Re: Want to move to all 208V for server racks

[Owen DeLong <owen () delong com>]

On Dec 2, 2010, at 8:46 AM, Leo Bicknell wrote:

> 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.
It is not uncommon for three-phase panels to be different and have
all three phases in the panel each phase feeding every third breaker
slot.

Owen