Power distribution in server racks come in a number of different varieties. It used to be that voltage and amps were the only considerations to make when selecting the right power. Over the last decade three phase power distribution in racks has become more common. But do you know the differences between single phase and three phase power and how it affects the power usage in a rack of servers that use single phase only?
Let's start with a simple overview of 208 volt single phase power distribution in a rack of servers. With this type of power distribution we have two load carrying conductors, or phases, in the circuit and each server that plugs into the power distribution unit (PDU/CDU) connects to both phases. Since all of the servers plugged into that PDU are connected to both phases; power draw (amps) across that circuit will be consistent across the two phases. This is why on a smart single phase PDU with digital power display; only one number will be shown for power usage.
Now let's take a look at what happens on a three phase PDU. A three phase PDU has, as you may have guessed, three load carrying conductors or phases. Since the servers in the rack require single phase power; the three phase power in the PDU is split internally into three banks of single phase power with each of these banks containing any number of outlets. Smart three phase PDUs will have three digital power displays; one for each phase.
In this configuration each server that plugs into an outlet of the three phase PDU only connects to two of the three phases. This presents the possibility of uneven power draw. If servers are plugged into the PDU unevenly across banks or servers with different power draw requirements are not plugged in intelligently; it is quite possible to over draw the circuit on one of the phases which could result in an outage.
Since most server racks have two or more PDUs; this uneven draw issue becomes a bit more complicated. The power fail over has to be planned out between all six phases (three per PDU). A good rule of thumb that many data center professionals have adopted is to determine a maximum amp draw number that each phase should be loaded to and manage accordingly. For example: if there are 24 amps of useable power total within the rack; then none of the digital load readings should read greater than 12 amps. This ensures that during a fail over that no single phase will reach more than 24 amps of power.