data center power balance

Is Your Data Center Power Balanced?

When building out a data center in an on-premise or co-location environment, it’s critical that your data center power is balanced and redundant. Let’s take a look at some common tactics to ensure you avoid data center balance issues.

1. Load the circuits in each cabinet to 40% of total circuit capacity

a. Do you know each circuit’s capacity?

b. You want to make sure in a fail-over scenario that you don’t exceed 80-90% of the total circuit capacity. Remember that if one side fails, the power from that side will be added to the other side.

Having each side loaded to 40% gives a nice margin for power in-rush. If you are less                              conservative, you can push that up to 90% in a failover scenario as well, but have less margin for power in-rush.

2. Data Center Power Panels in the RPP’s (or Room PDU breakers for Busway) should be balanced for redundancy

Each whip under the floor (or tap can in a set of Busway raceway) goes back to a 225A (or 225- 800A with the use of busway) breaker.

a. This translates to ~65KW de-rated. (225A x 208V x Sqrt 3 (1.732) = 81A x 80% = ~65KW)

b. Are the Panels being loaded to max capacity?

c. Be sure that each panel is loaded to less than 32KW per side, ideally on a 300KW Room PDU, 25KW per side.

3. PDU Breakers should be power balanced for redundancy

a. Even if RPPs or Busway are technically redundant, they could load the PDU above its redundant capacity. The total utilized load (PDU to RPP’s/busway to racks)  across a redundant pair of PDU’s should not exceed the 100 percent rating of a single PDU.

TIP: Be careful. If you have six breakers in a Room PDU and load each panel to de-rated capacity you will have an issue: 65KW x 6 = 390KW.

4. Power phases should be load balanced across the circuits, RPPs and Room PDUs

Power can easily be stranded if care isn’t being taken on how power is loaded on each phase.

a. In data centers running 208V 3phase power, there are 3 phases of power that make up each circuit (X, Y, Z).   IT cabinets configured with single phase power connections such as an L6-30 outlet only utilize 2 of the 3 available power phases.  When a large number of cabinets with single phase power connections are deployed, there is a potential for the phases to be loaded in an unbalanced fashion.

b. Many 3phase rack power strips have their power broken up in 3 groups of 2 phases (XY, YZ, XZ). If you load all devices in one group, it is possible you may load 2 of the phases more heavily or lighter than others, causing a stranding of power.

5. Know your load characteristics

Most data center power loads vary by the hour/day/week/month. You should also know the peak times and calculate the probable maximum utilization.

It’s also critical to understand the power supply redundancy modes of your IT equipment. Some dual power supplies share load and others switch sources from A to B or B to A in the event of an outage of one source. The loads may appear unbalanced between A and B power trains, but the total utilization should be maintained below 100 percent of the utilizable capacity. Also good to ask: Are the PDU and RPP breakers 100 percent or 80 percent rated?

Ways to avoid data center power balance issues:

  • Know the data center circuit capacities and design your rack power loads to fall within the de-rated power capacities.
  • Use rack power strips that have an LED that displays load, allowing your team to adjust into which phases each device is plugged.
  • Train your team to watch for power balancing issues while racking equipment.
  • Consider using outlet phase alternating power strips.
  • Use a basic DCIM tool to monitor loads and balancing with IP enabled power strips or
    Branch Circuit Monitor