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The Mathematics of Power

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The math involved in power is fairly straightforward, but it’s worth sharing some of the basics for datacenters so you’ll know how to shop for it when the opportunity presents itself.

First, look at the specs on your equipment to note the total power draw at maximum for each piece of equipment you’ll be bringing into the datacenter. Once you get those totals, divide the number of watts by 1,000 to get the total number of kilowatts (kW). Then take that number by .7 to get an average draw (The OEM specs are typically maximums, so you’ll draw less current on average). This gives you the total kilowatts, so when your service provider of choice (which is LightBound of course) asks you how much power you’ll need, you’ll be prepared with one of the numbers.

The next step is to understand what the power specifications are. The critical number is the voltage of the equipment. It’s commonly listed as 120V or 208V. Most modern equipment is auto-switching, so choose 208V since that’s the most efficient. We commonly get requests for 110V, but that’s primarily an old residential designation for the wall outlets in your house (remember the Christmas tree☺).

The next step is to understand the guardrails so you’re properly powered for redundancy in an A/B powered datacenter environment. Remember, it’s not just the total power draw, you’ll be given the ability to plug your equipment into primary and secondary power, commonly referred to as A/B power. Each one of these power feeds is backed-up by UPS and Generators, so don’t worry about buying or bringing in your own UPS units. We’ve got that covered.

Here’s the math:

You can refer to an online power calculator like this if you’re curious Power Calculator

Let’s say you need 4kW of power based on step 1 above.

Here’s a hint about power options from your provider…

30A/120V = 3.6kW

30A/208V = 6.2kW

Since you need 4kW, you’ll choose 30A/208. However, the devil is in the details. You’ll need to multiply 6.2 times .8 because the electrical codes say that you should not load these power circuits beyond 80% of the maximum draw or you’ll risk popping the breaker on your circuit…and your Christmas tree will be without lights. So, 6.2 * .8 = 4.96, which is why we say a 30A/208V circuit provides 5kW of power. Since you need 4, you should be fine, but the “how” you plug into the power strips is a handy piece of knowledge to avoid future disappointment. When plugging into the power strips, remember to distribute power equally on each of the A/B circuits because neither side can be overloaded at any one time. For example, if you lose a power supply for either the A or B side of the server, power will be transferred to the other power circuit. If you exceed the maximum limits of that other circuit, your server will experience an outage.


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November 04, 2016 / by Aaron Branham | VP Products