other, the system is a 4 Wire Wye, and the Common Grounded Conductor is carried to the Service Equipment for bonding purposes.
If there is one Line with a noticably higher Voltage than the other two, the system is a Center Tap Grounded Delta. Most likely, the Ground Bonding was done only at the Transformer, but the Conductor from the Center Tap may also be brought to the Service Equipment, then bonded at both the Transformer and the Service Equipment.
I have seen them done both ways for 480VAC 3Ø 3 Wire Grounded Deltas - Grounded only at the Transformer (via center-tapped, center transformer), and with the Conductor brought to the Service Equipment.
If you find that one Line has zero Volts to Ground as read by the Wiggy, and the DVM shows like 10 - 25 Volts from the same Line to Ground, + the remaining two Lines have a high solid reading to Ground; you have a Corner Grounded Delta.
If the Wiggy shows no readings, and the DVM shows really high ones - like 300 - 500 Volts to Ground, you have an Ungrounded Delta.
You can verify this by having someone place the Wiggy's leads L-G as you have the DVM's leads L-G, and observe how the Voltage readings on the DVM drop drastically.
If this Project is of a good square footage size, and was built circa 1950 to 1970s, you most likely have an Ungrounded Delta.
If there are customers sharing the Transformer which have Commercial Lighting loads, the system may be a 4 wire Wye.
I doubt very much that you will find any 480V Corner Grounded Deltas, but the 480V Grounded Center Tap Delta "Oddball" may test out as would an Ungrounded Delta (most of the time it will not, unless the PoCo's tap and pole mounted grounding electrode conductor have a crappy connection going on!)
Good luck.
Scott35
P.S.: In the above descriptions, with the term "Voltage To Ground" or "Line To Ground", the meaning for "Ground" is to reference Metallic Enclosures - like Switchgear and Panelboards and such; NOT to any Earth Ground or Metallic Water Piping systems.
S.E.T.
