My friend connected two wires to two "non-earth hole" in a 3 phase outlet (208V) to run a 220V single phase blower (Note: the outlet has four holes, one of them is earth). Basically, he used the potential difference of those two holes. I don't know why it is OK, so I used MATLAB to draw 4 curves (See picture)

Blue - regular sine wave 208V (peak is 208*sqrt(3)) where sqrt(3) is three phase peak ratio

Red - Same as Blue but with 2/3 pi shift

Green - The difference between Blue and Red

Black - regular sine wave 220V (peak is 220*sqrt(2)) where sqrt(2) is single phase peak ratio


Officially, the green peak is >1.73 times the blue one. If the blower uses 220V single phase AC, then the blower should be connected to

----calculation process-----

Peak voltage of 220V AC = average voltage*single phase peak ratio = 220V*1.41 = 310.2V

Peak voltage of two legs out of three phase = (two legs potential difference from diagram)average voltagethree phase peak ratio = 1.73*average voltage*1.73 = 3*average three phase voltage

310.2/3 = 103.4V

-----end of calculation process----------

103.4V three phases power outlet (Two holes of a 3 phase outlet). Note: I understand that there is no such 103.4V three phase power outlet.


Am I right?

enter image description here


1 Answer 1


Any cross phase wiring of a WYE connection will yield sqrt(3)* higher voltage. You have shown this in your simulation. This can also be determined via phasor analysis.

The current flow in these phases will no longer be balanced, which may or may not be a concern.

As you note the motor will be running at 208V instead of it's designed 220 (which BTW is actually 110 @ 0degrees and 110 at 180 degrees). This motor is designed to take two active phases so insulation issues should be safe.

Yes, you are right.

  • \$\begingroup\$ standard power has Line (110 Vac), Neutral (return) and safety. Neutral and safety should be around 0 V. North American house holds have 220 Vac ( I forget the exact term) which is 2 opposite phases (180 degrees apart) which are used by dryers and ovens. \$\endgroup\$ Apr 22, 2013 at 5:13
  • \$\begingroup\$ A small correction. US household voltage is 120V/240V not 110V/220V. As noted, 2 phases out of 240V 3ph yields 208V. Superhero, of the three, one is hot, one is neutral and one is ground. Neutral is eventually connected to ground, at the point that service enters your household, at the main service panel. Neutral should never be connected to ground at any other point as a failure of the ground/neutral wire could leave you with lethal voltages on the ground pin/the case of something plugged in. \$\endgroup\$
    – DoxyLover
    Apr 22, 2013 at 5:43
  • \$\begingroup\$ I changed the number a little bit. The graph is also updated. \$\endgroup\$
    – Superhero
    Apr 22, 2013 at 6:34

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