Anybody who deploys equipment in a data center knows that power can be a complex subject. There are lots of different ways to measure it, charge for it, and deliver it. I intend here to shed some light on the various aspects and hopefully give you some ideas of how best deal with power in your own colocation data center space.
Provisioned Electrical Capacity (PEC) – The most basic way to think about power. This is the amount of power that the physical circuits delivered to you can support without tripping any breakers. You can easily calculate it as the breakered amps of the circuit times the voltage [times 1.73 for 3-phase, see below], times 80% as an industry standard safeguard. Note: This 80% is somewhat arbitrary; It mostly reflects the fact that breakers aren’t really that exact and you shouldn’t push it. So for a standard 20A@120v circuit, you get 20*120*.8 = 1920W, or about 2kW. This may or may not bear any correlation to how much power you are allowed to draw on those circuits or what you are charged. But it’s a place to start. By wary here, because often data centers won’t call attention to the fact that they won’t allow you to draw all the power you have provisioned. It’s in the fine print somewhere that you have a power cap lower than what the circuits could handle. You might want to do this, but it should be clear to you up front. See below.
Primary or Redundant Circuits – Primary and redundant circuits are exactly the same from a power point of view. They are both live and able to be drawn from. For the sake of redundancy it is best to get these from different breaker panels or different RPDUs. The key difference that you need to be aware of is that redundant circuits don’t do anything to increase your power cap. Primary circuits do. See below.
Power Cap – The maximum amount of power you are allowed to draw in some defined space. This is usually the sum of all primary circuit capacity, but not always. For example, if you have 2 primary and 2 redundant 20A/120v circuits, you power cap is probably 2x 1920W = 3.8kW. In some cases it will make sense to have a power cap lower than what you could draw on all your primary power circuits. This usually occurs when you need circuits of a particular type – with a fairly high breakered amperage – but don’t actually need to draw all that power. You don’t want to pay for all the power, and you don’t need the data center to allocate it to you, so you ask them to deliver the larger circuits, but give you a lower power cap.
Circuits Types – Power circuits come in many different flavors, but in the US most fall into some combination of three parameters: 120 or 208 volts, 20 or 30 amps, and single or three phase. Which of those you choose depends on a number of factors:
- What type of power does your equipment require?
- What type of PDUs do you want to use?
- How much power do you actually expect to draw?
- Can you use vertical PDUs?
- Are you in open racks or closed cabinets?
Plug Types – WikiPedia has excellent information as always, but I’ll give you the short version. NEMA is the US standard, and it labels plug types as (L)XX-YY. L is optional and means that the plug twists to lock into place. The XX indicates the voltage, with 5 = 120v and 6 = 208v. YY is the amperage. You might also see a suffix of “r” or “p” indicating plug or receptacle. A very common one is L5-20, meaning a 20A/120v circuit, and the plug twists to lock in place.
Power Density Considerations – One of the most important criteria for choosing any data center space, whether a single cabinet or a 1000sqft cage, is the amount of power you can put into it. This will be limited by the data center’s policy, which is in turn limited by how much the facility is designed to support. I newer, more modern facilities you can expect to be allowed to draw 5-6kW per cabinet or rack, and 10kW or more in “high density” space. Don’t be confused between actual power draw and PEC, read above on that subject.
PDU Considerations – You will want to consider whether to use vertical or horizontal PDUs. My personal preference is to use one primary and one redundant vertical in each open rack, because it makes the cabling much easier. In cabinets I prefer horizontals simply because verticals won’t fit in standard-size cabinets that most data centers provide. I make it a point to always prefer my only option. I also always try to get the maximum number of outlets I can squeeze in, as this make it easier to maximize my draw. It’s irritating to pay for 16Amps of power, but to run out of outlets at only 13Amps.
I find the best tradeoff for cabinets to be two primary and two redundant 30A/120v circuits, with 16-outlet 2U horizontal PDUs. This allows me to draw over 5kW without trouble, but consumes 8U with PDUs.
In open racks, when I can use large vertical PDUs, I like one primary, one redundant 20A/208v/3-phase circuit, with 45 outlet PDUs. This makes cabling a breeze, and I can easily draw over 5kW in this case. Doing the math, my PEC would be 20*208*1.73*.8 = 5.7kW
I hope you’ve found this information useful. I’d welcome any other ideas you’d like to share regarding data center power.