Diggin' technology every day

April 30, 2010

Violate Electrical specs for more servers?

Filed under: Datacenter,General,Random Thought — Tags: — Nate @ 8:46 pm

As usual on big blog posts I often literally go back and re-read the post about 60 times and think about what I wrote.

Well I was reading my most recent post about Datacenter Dynamics specifically the stranded power section and the datacenter operators of hyperscale facilities wanting to draw every watt they can off the circuits to maximize efficiency and I got to thinking..

Would they go so far as to violate electrical specs by drawing more than 80% of the power for a particular circuit? I mean in theory at least if they construct the components properly they can probably do it fairly safely. I learned a few years ago from someone, that the spec in question is NEC Section 384-16(c). Which I think in part reads:

The NEC requires the branch circuit computed load for conductor sizing to be sized at 125% of the continuous load, plus the noncontinuous load (100%).

Which equates to 80% utilization. If you know your power usage levels that well, and your loads etc, do you think such hyperscale facilities would run at 85%? 90%? 95% of circuit load? Really with all of the other extreme measures being taken to maximize efficiency I wouldn’t put it past them. They’re going so far as to design special motherboards and have specific components down to the VRMs to lower power usage. I can see them investing in higher grade electrical gear allowing them to safely operate at higher circuit draws, especially when you take into account power capping as well. Afterall, if your spending the effort to shave literally single digit watt usage off your systems that extra 20% capacity on the circuit has to be very tempting to use.

I remember a few years ago doing a load test on one of the aforementioned lower quality power strips(they weren’t cheap, but the company’s QA wasn’t up to par), it was a 30A PDU. And I loaded it up with a bunch of systems, and walked away for a couple minutes and came back and was shocked to see the meter reporting 32A was being drawn. I immediately yanked some of the power cords out to get it back under 30A. After talking with the manufacturer (or maybe it was another manufacturer I don’t recall), they said that was not unexpected, the breaker has some sort of internal timer that will trip based on the amount of excess load on the circuit, so if your drawing 30A it probably won’t trip for a while, if your drawing 32A then it may trip after a few minutes, if you try to draw 40A it will likely trip immediately(I’m guessing here).

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