In a message written on Thu, Dec 02, 2010 at 11:32:16AM -0500, Jay Ashworth wrote: > No, I'm pretty sure he means "across the 2 high legs of a 120/208 3ph > Wye service", and I'd never heard that idea suggested before. I can see > why it reduces the amount of copper you need to run, but it seems as if > it would have compensating disadvantages, though I can't think precisely > what they might be at the moment.
In most residential / small business construction in the US you
will find "240V single phase with neutral". There are two hot wires
and a neutral from the provider. Hot to hot is 240, hot to neutral
is 120.
Most colos run their back end plant (e.g. UPS's, Gensets, etc) on
480v 3-phase power. The typical way they get 120v power is to
transform that to a 3-phase Y wired output, also known as 3-phase
4 wire. Each hot leg is 120v to the neutral (the fourth wire).
You can run hot to hot here as well, where the voltage is 208v.
The trick with 208v loads in this situation is you want to keep the
load across each pair of phases roughly balanced.
What can be particularly confusiong here is the panels look exactly
the same. The same physical panel layout your house gets with 2
phases in plus a neutral is now two of the three phases from the
three phase power go in, plus a neutral. Same breakers are used,
with hot to hot being 208 volt. The difference is, in the colo
there are three of them:
A N B B N C C N A
| | | | | | | | |
Panel 1 Panel 2 Panel 3
With A, B, and C being the 3 phases, and N being the neutral.
You may also find this arrangement in larger multi-tennent buildings
where they are fed with 3-phase power.
--
Leo Bicknell - bickn...@ufp.org - CCIE 3440
PGP keys at http://www.ufp.org/~bicknell/
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