[132864] in North American Network Operators' Group
Re: Want to move to all 208V for server racks
daemon@ATHENA.MIT.EDU (Owen DeLong)
Thu Dec 2 12:10:43 2010
From: Owen DeLong <owen@delong.com>
In-Reply-To: <20101202164628.GA20227@ussenterprise.ufp.org>
Date: Thu, 2 Dec 2010 09:06:19 -0800
To: Leo Bicknell <bicknell@ufp.org>
Cc: NANOG <nanog@nanog.org>
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
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=20
>> 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.
>=20
> 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.
>=20
> 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).
>=20
> 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.
>=20
> 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:
>=20
> A N B B N C C N A
> | | | | | | | | |
> Panel 1 Panel 2 Panel 3
>=20
> With A, B, and C being the 3 phases, and N being the neutral.
>=20
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
