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Went to a commercial job today, to install a receptacle. I'm told to come off a certain circuit, do so, add the box above the grid, go to test receptacle. all 3 of my lights are on my plug tester, so I use my multimeter 110v neutral to hot, 210v hot to ground.

I take apart the box, checking voltages notice I'm getting 210v from the hot side. notice others are getting 210v, decide to check panel.

ground to hot
breaker on position
phase a 0
phase b 210v
phase c 210v

neutral to hot
phase a 110v
phase b 110v
phase c 110v

between each phase 210v

I'm check ground and neutral connections into subpanel, neutral and grounds separated properly.

thinking the panel it's feeding has some type of lost neutral/ground or loose connection. one neutral, two grounds from conduit

I'm turning off all the panels seeing if two circuits are tied in, nothing

turn off sub panel main breaker with issue
ground and neutral
phase a 110v
phase b 110v
phase c 110v

soon as I turn breaker on
0v on phase a to ground
210v phase b
210v phase c

starting to wonder if the main breaker is bad at this point, decided to try another breaker. same issue just different phase 0v, 210v on other 2.

turn off all breakers
one by one checking
soon as I turn on a one circuit all issues start

someone else follows the conduit and determines an added receptacle is wired wrong.

How does a breaker on phase c, cause 0v ground to hot on phase a, phase b to get 210v, and phase c to get 210v?

Makes zero sense to me how a breaker that is on one phase can backfeed voltage to phase c and b, and eliminate voltage on phase a when tested via ground.

Someone explain the reasoning behind this, trying to learn from this.
 
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