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Apr 25 / Greg

Proper Power Planning And Usage

Working in a datacenter power delivery is a big part of what we do. We get regular questions on the subject, and I had a comment from Andrew recently that pushed me to write this post.

Our datacenter provides true A/B power…and what does that mean? What it boils down to is we hand you two power whips that are diversely powered. A power whip is just an electrical outlet built into a flexible conduit. When I say diversely powered I mean separate power substations that connect to separate transformers that connect to separate power rooms/switch gear/UPS/PDU. The idea is that if something happens to any one of these components in the chain, you will still maintain the other whip.

When we offer these to you, we offer them in a pair. These two whips are logically tied together. As per the majority of breaker and PDU manufacturers, you should only run a circuit at 80% of its rated power. Once you get past that 80% mark on a circuit, you never know quite where your PDU or breaker will blow. Also, when a piece of equipment first powers on you get a large in rush of power which can push you up quick enough to blow the circuit. So why sell them in a logical pair?

We sell you the two circuits and you are allowed to only use 80% of a single whip in that pair. You can load a single whip to 80%. You could split the difference and load 40% on each whip…you can get crazy and do 60% and 20% if you like. The idea is that if for some reason anything in that chain of power providing equipment were to fail that the remaining whip will be able to carry the load. This is generally referred to as concurrently maintainable. If you were to load 60% on one whip and 60% on the other whip, then lose power, the remaining whip would have to support 120% of its capacity which I somehow doubt it will. 😉

What can you do in your standard facility? Get some decent PDUs that will allow you to meter power. Tripplite and APC both have decent models, though Tripplite will most likely be your less expensive option. Graph your amperage and give them a quick check once a month. For devices that don’t have dual power supplies, get yourself an in-rack ATS. The above mentioned MFGs make decent ones. I believe Geist also makes some cheap and reliable models.

Visual Example

Standard server with two PSUs.

Properly Loaded Whip Pair

Running 40% on each whip which adds up to our total usable of 80%.

The B power fails and the A side picks up the entire power load of 80%. All is right with the world.

Improperly Loaded Whip Pair

Each whip is being run at 75%...this is a no no.

When the B whip fails in this scenario, the A whip tries to take 150% of it's rated amperage...and the breaker blows. Now you have neither whip.

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