Musings on North American / International power

  • Sat 04 January 2014
  • misc

This is the email that started things rolling. I suspect that Avi looped me in because I used to work at Afilias which has medium sized nodes sprinkled across multiple continents. The person I sent it to is a fellow from the UK who was asking about what kind of power he could expect to find in the USA. I made a few changes as part of adapting this for the blog, but it's more or less as I sent it.

Safest course of action when doing international stuff is to standardize on IEC C13/C14 cables to the PDU (you can supply your own at Equinix or they will supply them for you by default) and then chop of the cable end to put whatever you want on it to plug into what they provide on the rack. That's what we did at Afilias (we provided our own APC switching and current monitoring power strips that were good up to 250 volts and deployed them around the world).

US electrical outlet standards are done by NEMA.

The "normal" receptacle that you would be presented with if Equinix supplied your power is a 5-15R. Deconstructed, 5 means 120v, 15 means 15 amps, R means receptacle. There is also P for plug (the thing on the end of the cord that you stick into the wall).

On the cable trays above one's racks, if one is supplying one's own PDUs, one will find one of the following:

  • 120v 20a - L5-20R
  • 120v 30a - L5-30R
  • 208v 20a - L6-20R
  • 208v 30a - L6-30R

The L means twist lock.

For delivering a single phase, 208 (3 phase inter-phase) and 240v (split phase residential across the hot legs) typically use the same connectors in the US. Polyphase delivery to a rack is probably out of scope unless you're running some truly scary kit.

I recommend Hubbell connectors. No need to order from Amazon though; L5 and L6 connectors should be available at any "real" electrical supply store. They're a bit niche for Lowe's or Home Depot but I've been surprised before at what one can get there.

If you need to get cables stateside that have various IEC and NEMA connectors on the ends, I have had good luck with StayOnline. If you buy from them don't forget to include a couple of these or whatever the equivalent is for your native power cable to let you plug into a C13 power strip. Bloody handy in a datacenter. I keep at least one in each laptop bag and have ordered them to hand out as party favors.

If you're chopping off $EU_Connector to put on your own, DO NOT rely on black to be the hot lead in the cable - instead verify with a multi meter. "Black to Brass - Save your Ass" (implying white to silver and green to ground) is a great US mnemonic, but only works if everyone is on the same page.

Thermal breakers work pretty much the same everywhere. The rule of thumb is 80% of the breaker's nameplate rating is the maximum for a continuous load (more than 3 hour, which all datacenter loads are by definition). It's codified in NEC 210.19(A), in language that speaks of 125% which is of course the reciprocal of 80%. This is the threshold for "nuisance trips" likely becoming an issue. Nuisance trips on your microwave or toaster are not that big a deal compared to datacenter outages where the consequences might involve having to polish up your resume. And of course if you're dealing with redundant A/B circuits the whole load has to fit on one side or the other, which means effectively you'd better not be pulling more than 40% anywhere that you're redundant.