[rescue] Onyx Rack

[James Lockwood]

On Fri, 7 Jun 2002, Bjorn Ramqvist wrote:

> Isn't 2-phase 220V over in the US? If you have 2-phase, adding the thrid
> wouldn't be that much of a problem?
> I know you guys is waaay different from Europe, where we have
> 220V/phase, making 380V between phases. :-)
> (In sweden we're actually running 230V/400V)

Mini USA power crash course:

Power distributed to houses is usually single phase 240V, center tapped
and grounded.  Neutral is run from this tap, giving either 240V hot to hot
(usually used for electric ovens, dryers, air conditioners) or 120V hot to
neutral (for most everything else).  Power delivered to the pole is
generally 3ph Delta, with transformers taking two phases in and giving one
out for residential power.

3ph nowadays is usually supplied as 3ph Wye.  3 hots, 120V hot to neutral,
208V hot to hot.  Alternatives are 3ph Delta which is 240V hot to hot, and
the deprecated high leg (red leg) Delta which runs a neutral from a center
tap of one of the 240V Delta pairs.  This is bad because using the neutral
for 120V circuits tends to unbalance the load.  3ph uses transformers as
well even when voltage matching is not an issue to provide isolation and
short-circuit protection.

The power companies are set up to provide 3ph, but only to commercial
customers.  They tend to overcharge for installation in a residential area
and charge a high minimum monthly bill.  There are exceptions, some power
companies charge fair rates.  A friend of mine in southern California has
400A 3ph Wye coming into his house in Redlands and the price difference
was minimal.

"2-phase" 240V would mean two separate A/C waveforms 90 degrees out of
phase.  This hasn't been used since the 1960's.

-James