On Sun, 27 Jun 2004 11:20:48 +0100, "Andy Wade" wrote:
The conductors concerned are phase conductors of a single-phase AC
circuit. Neither is a neutral because of the earthed centre tap.

This was one of my debates with myself because considering that it is
a centre tapped supply, there are, in effect, two phases albeit not
with the conventional 3 phase 120 degree shift.

See sci.engr.electical.compliance /passim/ for interminable threads on
the
subject of "is it 1-phase 3-wire or is it 2-phase?". I prefer the
former,
reserving 2-phase to refer to two phases and neutral from a conventional
3-ph system, or two phases in quadrature.


Same looseness of terminology used here in Canada. Unfortunately.
If the conductors are NOT actually the one, two or three phase wires of a
three phase system, why refer to them as 'phase wires' at all?
Why not use the term 'legs'.
Two examples come to mind;
On UK industrial sites AIUI transformers are used to step down 240 to 120
volts to power electric tools. A centre tap of the secondary of the
transformer is earthed. Hence we have two conductors at around 60 volts to
earth, for safety. Neither of those conductors are 'phase wires', they are
two conductors of the same single phase. (Although not correct for AC one
can almost think of them as plus 60 volts and minus 60 volts!).
Also. In North America atypical is a SINGLE phase 13,000 volt feed to the
local distribution transformer where it steps down to 230 volts centre
tapped.
Three wires come from each distribution transformer to several houses. The
centre tap is considered zero potential and is earthed; the maximum voltage
to earth is therefore 115 volts (RMS). The three wires are all on the same
single phase. In fact the nearest place, in our case, that three phase is
available is some distance away. Three phase is rarely, if ever supplied to
a domestic installation..
After the three wire service enters the house one of the legs is generally
black and the other red. The 115 volt loads are distributed, hopefully
evenly between the two legs and neutral, using single pole breakers. Heavier
loads such as water heaters, clothes dryers, electric cooking etc. are
connected between the two legs providing 230 volts using double pole
breakers.
But it's all single phase! 200 amp service at 230 volts (46Kva) is normal
these days for a typical house. Although I have seen one very large house
that had two such 200 amp services; they must have extra load such as a
indoor pool and/or some kind of elevator. (Growing marijuana also requires
much light btw!).