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.
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. U.S. three-phase delivers 120 V per leg to neutral, but 208 V between any two legs—PDUs draw between two phases to deliver that 208 V voltage commonly used by IT gear.
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. Modern SMPS designs already handle single-phase rectified input efficiently. The real benefits are more about distribution efficiency and inverter losses than dramatic single-rack PSU efficiency gains. The fewer changes you need to make to your power in the network from generation to consumption – the less loss you’ll experience from conversions. No transformer or distributor network is 100% efficient (although the superconducting power lines feeding Long Island, New York come very close).
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 208V!
When the power is delivered to the PDU, the three phases are separated into 120V phase-to-neutral and 208V line-to-line sockets on the PDU. The example below shows three branches of line-to-line 208V (single 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. Proper PFC and power factor calculations are essential to help balance your power consumption over all branches to help properly balance your load on the UPS / generators.
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 208V then?
Remember with 208V 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 120v of power, and with that increase in voltage, that brings up the volt amps up with it, allowing that circuit to more actual work.
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.
A Solution Architect’s Notebook
Copyright © 2023 A Solution Architect’s Notebook | Powered by A Solution Architect’s Notebook
