Powering your server racks

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power_cordCongratulations! You have the opportunity to set up some new server racks and you want to do it “right.” This doesn’t happen very often for most techs; they usually have to live with someone else’s choices that came before. But when you have the opportunity to start from scratch, it can feel like the problems of the former racks will soon be a distant memory.

Once you have decided on the manufacturer and model of the rack you want to buy, you need to start thinking about the accessories that will be installed in the rack. One of the first accessories you need to consider is power distribution units (PDUs). There can be an overwhelming selection of volts, amps, and how many rack units (RUs) a particular PDU uses.

To start with, I like to use zero-RU PDUs as much as possible. Most of the better racks have this option. How can you have something installed in a rack and take up no space? Simple, it’s mounted vertically in the rear of the rack. The better racks will have multiple vertical channels to allow for multiple accessories to be installed, most often PDUs and cable management.

Many techs will immediately look for 120 volt (V), 15 amp (A) PDUs. After all, this is the most common power outlet in homes and offices in North America, so it’s no wonder they look at these first. But there are better choices for a server rack.

As far as voltage, I am going to suggest you put 208/240V PDUs in the rack. (The exact voltage is determined by the type of transformer feeding your building; check with an electrician before buying your PDUs so you get the right ones.) Here’s why:

  1. Almost all modern IT equipment has an automatic-sensing power supply. If you look at your computer’s power supply, it probably lists the voltage rating as something like “100V-250V”. At that point, the power supply doesn’t care.
  2. 208/240V allows you to achieve higher equipment density from a single circuit. It works like this: 208/240V uses half as many amps as 120V to accomplish the same task. So a 120V 30A circuit is equivalent to a 208/240V 15A circuit. Many uninterruptible power supplies (UPSs) and PDUs have a 208/240V 30A breaker, so you can effectively put twice as many servers in the rack on the same circuit. (Consult Ohm’s law for more about converting amps to watts at certain voltages.)
  3. 208/240V still uses 3 wires from the breaker panel to the plug, so it isn’t more wires for an electrician to install. But it might be a different size wire, and it will require 2 slots in the breaker panel instead of 1, so there may be some extra expense.

Yes, you will invariably have a few devices that will only work at 120V, so install a small 1-RU PDU in the rack for them. I would wager that you will not use more than a couple plugs on that PDU.

When I have the ability to set up a new server rack I do it like this:

  • Two 208/240V 30A circuits per rack, preferably connected to two different UPSs
  • Redundant power supplies in servers, one connected to each circuit
  • Redundant power supplies in network gear, one connected to each circuit

This allows many points of failure to occur without necessarily taking down your servers. A circuit breaker could trip; the servers keep running on the other circuit. A UPS could fail; the servers keep running on the other UPS. A PDU could fail; the servers keep running on the other PDU. Etc.

Lastly, you will end up with a few devices that only have one power connection, even if it operates at 208/240V, such as a KVM switch. Just try to balance the load of these devices between the UPSs so you don’t unintentionally overload one.

Properly planning the power feeds and distribution to your server racks will help keep your new racks functional for today’s needs, as well as provide for tomorrow.

**Note: This post is geared to North American readers, where 120V 60 Hz AC is most common. This may or may not apply to other global regions with other power network standards.

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