Want to move to all 208V for server racks

Chuck Anderson cra at WPI.EDU
Fri Dec 3 07:54:37 UTC 2010


On Thu, Dec 02, 2010 at 01:59:33PM -0500, Alex Rubenstein wrote:
> A couple of reasons.. Neutral current, more power delivered using 
> less copper, etc. Personally, I like delivering two L21-30's per 
> rack and call it day - allows for a comfortable 8kw per rack in 2N+1 
> redundancy. And, it still has a neutral if it's needed, which we 
> hope it isn't.

Here's a question for you.  How do you calculate the total current & 
power capacity of a L21-20 or L21-30, and how do you do the 
calculations in order to balance the load between the phase legs?  
This seems like it would be a trivial thing to do, but given that the 
three legs are 120 degrees out of phase with each other, I don't think 
you can just do normal addition.

For example, I have APC AP7961 3-phase PDUs with L21-20 plugs.  The 
management interface claims a maximum load per phase of 16A (which I 
believe is the 80% derating of 20A required by NEC).  Does this mean I 
can draw 16A * 3, or 48A total if I have a perfectly equal balance?  
Also, how does this relate to power, i.e. how many kVA or kW does this 
provide?  16A * 208V * 3 phases ~= 10 kVA?

On another note, how do you calculate N+1 power feeds in your racks?  
If you have 2 PDUs fed from two different branch circuits/UPSes/etc. 
do you just set your PDU load alarm thresholds at 50% of the max 
rating of each PDU and never load them beyond that point, so that if 
you lose one PDU/branch circuit/UPS and the dual-power servers 
transfer their load over to the other side, it doesn't get overloaded?  
That would be 8A on each phase in the case of my AP7961's.  Of course, 
things get complicated when you have a mix of single- and dual-power 
servers, especially if you have server admins who don't keep you 
apprised as to the types of equipment that are installed there as 
things change over time...




More information about the NANOG mailing list