On 2/9/2009 5:38 PM Pete C. spake thus:

David Nebenzahl wrote:

But here's the weird thing: there are some 120-volt outlets apparently
attached to this panel that function correctly. When I turned some of
the breakers off, the outlets lost power, so I assume that they're
directly powered from this panel.

How can this be? How do you get 120 volts from 240? (No big fat
transformers visible anywhere.) What am I missing here?

In case anyone's wondering, no, I don't mess with 3-phase power or any
other kind of heavy-duty commercial/industrial stuff. My customer's
reaction was "I'll call a licensed electrician", which would have been
my advice. It appears the entire panel needs replaced.

I'm just curious about this situation.

If you read 240V and not 208V it's most likely a three phase delta
configuration. To get single phase 120V service out of it one of the
three distribution transformers in the three phase configuration has
it's secondary center tapped to provide a neutral that gives 120V
relative to each of that transformers poles. Since this transformer
represents one side of the delta configuration and is connected to two
of the three phase legs, that third phase leg is *not* 120V relative to
the neutral and is normally color coded orange to identify it. The third
leg is ~208V relative to the neutral and is typically called the "wild
leg" or some less PC terms. I'm sure a 'net search on "wild leg" and
"delta" will turn up some diagrams.

Yes, it was 240 volts. Thanks for the explanation, by the way.

Found a diagram that shows a delta configuration with a wild leg. Makes
sense.

But that panel only has 3 big fat wires coming into it, so apparently
there's no neutral (or no center-tap) connection. Is it possible that
this wire was upstream of this panel and connected elsewhere?

Not going to lose any sleep over this; just curious.


--
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- lifted from sci.electronics.repair