Why choose 120 volts?

Jay Hennigan jay at west.net
Tue May 26 16:07:45 CDT 2009

Aaron Wendel wrote:
> Our power is handed to us at 480v.  We then deliver it to the customer at whatever they need.  The nice thing about 120v is that everything uses it.  No odd cords (as mentioned before) or expensive PDUs.
> I've had a lot of people suggest that running our servers at 240v would save us money because we'd use less amps.  Last time I looked at my bill I was being billed by the kWh, not amp and 240v at half the amps is still the same wattage.  I've been told this so many times though that I'm starting to doubt myself.  If anyone can present a reason for me to switch to 240v I'd like to hear it.

Some servers (HP/Compaq comes to mind) and Cisco switches have 
limitations in terms of performance and/or capacity on 120v circuits.
Yes, it all gets crunched down to 5VDC and similar low voltages in the 
power supply.  The limitation is likely due to the gauge of wire used 
and copper losses in the input circuitry.  Higher current connectors and 
switches, larger copper conductors, etc. are costly.  If you have an 
application that needs that kind of power, higher voltages make sense.

This is just as true if the application is a server as it is if it's an 
electric stove or clothes dryer.

Most of the rest of the world has 240v as conventional domestic power, 
and most server rooms or datacenters supporting >2KVA single devices 
have 208 or 240v available, so it makes sense for manufacturers of 
high-power gear to save the money on copper and connectors and insist on 
higher input voltages for full spec output.

Yes, it would be nice to be able to plug in your laptop charger, etc. 
And the voltage on that charger is likely compatible with anything from 
100 to 240V.  Wiring a NEMA 5-15 with 208V is just wrong, though.  I 
have an IEC male to NEMA 5-15 female pigtail (old-school "monitor cord") 
with a big sticker saying "208V - Be very careful what you plug in here" 
for just that purpose.

Jay Hennigan - CCIE #7880 - Network Engineering - jay at impulse.net
Impulse Internet Service  -  http://www.impulse.net/
Your local telephone and internet company - 805 884-6323 - WB6RDV

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