Right now, if you happen to be in Noth America, chances are pretty good that there’s at least one little face staring at you. Look around and you’ll spy it, probably about 15 inches up from the floor on a nearby wall. It’s the ubiquitous wall outlet, with three holes arranged in a way that can’t help but stimulate the facial recognition firmware of our mammalian brain.

No matter where you go you’ll find those outlets and similar ones, all engineered for specific tasks. But why do they look the way they do? And what’s going on electrically and mechanically behind that familiar plastic face? It’s a topic we’ve touched on before with Jenny List’s take on international mains standards. Now it’s time to take a look inside the common North American wall socket, and how it got that way.

Hubbell’s Plugs

Consider the problems faced by engineers and designers in the early days of the electrical age. They were literally inventing an industry from the ground up, with very little to go on in terms of prior art. Not only did they have to invent the means of producing electricity, they had to come up with absolutely every component that would connect together to create useful circuits for paying customers, preferably without killing them.

One thing customers, particularly residential customers, would need would be a means to temporarily attach electrical devices to the mains supply, without requiring a visit from an electrician to connect them to the fixed wiring of a house or office, which was typically dedicated to sockets for light bulbs. The requirements were simple: provide two contacts, one for the line conductor and one for the neutral, that could remain firmly connected but easily interrupted at need.

Imaginative minds worked on this and similar problems in the late 19th and early 20th centuries, and various solutions were adopted. But it wasn’t until 1903 that Harvey Hubbell, an inventor from Bridgeport, Connecticut, patented his “Separable Attachment Plug,” a device that we’d recognize as a plug and socket. Hubbell’s first pass at a design used round conductors that looked a bit like the plugs used in manual telephone exchanges to make connections, and might have been inspired by them. The detents at the tip of the pins were retained by the spring action of the contacts inside the socket.

The device worked well, but the manufacturer and businessman in Harvey saw problems. Foremost was the costs behind those round pins, which would have required machining to achieve the tip and detent. Harvey would have known that parts stamped from sheet metal would be cheaper and easier to manufacture, and so he scrapped the round pins in favor of flat metal blades in 1904. Like the round prongs, the flat blades had a detent for retention, and were arranged in a line. Catalogs from the time list dozens of variants of the “Hubbell Attachment Plug,” and the prices shown for each device suggest that Hubbell’s company fared well in the early 20th century.

For reasons unknown, though, Hubbell altered his design in 1912. The two blades were no longer in a line; each blade was twisted 90° to form the familiar parallel arrangement we see to this day. Hubbell continued to sell both styles of plugs and sockets, and by 1915 had sold something like 15 million units, enough to ensure that Hubbell’s design would be adopted as a standard, even without the millions of units also sold by Hubbell’s imitators.

Standardization

The specifications for the standard wall outlet we know and love today in North America are determined by the National Electrical Manufacturers Association (NEMA). NEMA standards cover a bewildering range of electrical products; we’ve covered their enclosure and weather-resistance standards before. The standard 120-volt, 15-amp outlet is a NEMA 5-15. The third conductor, the ground pin that completes the outlet’s face, is a round or U-shaped prong. It was added to some outlets as early as the 1920s as a safety feature and is now required for all outlets by the National Electrical Code.

The ground connection is interesting. You’ll notice that on three-wire plugs, the ground pin extends further out from the insulated cord body by about 1/8″. The idea here is that the ground circuit will be completed before the line and neutral connections are made when plugging the cord into an outlet, and perhaps more importantly, will be disconnected last when unplugging. That ensures that there’s a path to ground any time a circuit is plugged into the outlet.

Note too that the NEMA standard says the ground pin is actually located above the slots for the line and neutral pins, turning that frowning face upside down. There’s some logic to that — if something conductive should drape across a partially unplugged cord, it’s safer to have the line and neutral blades physically blocked by the ground pin. In practice, though, most outlets in residential and business settings are installed with the ground plug down. But look around the next time you’re in a hospital; chances are, the outlets there are all installed the correct way.

Behind the Face

The internals of a NEMA 5-15 outlet vary by manufacturer, of course, and even within a brand, there are different grades of outlet. The picture below shows two different grades of outlet taken apart. They’re similar in that both the line and the neutral connections are formed brass bus bars, with screw connections on the outside for connection into a building’s wiring, and springy contacts to grip and retain the mating plug. The industrial-grade outlet has thicker bus bars, better contacts, and stouter plastic in the body. You’ll notice too that both grades have the ground pin directly connected to the metal frame of the outlet, which would also be in contact with a metal wall box, if it were mounted in one.

Considering how much else has changed in the last century, it’s pretty remarkable that Harvey Hubbell’s original plug and socket designs have remained pretty much unchanged. They’ve been tweaked, for sure, and the original idea has been extended to a panoply of configurations for every connection imaginable. There’s no doubt that the design has some deficiencies, but in the end, Harvey’s ideas seem to have won the day by addressing the basic needs.