Why choose 120 volts?

Owen DeLong owen at delong.com
Tue May 26 16:06:57 CDT 2009

On May 26, 2009, at 1:48 PM, Alex H. Ryu wrote:

> Also, adding followings.
> 5) availability from local power provider(s)

I don't know of anywhere in the US/Canada where power comes into the
building as strictly 110-120V.  That is almost always delivered either  
1 leg of a 3-phase 208 service (most commercial/industrial deliveries)
or as two hots (240V across the two hots) and a neutral (120V from
either hot to neutral).  Most datacenters are taking much higher voltage
feeds from their utilities and most of the readily available step-down
transformers and UPSs will produce 208 three-phase or 240V as
described above.

I am not an expert on power outside of the US, but, to the best of my
knowledge, Japan's 100V/50Hz is one of the few other countries
using less than 208V as their standard.
> 6) local regulation such as fire department safety rules...
I seriously question this one.  Can you point to any examples?

> 7) for your own safety... (120V may not kill people, but 240V can  
> do...)
It's relatively easy to kill someone with 12V, so, I don't see how 10x  
is significantly less dangerous than 20x.  Sticking your fingers in a  
socket is going to hurt regardless of the voltage.  Yes, 240V can hurt  
and faster, but, at the end of the day, it's not significantly more  
to kill you than 110.  Fortunately, most servers don't have light  
in the high voltage portion of the server.

> If you want better, why not just have everything to DC power ?
> Something like 48V...
There's a whole host of reasons, but, the biggest one boils down
to cost... Cost of the larger wires, cost of the increased line losses,


> Alex
> Wayne E. Bouchard wrote:
>> 1) Equipment used to not be dual voltage
>> 2) For smaller scale, 120V UPS and distribution equipment is usually
>> cheaper
>> 3) 120V embedded itself into operations as a result.
>> 4) We're all lazy and hate change.
>> On Tue, May 26, 2009 at 12:39:10PM -0700, Seth Mattinen wrote:
>>> I have a pure curiosity question for the NANOG crowd here. If you  
>>> run
>>> your facility/datacenter/cage/rack on 120 volts, why?
>>> I've been running my facility at 208 for years because I can get  
>>> away
>>> with lower amperage circuits. I'm curious about the reasons for  
>>> using
>>> high-amp 120 volt circuits to drive racks of equipment instead of
>>> low-amp 208 or 240 volt circuits.
>>> ~Seth
>> ---
>> Wayne Bouchard
>> web at typo.org
>> Network Dude
>> http://www.typo.org/~web/

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