On Thu, 28 Jun 2007 01:45:44 GMT, Gary Tait
wrote:

"Tam/WB2TT" wrote in
:

For instance the wiring to an electric clothes
dryer will have two 20 Amp hot wires, a 20 Amp neutral wire (from
center tap of transformer), and a 20 amp earth ground wire connected
to a cold water pipe or ground rod.

Actually a typical US clothes dryer uses a 120/240V 30A circuit.
And the ground wire always connects back to the services neutral, where
things are grounded with rods and/or plumbing.

In some localities, the neutral
and ground wires can be tied together at the appliance.

They way is was, is that it was permitted to run just a grounded
conductor (neutral) to the appliance, and bond that to the case.

Since 1996 or so, that has been prohibited, so separate neutral and
grounding conductors must be supplied to a 120/240V appliance.

I think the
only reason for the heavy neutral and ground wires is to make sure the
circuit breaker trip in case of a short. As recently as about 20 years
ago, a much smaller earth ground wire was used.

For the safety ground, yes. For neutral it is assumed, to the terminals
in the appliance, that the appliance could draw the full current on the
neutral.

The electric use meter must be more expensive than an unbalanced
single phase one.

If you mean the US ones, 2-wire 120/240 ones are the norm, so are made
in quantity enough not to be expensive.


Most US electric meters, at least the electromechanical kind, have one
voltage coil (240 volts, l-l) and two current coils, one in each of
the 120 volt phases. That computes power based on an assumption of
voltage symmetry, usually a reasonable bet.

John