Here in the USA, most consumer power strips seem to make use of NEMA 5-20R connectors - they look something like this. However, in the rackmount/datacenter world, I see a lot of power distribution units that accept IEC C14 connections instead, which look like this.

I know there's no significant difference electrically; I'm wondering why I should prefer one over the other. In my limited experience, I've found that connecting C14 connections to a PDU results in looser connections that I'm more likely to have fall out without securing them with zipties or velcro.

up vote 6 down vote accepted

I think it's a question of voltage. NEMA 5-20R can only support 125V (the common household voltage in the US...minus your electric clothes-dryer, of course). IEC C14 can support up to 240V, a more common voltage outside the US.

So why would 240V be better than 125V? The higher the voltage, the more efficient the power supply (I believe because it doesn't have to be stepped down as much from the building's electrical feel...which is 480V (or higher, with even larger buildings).

By way of a real-world example, we've switched one of our DataCenters to be entirely 3-phase 208V power (using IEC C13/14 and C19/20 outlets). We're expecting to lower our bill by almost 50% compared to the 125V power (not accounting for overall growth).

  • I didn't realize that NEMA 5-20R only supported 120V... that makes a lot of sense. We're actually running 208V on our equipment already (it's cheaper overall) so I guess there's no question which form factor I should be looking for. Thanks! – natacado Jun 4 '09 at 23:54

Guamaniac hit on the difference, but his numbers are slightly off.

If you are in a commercial building with 3-phase power, then you may be able to run your servers at 208V. (240V will not be available. That is only in residential 2-phase power. None of the racks I looked at were certified to run at 240V in North America.) Most power supplies in modern servers are auto-sensing, and will run more efficiently and cooler at the higher voltage.

Since you won't want people to come along and plug in devices that require 120V (also known as 115V), the electrical code requires using C14 instead of NEMA.

For our server room, I chose to go with 120V. Since my servers are generally lightly loaded, I wouldn't save much on electricity anyway. It was more important to be able to be able to plug in devices like monitors, KVM units, and small switches that we already owned, anywhere in the room without having to run double the number of circuits. So we just ran NEMA everywhere.

  • 1
    All of our monitors and switches already supported 208v. – Karl Katzke Nov 3 '09 at 5:25

I'm in the UK so it's C13/14 or C19/20's only for me, we don't do NEMA here. The 13/14's are a bit loose you're right - that said I'm pretty lucky, I tend to just use blade enclosures so even if one fell out if wouldn't really effect our machines. If you have the choice then go with whatever makes you sleep best at night.

Use the higher ~208V range for efficiency. Volts * Amps equals Power; if you can increase the voltage for a particular power requirement, you're lowering the amperage. Most modern equipment with C14 connectors will run at the higher volt range. Equipment with C19 connectors (like large servers and blade server enclosures) usually require the higher volt range.

Use PDUs with IEC connectors for the best density and more componentized and flexible power routing. As you increase in scale, you'll probably want to scale battery backup and power conditioning beyond single systems and racks.

Regarding the relative "looseness" of the IEC connectors: if you have dual-sourced equipment and limit access to racks, I've found that isn't a problem.

In Canada, in just about all machine rooms I see the NEMA L5-30. Its very popular on APC rackmount UPS systems, which then translates power to standard (at least to NA) NEMA 5 stinger plug which plugs in Dell, HP and other 3 pronged rackmount gear.

I believe there's a bit of politics and money involved as well. NEMA is for the most part an American-centric standard (with wide adoption in Canada and Mexico as well) while IEC is an international standard. The NEMA "consortium", which is a member of the IEC is pushing to have their standard accepted more on a global level and the IEC in turn is making inroads into the American market. I'm finding more and more that IEC style cords and PDU's are easier to come by and cheaper than their NEMA counterparts.

"Most power supplies in modern servers are auto-sensing, and will run more efficiently and cooler at the higher voltage." And those that don't generally have a switch near the power supply to set this, so before plugging anything in, look for one and if it is there, make sure it is set properly!

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