In this exciting age of virtualization, cloud, SDN, and other hardware abstractions, we often take for granted the most important technological advances of the 19th century – electricity. While some of us are just able to spin up another EC2 instance from the ether, most of us still worry about physical capacity, even if it’s going to our own internal virtual infrastructure. So that means at the end of the day, in order to add capacity, some poor soul somewhere is going to have to heft 60 pounds of metal into a rack and plug it in.
In its most basic form, this moment of truth is a non-event. The server comes with a couple cables, the PDUs are already mounted in the rack by the last person, just plug in each end and move on. Easy enough. But what if you had to order more of those cables? What the heck is the difference between a C14 and a C19? What if you had to order a new PDU and UPS to build out a new closet or rack? What if you’re standing in the server room right now talking to that kindly gentleman wearing overalls trying to figure out just what sort of socket needs to go on that wall? These are the sort of low-tech questions we’re going to answer here, because the day will come when you’ll be asked to “just” plug something in.
Let’s start with the common power cable. Excepting proprietary oddities from a few manufacturers and regional regulations, you will mostly encounter the same 4 types of cables:
NEMA 5-15P-C13 (mostly North America):
So now that we can identify these cables, where do we plug them in? The most common AC output voltages from UPSs will be 120v and 208v. For the remainder of this post, I will refer to 208v as it is most prevalent in North America, but it could be anywhere from 208v-240v internationally. Most modern datacenter gear will accommodate voltages from 120v-240v AC, however it is always advised you actually check the power supply of the gear to verify there isn’t an old-school voltage selection switch before plugging it in. Unless you like the sweet smell of fried PCB in the morning.
This also applies when you’re spec’ing out UPS and PDU gear for a new rack build-out. The first order of business is to make sure all of the gear going into the rack will support the output voltage of the UPS. (Or for that matter, that the PDU does as well.) 208v is more efficient than 120v, so if you have a choice in the matter, go with 208v. Secondarily, choose a PDU (or UPS) with a sufficient number of the desired type of sockets. The choice in output socket is primarily driven by your voltage selection and device amperage requirements, so if you’re going with 120v in North America, NEMA 5-15 is most common. Higher draw devices (read bigger) will likely call for C19 sockets as they can handle ~60% more juice than C13. C13’s also have proclivity to not fit as snug and secure as one might hope, so it may be a worthy investment to find PDUs / cables with locking latches (APC has some decent ones) to prevent accidental downtime.
On the other end of that UPS (or PDU) you will have a whole new set of decision points to determine what you need. Apologies in advance for the focus on North American plug types here. High-res pics of other standard (IEC) types would be very welcome in the comments section below. The common input receptacles you will see (or ask your electrician for) in North America will be:
Less common, and usually seen with big 120V UPSs will be a L5-20. Ever wonder what that horizontal slot was for in commercial building sockets? Now you know. If you’re going 208v, you may also be asked by your electrician if you need a “three-phase” circuit. The short answer is that three phase circuits can handle a heavier load, and that you need a UPS that supports it. The long answer may be found here: http://en.wikipedia.org/wiki/Three-phase_electric_power
Speaking of UPS- how does one select the appropriate model? Firstly, let’s talk about one of the dark mysteries of the electrical world- kVa and how it differs from kW. We won’t go too far into the weeds on this, but the reason kVA maps so well to kW is that kVA is “apparent” power and kW is “actual” power. Represented mathematically:
kW = kVA * pf (power factor)
Power factor is basically the efficiency (loss) of the power load. Nearly all modern high-efficiency server power supplies should have a power factor close to 1.0, so kVA tracks very closely to kW. This is what you will use to size the rated output load of the UPS. As far as runtime goes- pardon the obviousness, but the more batteries, the longer the UPS will run for. This varies by vendor and model, but will usually be stated on the product specs under min/max/average load. Make sure the UPS has the appropriate input connectors for the wall plug you chose, and the matching connectors for the PDUs, and you’re in business. Next step is to wait for a few hundred pounds of gear filled with lead-acid batteries to arrive on a pallet and hoist it into the rack.
Until the day comes that Tesla’s dream of wireless power distribution becomes a reality, there will always be a need for someone to push a plug into a matching receptacle of appropriate voltage and with sufficient amperage. If that someone is you- I hope you found this post useful.
In the comments below, we would love to hear how you currently manage your power / cooling / environmentals. Do you have them in NPM or are you using something else? How do you model your racks- Visio? Dare to dream of using something better?
We also have a quick 4-minute survey that gives an opportunity for even more feedback on rack diagramming and power / cooling management. We'd love to hear your thoughts: Rack Diagramming Survey
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