Newbie datacenter lesson: The implications of 120v and 220v

So I've never initiated a fresh colo build before, and it's showing.  And before you get all pedantic, I know 220v is probably not the right term (208, 240, etc), but it's what I'm using.  So there.  Newbie, right?

Problem: Not getting super specific about what our power circuit will be.

Assumptions by both parties caused this, but the fault really only matters on my side.  It ended up being that I ordered the wrong PDUs (well, right if you assumed 220v), and moving to a 220v circuit would have been an additional monthly cost.  I incorrectly assumed that we would be getting a 220v/20A circuit.
  • The difference between 120v/20A and 220v/20A is significant
  • The difference between L5-20 and L6-20 is significant
  • The difference between 20A and 30A is significant
Let's address these, because it's been a heck of a learning experience for me.

120v and 220v

Why is it a big deal?  To sum it up really quickly - 220v basically means you can house twice the equipment.  
  • Amps = Watts/voltage
  • get the amps of a 750w power supply, you take 750, divide by 120, and bam: 6.25A
  • Using 220v: 3.4A
So that minor difference in numbers (120 vs. 220) means you're in an awkward position if you've done power calcs based on 220v input and they give you 120v.

Another gotcha is the thought 'no biggie, just upgrade to 220v!'.  Well, now you need new PDUs.  Congratulations.

L5-20 and L6-20

If you look up the wikipedia page for NEMA standards, the key difference between L5 and L6 is voltage. L5 is 120v, L6 is 220v.  Plus, they are physically incompatible (because magic smoke).  What this adds up to is buying a PDU with an L6 plug means you are sending it back when you discover you've been given an L5-20R to connect to.

We bought some fancy APC metered/switched AP8659NA3 PDUs, but now they're going back because they are 220v only.

20A and 30A

This is less siginificant, but still an issue.  When you buy a PDU, you have to specify plug type, plus max amperage.  So you've bought a PDU rated for 20A, and you upgrade circuits to 30A - now your PDU is the limiting factor.

Side notes

20A is actually 16A.  So if your power calcs come out to 16A and you're all 'phew, 75%!'...wrong - 100%!  For whatever reason, a 20A circuit in North America is actually specified as 16A maximum.

Also fun - while your Dell iDRAC will tell you actual power consumption, your much more expensive EqualLogic array has no capability to do this (as of Jan, 2015, anyways).  The solution for those without nice metered PDUs?  Kill-a-watt.  Yep - sanctioned by EqualLogic support, even!

For a full rack (assuming you're not just storing paper and hot air in it), I simply cannot see how you can get away with 120v/20A.  My middle-of-the-road calcs for 6x R620s, 1x PS6510, 1x PS4210, and networking gear comes out to 12A.  Remember, 12A out of a max of 16A!!  That's a 75% load right there...and we're only talking about 24U of equipment!

So yeah.  Pay attention to power specs.  Important stuff to get right from the start.


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