Server room power: Plan for redundancy and flexibility

A general explanation of the ways that power can be supplied to a server room and basic guidelines on such areas as long-term planning, voltage and amperage, and physical cabling.
Written by Scott Lowe, Contributor and  Staff , Contributor
Without sufficient power, it would obviously be difficult to run the multitude of servers that most organizations need. But providing enough power isn't enough. Organizations also need clean power with redundancy and backup built into the power system design. In this article, I will provide a general explanation of the ways that power can be supplied to a server room. I will also give basic guidelines on such areas as long-term planning, voltage and amperage, and physical cabling.

Planning for the long term
Before any electrical cabling is run or any circuit breakers installed, you must develop a detailed, long-term electrical plan. There are two common ways to develop such a plan. First, you can get the actual maximum load ratings from all the hardware you support, determine what hardware is likely to be added in the future, and develop load estimates based on this information. However, with this method, it's likely that you will overbuild the power system because you're using the maximum load ratings to determine the power requirements. Often, equipment specifications rate the maximum power used by a system rather than  the normal operating power. While an electrical system designed in this way will easily be able to handle the load placed upon it, it could end up costing quite a bit of money.
A second approach is a more customized system. Develop your plan using the nominal operating power requirements instead by adding up the current usages of each piece of individual equipment along with plans for future equipment. The only drawback to designing your system this way is that it results in the inability of all equipment to start up simultaneously because of the increased power draw that systems use at startup time.

Power system design: A wide range of possibilities
Power can be supplied to your server room in a variety of ways. Most of your design will depend on how mission-critical the server room is to the organization. For example, if you have workgroup servers that are needed only eight hours a day, you could get away with having servers with redundant power supplies connected to a UPS. In a larger server room situation—say, where you have infrastructure servers such as domain controllers, DHCP servers, and DNS servers—you would need servers with redundant power supplies connected to UPSs on separate power circuits.

Even further, if you have mission-critical servers (such as e-commerce Web servers that run 24/7), it would be good to have servers with multiple power supplies connected to UPSs on separate power circuits, which are in turn backed up by a diesel power generator.

Bear in mind that these scenarios are meant to be examples, not to serve as a guide, and this is by no means a complete listing of the range of power supply needs. When planning power supply options for a server room, you must analyze your environment to determine what level of power your organization requires.

Don't overlook the need for backup
With all of the complexities involved in building the proper IT environment, determining exactly why certain equipment is needed can be difficult. For example, if you have backup generators, why do you need a UPS?

The purpose of a UPS is to provide conditioned, clean power for that short period of time between when a power failure occurs and a backup generator kicks in. So installing a UPS that can handle your equipment load for 15 to 20 minutes is wise and economical. For true high availability, a second UPS should be installed as a backup to the first. This not only provides redundancy in the event of a failure on the primary UPS, but it also allows a UPS to be serviced without affecting the equipment connected to it.

The purpose of a backup generator is to provide power for an indefinite period of time—assuming that it's refueled from time to time—in the event of a major outage. For true high availability, a company should have refueling contracts set up in order to maintain the generator.

Power quality
The quality of the electricity powering server room equipment is important to the equipment's long-term reliable operation. When you install new power systems, check the quality of the power coming into the building to make sure that it is within American National Standards Institute (ANSI) specification. There are power monitors (such as the ones found at Power Quality Inc.) that are specially designed for this job. If the quality of the power doesn't meet those specs, you may need to install power conditioning devices (such as Tripp Lite’s LC 2400 Line Conditioner) in the electrical system.

Voltage and amperage
Most servers in North America use 120V power connections, and their amperage ratings vary from server to server. If using dense server racks, make sure that there are enough power circuits to support each one and that the servers are distributed evenly across the board so that no one circuit ends up overloaded.

Power whips and power strips
If you have a raised floor in your server room or data center, I would consider using power whips, power outlets on the ends of flexible cabling, instead of fixed outlets. Since your power requirements will almost certainly change over time, using power whips with power strips in the server racks provides for flexibility and expansion of your data center.

In my most recent major data center project, I installed three racks with 16 servers each. Since each server had a redundant power supply and needed to be highly available, I had to double the power requirements. So in each cabinet, I ended up with 16 servers, each with three power connections, including one power connection for the remote management card. I installed six 8-outlet power strips with locking power cords, which I ran into the floor under the cabinet and connected to the incoming power whips.

I was careful to make sure that each power supply was plugged into a different power strip from the others and that no server had both of its power supplies connected to a single circuit. That way, I could rest easy knowing that a single circuit failure would not affect my equipment. This data center was also equipped with redundant UPS systems and a hefty backup generator that would power the entire operation in the event of an extended outage.

Server room or data center power systems aren't to be taken lightly. Many individual factors can determine the type of power supplies you need, so it's important to calculate the amount of power you require and to allow for future expansion. That will enable you to build a system that meets the needs of your business without breaking the bank.

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