Power in the data center, when three phase and single phase makes sense.

Why do we run three phase power in the data center?

Why three phase?!

The TL;DR: it’ll cost you less and provide many advantages.

1. Three phase circuit will provide higher average power delivery with fewer wires than a single phase circuit for the same power requirements. Fewer cables, fewer PDUs, lower cost.
2. Three phase service can provide 2 phase power to the gear in the racks that can use it – which is probably all of it. Look at the power supply, if it has a sticker that says 100-240V AC, you’re in business! Check your older gear – they used to put a slider switch to select 110V or 208V AC… guess what happens if you plug 110V set power supply into a 208V AC socket. The magic smoke escapes or if you’re lucky an internal fuse or circuit breaker pops.
3. UPS load balancing made easy. Having all three phases in the rack allows you to even out the load in the rack by simply moving power cables to different sockets on the same PDU.

What is 3 Phase Power?

Here’s a visual of what a three phase sine wave looks like. Each one of those waves represents one phase of the electricity wave form being delivered to the PDU in the rack. The energy in data centers in the US would be sending 120V AC on each phase at 60Hz.

60Hz means that sixty times a second a phase of electricity cycles from 0V to +120V to 0V to -120V and back to 0V.

Now offset two more phases by 120° and you’ll get the three sine waves shown below.

With a positive voltage or negative voltage your IT gear can actually do work. The more often there is a positive or negative voltage on the wires, the more work it can do. When it’s at 0V, your gear can’t do any work.

A side node: Your IT gear runs on DC (direct current) and uses power supplies that convert the AC into DC power. Besides doing that important job of AC to DC conversion, they have large capacitors to ride out the dips in the waves – they soak up the extra energy on the upswing and release it on the downswing. The capacitors ensure that your servers have a constant supply of direct current energy to keep running happily along.

A side side note – some telecommunications closets, distribution huts, and data centers distribute power as 48V DC to the racks, so they have to order special power supplies for that gear. This is a carry over from olden times with really old telephone equipment…

When does Single Phase make sense?
Single Phase makes sense in small racks of gear, especially those found in wiring closets, office IDFs, and other places that probably have no requirement for more than one or two 120V 20A circuits of power.

I know my equipment in the rack can’t run on three phase power… why bother?

You’re absolute correct – but it can and should be running on two phases!

When the power is delivered to the PDU, the three phases are separated into single – and two-phase sockets on the PDU. The example below shows three branches of two phase. Branch 1 using L1+L3, Branch 2 using L2+L3, and Branch 3 using L1+L2.

There are hundreds of types of PDUs that provide different connectivity options based on your needs, but that’s a basic setup for a 3Ph PDU.

The sockets on the PDUs are labeled to signify their connectivity to help you balance the loads – for example L1+L2, L2+L3, L1+L3, or just L1, L2, L3 for single phase if the PDU has this option.

Most of the gear you carry today can be powered by 120V or 208V power in the data center. So if you’ve ever ran out of data center to go charge your phone or laptop, stop. You can just plug into a rack PDU – I keep a C14 to NEMA 5-15 adapter in my bag for such occasions!

So, what are the benefit of two phase then?

Remember with 2 phase power you’re supplying power using two of three available phases of power – one phase on each leg or connector. This is providing 208v instead of 110v of power, and with that increase in voltage, that brings up the volt amps up with it, allowing that circuit to more actual work.

With two phases flowing through the power supply, the swing of power isn’t as large – this is where some of the savings comes from.

That’s it! (No, not really, please check the sources below for much more detail into power and the math behind it)

Over the same cycles, more power is delivered at the same time – allowing your IT gear to operate in a more efficient manner.

This is the difference between 120v and 208v power in the racks but it’s not just a bump in voltage, it’s a bump in average current.

Your equipment will operate more efficient and cost you less in power and heat… the savings won’t be massive, but 3-5% is still a nice note in your eco sustainability report. Most IT gear can operate in 120 or 208V – but aim for 208V if you can to save money.

Some gear can’t operate at lower voltages because there’s just too much work to do. The 60 disk drive shelves from NetApp, the Cisco UCS 5108 blade chassis, for example require 200-240V so this works out well for them already.

Math alert:

Finding the voltage of two of three phases is figured out by taking the volts * √3 so…

120 *√3 = 207.846

Yes we rounded up to 208v, shocking isn’t it?

Residential Power:

If three phase gives us 208V, why is my dryer or stove in my home running at 240V?

Great question – totally different power distribution is used in residential areas. Residential gets single phase power because it’s easier to work with and delivers 100A of 120V energy on single cable.

Residential electricity in the US is called a split-phase (240/120V) power system. Normally there are four cables run to a home – two 120V sources, a neutral connection between the two, and a ground for safety.

On the pole for your utilities is a transformer that is designed to “step down” the high voltage in your neighborhood to 120V AC for delivery to your home. Homes typically get two single phase 120V feeds, so when combined for high energy appliances the circuit provide a total of 240V power, simple as that.

Would 3 phase work at home? Sure, in theory it would take rewiring most of your distribution panel.

For example one circuit could be on one phase, another circuit on another, providing your nominal 120V in the house. Your electrical appliances that run on 240V would require a hefty transformer to change up the 208V of two phases to 240V single phase. You lose some efficiency in the conversion so your appliances would be more expensive to operate.

But it wouldn’t available in most residential areas unless your power company had 3 phase equipment in the area for businesses.

If you want to go even deeper into single and three phase power: