For the sake of younger US readers—
Some probably are not aware that 100 years ago the US ‘standard’ receptacle was an “Edison- based screw shell” that would accept an ordinary [e.g., 40-100 watt] incandescent lamp (light bulb.) I’ve only seen one in a ~1910 house bedroom, and it was mounted in a bedroom wall about 42 inches from the floor. The ‘socket’ appeared to be integral to the brass plate, and had an integral round hinged cover to save unpleasantries if you were sloppy in attempting to find it in the dark and got am unanticipated poke.
So-called straight-blade wiring devices eventually replaced them. Blade polarization was a later adaptation—with switched appliances and lamp screw shells. There are now both NEMA 1-15 (two-pin) and “U-ground” 5-15 (three-pin) configurations. The 5-15 female connector male is inherently ‘backward compatible’ with the 1-15 male device.
T-slot/opposed blade is offered in place of (15A) flat blade for 20-ampere wiring devices—NEMA 5-20 and 6-20 designations. Twenty-ampere female devices are considered ‘backward compatible with 15-ampere male cord caps.
Fused caps are rare except in series Christmas light strings (5x20mm.) Fused receptacles are still sold, and, in some regions were once de facto for garbage disposals and gas furnaces. They used 125V Edison-based {like incandescent lamps} plug fuses or non-tamperable type-S versions. At one point, integral thermal overload protection was not common in single-phase appliance motors.
Face in the female connector-body end of an 2-wire extension cord is oversized so that you can’t by cheat inserting a male 5-15 with a ‘floating’ and potentially hazardous ground pin. Face of female 1-15 cap was made larger to prevent finger contact—especially for little kids.
The US 2-to-3-pin adapter with ~3-inch flying lead was removed from market because flying lead- end spade could contact ungrounded adapter blade and ‘heat up’ supposedly “grounded-metal” case. Understand Canada has outlawed these across the board.
As a precursor to U-ground connectors, I remember as a kid my dad having a metal-cased drill motor with a 3-wire cord but a two-blade cap and a loose green lead with a 6-32 machine thread that screwed in place of a (grounded?) receptacle-plate screw.
The ‘swing-away’ U-ground pin came and went. Have an old military tube tester with one. At the time it was an accepted compromise for use in older (2w) and newer (3w) facilities.
SPT2 or ‘zip’ cord ’ has a ridge on the jacket of one of the conductors, and is formally referred to as “the identified conductor” that we all equate to the neutral, white wire, and grounded circuit conductor. It’s the one that connects to the screw shell in a light socket and the wide blade of the ordinary NEMA 1-15P cap.
“Ring circuits” are not strictly illegal in the US, but inspectors could make things miserable if you applied the practice most anywhere. It could be argued NEC 310-4 parallel “electrically joined” does on prohibit a loop, and could be advantageous for 120v multi-outlet circuits. But, seemingly unlike the UK practice, if the typical receptacle circuit was fed from a 20-amp fuse or circuit breaker, 20-amp (e.g., 12-AWG) wiring is required in to loop. It would be a misapplication to use 15-amp/14AWG wring served from, say, a 20-amp breaker, where it sounds like is effectively permitted in the UK ring circuit. In the US’ situation, because loop current would not always divide equally, that would not of concern with US ‘fully rated’ conductors. To update, GFCIs and AFCIs would not necessarily complicate the issue.
I have never come across any, but apparently during World War 2, two-w1re romex with a bare neutral was temporarily permitted in residential construction—to save rubber and plastic for the patriotism and the “war effort.”
In the early 1960s when 14/2 WG and 12/2 WG “with ground” first was required, the bare ground was not the same size as the insulated conductors. I believe insulated 14s had an 18-gauge ground, and 12s had a 16-guage ground. I’m not sure if 10-gauge romex use a 14-gauge ground wire.
