On Tue, 01 Sep 2015 10:55:01 +0100, NY wrote:

As a matter of interest, what's the typical voltage of the three wires
connected to porcelain/glass insulators, mounted on wooden poles, that
feed the transformers (often pole-mounted) that step down to 240V for
overhead wiring to houses? I can't find anything on Google, using
phrases such as "overhead power line 240V" or "low voltage overhead
power line".

I presume the 240V overhead wiring is 4-wire (ie a star with a wire at
each tip and one at the centre) to allow one of the wires to be
designated as neutral and thus able to be earthed at each house just
before the "electricity board fuse". Does this neutral wire have to be
thicker than the other three because it is carrying the return current
for all three phases, or do the three return currents usually cancel
each other out (at least for the ideal case where all three phase
currents are the same magnitude and exactly 120 degrees apart)?

Having had some feedback and googled to verify 'my facts', I've decided
to revisit your questions.

Your mention of glass insulators and pole top transformers (mini sub-
stations) rather implies an 11KV distribution[1] which doesn't normally
include the neutral wire at this Medium Voltage level. The transformers
employed are normally delta/wye types where the 11KV primary is delta
connected, obviating the need for a neutral connection, with a wye
connected 240v secondary to provide a locally grounded neutral return for
each customer connection.

Your concern over the rating of the neutral wire only applies when the
sub-station transformer is at the remote end of a 240v LV 3 phase
distribution cabling network (either underground in urban areas or else
on poles in more rustic areas where you may well see all four wires
strung from pole to pole).

If your property is fed from a pole transformer (possibly also serving
two neighbouring properties), it seems unlikely that the 11KV line will
include a neutral wire. Otoh, if your incoming supply is via a drop line
from an overhead LV 3 phase line from a remote sub-station transformer,
your connection will simply be a live and neutral 2 wire drop connecting
to the common neutral and one of the three phase wires.

If you care to walk the route of such an LV line, close observation will
reveal that every property connects to the neutral (usually the topmost
or bottommost of the four wires) and one phase wire, usually in a cyclic
sequence so that each phase wire only connects to every third property.

If you're observant enough, you may also spot that the neutral is no
thicker than any of the other 3 phase wires (indeed, if the phase wires
are insulated, you might gain the impression that the neutral is thinner
than the phase wires).

Your surmise that under balanced loading conditions on all 3 phases that
the neutral return current is cancelled out is quite correct. What's
important in this case is that even under the most extreme conditions of
unbalanced loading conditions, the neutral has to carry no more load
current than any of the phase wires, hence it being sized exactly the
same as the phase wires.

[1] 11KV is the usual voltage level in this part of the distribution
network but lower medium voltages might also be used such as 3.3 and 6.6
KV. Not as high as the more usual 11KV but still high enough to warrant
'glass insulators' and elimination of a neutral wire, courtesy of the
pole transformer's delta primary windings connection topology.

--
Johnny B Good