[133093] in North American Network Operators' Group
Re: Want to move to all 208V for server racks
daemon@ATHENA.MIT.EDU (Owen DeLong)
Sat Dec 4 01:29:42 2010
From: Owen DeLong <owen@delong.com>
In-Reply-To: <20101204051724.GA11783@skywalker.creative.net.au>
Date: Fri, 3 Dec 2010 22:28:55 -0800
To: Adrian Chadd <adrian@creative.net.au>
Cc: "nanog@nanog.org" <nanog@nanog.org>
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
On Dec 3, 2010, at 9:17 PM, Adrian Chadd wrote:
> On Fri, Dec 03, 2010, Joel Jaeggli wrote:
>=20
>>> (OK, so it's not as practical when you have other customers to worry
>>> about... but it might not be so crazy when you're looking at the
>>> efficiency numbers for 100,000 small 1u power supplies vs a set
>>> of much larger ones.)
>>=20
>> Ohm's law is a bitch. 10kamp -48v DC plants are bad enough as far as =
the amount of copper required, running 12v for significant distance is =
comical, this is the reason small boats airplanes and diesel trucks =
adopt 24v systems. There's probably some model where top of rack =
rectifiers makes sense but that's really pretty much what a blade server =
is. When you look at a motherboard in a server a big chunk of of =
real-estate is devoted to taking 12v and switching it down to 1.2-1.8 =
for distribution to the CPU/memory, a 4 socket server might have to =
carry 400amp around in a space of around 300cm^2 on a layer of the pcb.=20=
>>=20
>> The justification for running 208 or 480 all the way to a cabinet is =
all about smaller conductors.
>=20
> Isn't this one area where Google have already (re-)pioneered recently?
>=20
> Besides, there's a reason why AC won over DC for carrying 0 < x < few =
hundred=20
> (or thousand? Amps) over a reasonable distance. IANA-PowerEngineer, =
but
> ISTR the behaviour/efficiency of voltage/current over distance for =
both
> AC and DC is well understood. (And no, ISTR it isn't "AC wins." :-)
>=20
> If you're at all serious about discussing this, I bet spending 15 =
minutes
> doing some research and then an hour or so crafting some simultaneous =
equations
> to solve/graph would be very very eye-opening.
>=20
> Come on guys/girls, you're a bright bunch, post some models and =
discuss
> those rather than un-substantiated datapoints! :-)
>=20
> 2c,
>=20
>=20
> Adrian
>=20
This isn't rocket science and doesn't require much math...
1. For long distances, you need higher voltages to overcome line =
loss.
2. For larger loads, you want to use higher voltages to have lower =
amperages so
that you can use reasonable wire sizes.
3. It's a whole lot easier to change AC voltages than DC.
The system that won is a system of very very very high voltages for the =
core
distribution with transformers converting that to intermediate =
distribution
voltages which are then further transformed down to even lower voltages
for service delivery. This is easily done with AC and would be quite =
complex
and inefficient (especially with the technology available at the time =
this
decision was made) with DC.
It would probably be more efficient to run the entire country on 200,000 =
VDC,
but, the dangers of exposing the general public to that kind of voltage =
are, well,
probably just one of the reasons we use 110 VAC instead.
Owen
