>> No, because the return currents from the two 110V legs are
>> out of phase and will therefore cancel.  The neutral wire
>> only handles the difference between the two 110V hot wires,
>> which means worst case it needs to handle the same current as
>> a hot lead if the entire load is only on one of the 110V phases.

> Regarding house wiring, you are just using 2 leads of the three
> phase line and then using a center tapped transformer to create
> the neutral and therefore 110 Vac for most home applications.

When I read this, it implied (to me) that the electric company
was bringing 2 legs of the 3 phase power that appears on the pole
into the residence.

In all the cases I've worked on (southwestern US), the electric
company uses 3 phase delta wiring on the pole (or underground);
3 wires, each hundreds or thousands of volts, and 120 degrees
out of phase with the waveform on the other 2 conductors.

Any 2 legs are used as the primary feeding a transformer that
serves 4 to 16(?) houses.  (Ttransformer is frequently called a
pole pig.)  Primary side floats.  If done correctly, the next
transformer is connected to the "next" leg pair so that the
overall load for a city (as seen by the generators) is equal
across all 3 phases.

Secondary side is 240V, single phase, center tapped.  Center
tap is tied to ground at the pole.  It should also be tied to
ground at the breaker panel.  Each hot leg has 120V measured
to the center tap (neutral/ground).  240V center tap power is
distributed to adjacent houses via an additional 3 wires on
the same poles (but seperate than the 3 wires carrying 3 phase
primary power on those poles).

Very odd results when the neutral line breaks.

Rambling personal story, hopefully amusing, follows.

After a wind storm, appliances and lights in the house across
the street were behaving very oddly.  Electric utility had so
many repairs they were estimating 4-5 days before they could
get to it.  The neighbors asked me to look at it.

I was seeing 60 volts to 150 volts on various outlets in the
house.  And it would vary depending on what lights and/or
appliances were running.  I suspected a bad neutral.

Power drop from utility pole was 3 conductors -- 2 insulated
hot lines wrapped around an uninsulated steel bearer cable.
Transformer center tap on pole is tired to ground and is
carried by steel bearer cable.  House breaker panel's neutral
was NOT tied to local earth ground.  Uninsulated steel bearer
cable was the house's only neutral connection.

Wind had flexed bearer cable until it broke.  House had 2 hot
lines coming in with local (breaker panel) neutral bus bar
floating at a level set by ratio between active loads of the
house's lights & appliances (i.e. a big resistance divider
with hundreds of volts & a couple dozen amps).

I used a rope to physically relieve stress on hot conductors
by pulling drop cable towards house entry point.  And I used a
car battery jumper cable to electrially bond across the break
in the broken steel bearer wire.  Problem solved.  House ran
that way for about 4 days.  Edison (electric utility) service
man thought it was creative when he arrived and properly
repaired the drop cable.

I explained to the owners that they should fix the lack of a
local ground on their breaker panel.  Not sure if they ever
did so.

						Lee Jones

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist