208V vs 110V Power
Jim Pizzirusso
Data/Telecommunications equipment (routers, switches, etc.) via 208V
power vs 110V power. I know that most carriers power there equipment
with -48 VDC, but I am working in an AC only environment so this is
not an option. In previous lives, I have used 110 power only in AC
environment. I am not a power guy so, but I do not believe there is an
issue with 2 phase vs 3 phase power for this equipment.
Thanks,
Jim
Jay Hennigan
Advantages are that for the same amount of power consumption you can
use smaller wire (less amperage). Disadvantages are non-standard
plugs and the risk of frying something that's designed for 120V (like
lights).
Most commercial buildings are fed with three-phase power, 120 volts
from any one leg to the common neutral (ground). Because the phases
are 120 degrees apart instead of 180, the voltage between any two of
the three hot leads will be a nominal 208.
A single-phase load will typically be designed for 120 volts or 240,
rarely 208. Three-phase loads are typically for heavier current uses
such as air conditioners, motors, etc.
I'd recommend 120 volts unless the load device is over a kilowatt or
so. You'll find it easier to get spares, etc., and UPSes and the like
are a lot cheaper.
Jay Hennigan - CCIE #7880 - Network Administration - j...@west.net
NetLojix Communications, Inc. - http://www.netlojix.com/
WestNet: Connecting you to the planet. 805 884-6323
Robert Casey
wire back to the power company. The switching power supplies
in these loads pull pulses of current at the top of the powerline
voltage waveform and no current between peaks. In a 3 phase
powered building, each hot (120V line) sees the pulses from the
loads it supplies. The neutral return (in a "Y" configuration of
power) sees return pulses from all three 120V hots. That can
overload the neutral without any circuit breakers popping.
If you connect each load from one 120V hot phase to another
120V hot phase, you avoid this neutral issue. In this case, the
load sees 208V, so configure it accordingly. Some switching
power supplies can accept anything from 100VAC to 250VAC,
some require setting a switch. Set such a switch to "220VAC".
I've run PCs from 208VAC and it works fine. This avoids the
PC from blowing up if a neutral wire ever fails in a 3 phase
electric power system.
"The phone company's got your number!"
danny burstein
Pizzirusso) writes:
> I was wondering I there is an advantage to powering
> Data/Telecommunications equipment (routers, switches, etc.) via 208V
> power vs 110V power. I know that most carriers power there equipment
> with -48 VDC, but I am working in an AC only environment so this is
> not an option. In previous lives, I have used 110 power only in AC
> environment. I am not a power guy so, but I do not believe there is an
> issue with 2 phase vs 3 phase power for this equipment.
Aside from the general niceness of getting more power transmission per
set of wires (thus reducing the number of wire runs, circuit breakers,
etc.) there's a specific practical reason I've noted.
I've found that a fully populated 19 inch rack (the standard width
stuff) will exceed 16 amps [a] when running at 120v, meaning either a
second power strip circuit has to be run [b] or some painful things
involving higher amperage supply are needed.
Using 208 (or better, 240) means that the amperage draw is cut in
(roughly) half, leaving a fair amount of spare capacity per circuit.
[a] in general, a continous duty circuit is supposed to be derated by
twenty percent. So your standard 20 amp line gets cut to 16. (and let's
not even think about power factor issues).
[b] that's above and beyond any desired redundancy.
Knowledge may be power, but communications is the key
dan...@panix.com
[to foil spammers, my address has been double rot-13 encoded]
Dale Farmer
> I was wondering I there is an advantage to powering
> Data/Telecommunications equipment (routers, switches, etc.) via 208V
> power vs 110V power. I know that most carriers power there equipment
> with -48 VDC, but I am working in an AC only environment so this is
> not an option. In previous lives, I have used 110 power only in AC
> environment. I am not a power guy so, but I do not believe there is an
> issue with 2 phase vs 3 phase power for this equipment.
Power is power. *shrugs* Make sure that the equipment has power
supplies that can use the 208VAC. You may have to order a different
sub-model of the boxes, although nowadays lots of gear have
autoselecting powersupplies that figure out the right thing to do.
You will need a different power cord to plug into the 208 outlet, an
expensive one.
The only real issues are does it provide enough watts to run your
gear, and is the supply reliable enough for your application. I've
never heard of a router or switch large enough to require being wired
up with three phase power. I have seen PDUs ( power distribution
units ) that take three phase on the input side.
Higher voltage power allows you to deliver more watts over the
same size wire. There are other things that it allows, but are not
worth bothering with here. I would be very surprised if you were
getting two phase power, what you will find delivered in virtually all
of the US is three phase used for distribution and in industrial
settings, and single phase power for all the other end users.
What I have done is run the 208 into the UPS unit in the bottom of
each rack, and then get output plug modules for the UPS that delivers
110. This assumes you are providing a UPS for your installation. One
reason you may want to use 208 for your UPS outlets to your gear is to
discourage folks from plugging other gear into your UPS, eating into
it's runtime or overloading it. It really sucks when some workman
plugs his hammer drill into the handy plug on the back of your UPS and
starts drilling holes in the wall for some future installation. Then
gets all upset when the UPS dies and his drill stops working.
--Dale