Why choose 120 volts?

Wed May 27 04:02:46 UTC 2009

Seth Mattinen <sethm at rollernet.us> writes:

> 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 spent the last several days going back and forth with salespeople,
trying to find a rack with 208v power in the south bay, or a cheap 100M
connection from market post tower to heraklesdata in Sacramento.  (where
I have cheap 208v power)   From what I see, most places in the bay area 
just can't handle the kind of heat density that a 30a 208v circuit per rack 
would bring.  (they won't sell me more than 2 20A 120v circuits, either, and 
many will only sell me a single 15a circuit per rack.  I assume that's an 
effort to keep the heat output within cooling system capabilities.)  But 
that still doesn't explain why they don't hand out 10a  208v circuits.  

I've also seen employers pick 208v over 120v even after I pointed out
the cost per watt advantages of 208v, even without counting efficiency 
gains.  In one case they provisioned one rack with 208v, because the 
vendor of some particularly expensive bit of equipment recommended it, 
then they left all the commodity servers on 120v.  Why didn't they put 
everything on 208v?   I pointed out that the cost per watt was lower.    
Maybe I blew my credibility by wanting to research 48v power supplies for
our kit before that?  (it was a telco facility, after all, and I was
young.)  

30a 208v is about perfect for a rack, if you ask me.  (I imagine
the guys who have to deal with cooling feel differently, but at my
scale, that's all priced into the power.)  

-- 
Luke S. Crawford
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