[EDIT] Can someone who understands what I wrote explain this?

[EDIT 2] Sorry I didn't say I'm in the USA, the power is 110/220vac 60Hz. The input to the main power transformer is 3-phase, underground, the output is 220 with (normally) a center tap for neutral/ground. So lights and plugs are on one side or the other for 110, and heating and cooking range are 220. In this normal configuration it is impossible for loss of power on the input to the transformer to cause only one side of the 110 to be lost. Yet that is what happened to all the units in both buildings. [END EDIT]

I live in one condo unit of a two building 72 unit complex. The entire complex, both buildings run on a single 3-phase power transformer. There is some new construction down the road and their digging equipment hit some wiring and caused a strange power outage that affected many homes around the area as well as our condo units. What is strange is that all the units lost power on only one side of the 220V panel giving 110V on one side and 0V on the other side. I measured the voltages in my own panel. How is a commercial 3-phase transformer wired so that this is possible.

I am an electrical engineer and am familiar with all the normal delta and Y power transformer connections. But I have not found anything in my searching that can explain this outage. The same thing happened about a year ago, same cause. And this one again today.

  • \$\begingroup\$ Can you clarify more what supply you have? I'm confused by the references to "one side" and "the other side" of the 220V panel. There are so many different wiring systems and voltages around the World. \$\endgroup\$
    – Simon B
    Commented Oct 23, 2018 at 12:59
  • \$\begingroup\$ @SimonB Added clarification in [EDIT 2]. Thank you. \$\endgroup\$
    – Harvey
    Commented Oct 23, 2018 at 20:20
  • 1
    \$\begingroup\$ How do you know the problem is on the transformer input? Can you see the transformer? In rural/suburban areas the transformers are often on the pole / underground at one end of the block, then distributed at low voltage down the street. \$\endgroup\$
    – LShaver
    Commented Oct 23, 2018 at 21:02
  • \$\begingroup\$ @LShaver My edit says "The input to the main power transformer is 3-phase, underground" It was not disturbed at all either time this happened. The problem was a couple blocks away while they were digging at a new multi building construction site, both times. - Not our property! \$\endgroup\$
    – Harvey
    Commented Oct 24, 2018 at 21:44
  • \$\begingroup\$ Are you sure your supply is not 120/208? AIUI it's rare in single family homes, but more common in commercial buildings and apartments. \$\endgroup\$ Commented Nov 4, 2022 at 21:18

1 Answer 1


In North America, most homes get 120/240 volt "split phase" - the local distribution transformer is single phase, and has a center-tapped secondary. The center tap is grounded and becomes the Neutral. There will be 240 volts between the two end terminals of the secondary, and 120 Volts from either end terminal and the center terminal.

It sounds like one of the two "hot" wires is broken. Around here, such a fault would only affect the 8 - 10 (or so) homes fed from one transformer - but the number of customers affected would depend on the local distribution system.

(The high voltage distribution system here is 3-phase, and the step-down transformers are distributed between phases in an attemp to keep the phase loading balanced.)

  • \$\begingroup\$ I understand that but that is absolutely not the case here. Read my post more carefully. There are homes and buildings all around the area that were affected which are not connected to our 3-phase transformer. There was no problem with our transformer or our wiring. The utility company did not need to come on our complex property at all. The problem and fix were about two blocks away. \$\endgroup\$
    – Harvey
    Commented Oct 21, 2018 at 10:39
  • \$\begingroup\$ This is not the answer for my situation. Anyone else understand my post? \$\endgroup\$
    – Harvey
    Commented Oct 23, 2018 at 10:19

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