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I have two power questions that would be greatly helpful.

  1. Just wanted to double check and confirm that I'm not assuming on nomenclature, but what is meant by 120/208V. My device is a UPS and is split phase, so does 120 mean the Line to GND voltage while 208 means the Line to line voltage? Similarly, is there a nomenclature to identify single from three phase 208? I've seen some spec sheets that just say 208V and I've had to dig deeper or ask the rep which phase it is.
  2. With my split phase UPS, I unfortunately have a three phase outlet. Is there any efficient way to convert the three phase to match with my split phase? Seems like I could maybe do this Can North American split-phase 220VAC supply power to a device designed for single phase power? and convert my outlet to single phase first, but I imagine there would be a lot of losses which would not start up the UPS. I also couldn't seem to find any converters or isolation (?) transformers? My device requires 10 kW.

Any guidenance would be appreciated thanks!

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    \$\begingroup\$ 120/208 means three-phase, 120 V line-to-neutral and 208 V line-to-line. It is not split phase. If you have a 120/208 service that has two hot lines and a neutral, you have two lines and a neutral from a three-phase service. It is not unusual for one tenant of a large building to have that service. It is not the same as a split-phase service. You need to clarify. What is your electrical service? What is the UPS input requirement? What output does the UPS require? What do you want to connect it to? \$\endgroup\$
    – user80875
    Jul 23, 2021 at 20:39
  • \$\begingroup\$ @CharlesCowie so my UPS is the Eaton 9PX10KSP. It says it's input is 120/208 (which according to you would mean three phase), but upon further inspect of the manual, its split phase (L-L-N). Am I able to route split phase from three phase? My wall power is 208 three phase \$\endgroup\$
    – bchang32
    Jul 26, 2021 at 2:02

1 Answer 1

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In a Y-connected 3-phase system, if the line-to-neutral voltage is 120 V, then the line-to-line voltage is 120 * √3 = 208 V.

In a single-phase system (common in North America), the 240 V winding has a center tap that is normally grounded to provide two branches of 120 V each.

Only you can determine whether your device designed for 240 V nominal will operate on 208 V.

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  • \$\begingroup\$ So from my UPS' datasheet, the electrical input is "120/208V default (120/240, 100/200, 110/220, 115/230, 127/220V). " Does this mean it will accept 2 live wires of three phase and I can just have one unconnected? (from the manual, its got L-L-N-G inputs). or do I connect the third L to N? \$\endgroup\$
    – bchang32
    Jul 26, 2021 at 2:05
  • \$\begingroup\$ No, do not connect N on the UPS to the third L. Connect it to N. That's what it's for. \$\endgroup\$
    – Dave Tweed
    Jul 26, 2021 at 3:48
  • \$\begingroup\$ so I guess that goes back to my original question. Is there a way to convert three phase to split phase? I imagine if I leave one of the legs floating, then I would be losing power and the UPS probably wont start up? \$\endgroup\$
    – bchang32
    Jul 26, 2021 at 12:15
  • \$\begingroup\$ Why wouldn't it start up? You say that the datasheet says that it's rated for operation on 120/208V. Just leave the third L unconnected. \$\endgroup\$
    – Dave Tweed
    Jul 26, 2021 at 12:40
  • \$\begingroup\$ Sorry, I'm a rookie to this and leaving a leg unconnected seems unsettling? So out of curiosity/learning, what effects does leaving the third leg unconnected? From researching online, it seems like it'd lead to an unbalanced system (and have my equipment be generate heat/be lossy/wear out faster?). With it being 120 degrees different, My system would be getting 208V (instead of 240) right? \$\endgroup\$
    – bchang32
    Jul 26, 2021 at 13:07

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