# NEMA plug types

As I mentioned in my “Five years in the lab” post, over time I’ve become the de-facto system administrator for the lab. With no formal system administrator training (though I’ve taken many computer science and computer engineering courses), every once in a while I’ve had to overcome a major hurdle of a knowledge gap.

The most recent hurdle, encountered when we moved into our new building and server room, was plug types. There are a ton of plug types, and even my background in Circuits I and Circuits II was not enough to make things readily apparent. I actually find it pretty interesting — enough to write a post about it.

There are four parts to a NEMA plug type (or connector) name:

1. Locking: If the plug can be twisted to lock it in the socket, a capital “L” is appended to the beginning of the name
2. NEMA number: This is an arbitrary numbering scheme, and indicates the voltage and phase. For example, “5” means 125 volts with a ground wire and “6” means 208 to 250 volts with a ground wire. “21” is a more exotic example, indicating three-phase power.
3. Current Rating: The maximum current in amperes that the plug and circuit are rated for. Typical numbers are 15, 20, and 30.
4. Plug or Receptacle: A “P” for “plug” or “R” for “receptacle” may be tagged on to the end of a designation when the corresponding part is being referred to.

To put these all together for some common examples, we have:

• 5-15P: The common plugs found on most modern (and grounded) appliances
• 5-20R: 120V receptacles that can supply up to 20A. Often these have a “T”-shaped slot that will take either the 5-15P (above) or a 5-20P (one prong rotated 90 degrees). Most of the receptacles in the new CSE building have this “T”-shaped socket.
• L5-30: Locking plug that is the standard for shore power for boats (120V/30A). Note that the “L” does not refer to the black screw-down ring usually present on these cords. That’s an optional add-on. It refers to the locking done by twisting the plug within the socket.
• TT-30: Cementing the arbitrary nature of the NEMA “number”, the “TT-30” is the standard 120V/30A plug for “all but the largest” recreational vehicles. This is also known as an “RV-30”.

Our new cluster takes 8 L21-20 receptacles and a few L5-15 or L5-20 receptacles (for the battery back-ups). We have a really old battery back-up that we plan to use again, and for that we need an L6-30R installed.

While trying to sort all of this out, I really wished that the NEMA designations were more directly informative about voltage and phase. Would 120-1-30 be so much more difficult to write than 5-30? What about 250-3-20 vs. L21-20? Have you ever encountered an uncommon plug type in a data center or other building?