3-phase power

Noel Chiappa jnc at mercury.lcs.mit.edu
Tue Jan 4 10:03:42 CST 2022

    > From: Scott Quinn

    > I have seen some roads where the utility has 2 of the phases plus
    > neutral going down them, not true 2-phase power, but 2 phases 120/240
    > degrees apart with the third phase just not present.

My street has that. The subdivision as a whole has all 3 phases (down the
main road through it), but individual streets off of it have only 1 or 2.
(The whole subdivision is on poles, so it's easy to see.) On the ones with 2,
some houses are connected to one, some to the other.

    > I guess they figure twice the loads for only one more wire.

No, because most homes are only connected to one phase. I think the main
reason to do it is that it allows the total load (of the entire subdivision)
to be somewhat balanced across all 3 phases.

    > Can't remember what it was called but I do remember seeing in some book
    > somewhere about a "phantom 3rd leg" or something

My house has something like that; the previous owner wanted '3-phase service'
for machine tools (I think - could have been a compressor, or something) in
his basement workshop, so they sold him a pseudo-3-phase service. I forget
the exact details of how it works, but the 3rd phase is at 170V to neutral,
or something like that. (So I can't power any 110V outlets off the third

I think the way it works is that the two 'main' phases are 220V to each
other, 110V to neutral (I think from the usual center-tapped transformer off
one of the three main feed phases, i.e. 180 degrees to each other). The third
'pase' is generated by a second, smaller transformer connected to the other
feed phase in some arcane way I forget the details of. So it's 120 degrees
away from the other two.


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