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Old-style U.S. Plugs (compare to Paul’s UK Plugs) #10213 06/01/02 11:01 PM
Joined: Apr 2002
Posts: 2,527
Bjarney Offline OP
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.

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Re: Old-style U.S. Plugs (compare to Paul’s UK Plugs) #10214 06/02/02 06:17 AM
Joined: Aug 2001
Posts: 7,520
pauluk Offline
This is interesting stuff -- Many thanks for posting it.

Re the 2-wire 120V extension cord with enlarged face on the connector to prevent use with a grounding plug, do you know when these were introduced?

I ask because I have a 2-wire cord (SO type, I think) which doesn't have this feature. I also have some 3-wire 5-15 extension cords where the face of the socket is such that you could connect a grounding plug the wrong way and leave the U-ground pin exposed. A couple of these are still in their original sleeve, and from the style of it I would guess 1960s era. (8' 16AWG, made by Pacific Electricord Co. of Gardena, Calif., cat. no. A2316-008, priced $2.49 if that's any clue.)

Our "twin & earth" cable still has the ground wire one size smaller than the other conductors, except on the smallest size.

Re: Old-style U.S. Plugs (compare to Paul’s UK Plugs) #10215 08/20/02 01:01 PM
Joined: Aug 2002
Posts: 1,691
SvenNYC Offline
Hi Bjarney,

The grounding adapter plugs with the 3" flying lead are still widely available for sale here in hardware stores in New York City and surrounding areas. In fact some of the stores have separate bins for the adapter plugs with leads and with tabs.

I have a number of these at home for when the odd occasion calls for them. The one here in use in my office is some no-name thing made in China (not even UL listed).

These "cheater plugs" are technically outlawed in Canada (according to CSA) but the past few times I've been to dollar stores in Montreal, I've seen them for sale (packaged with the little two-pin cube taps all for "un dollar").

Not that there's much use for them. Every standard 15A 110-volt wall socket that I've seen there is 3-hole, even in the oldest of houses (where the thing has about 500 coats of paint glooped over it). Whether they're grounded, that's another matter...

I did see one ungrounded two-hole wall socket in a Montreal deli that had a rotating safety cover on it. Really unique looking thing. This was in Ben's Deli. The thing is probably still one of the window sills (for plugging in neon signs and stuff like that).


Re: Old-style U.S. Plugs (compare to Paul’s UK Plugs) #10216 08/20/02 02:54 PM
Joined: Aug 2002
Posts: 1,081
ThinkGood Offline
There is a book entitled Old Electrical Wiring Maintenance and Retrofit by David E. Shapiro (Was published in New York by McGraw-Hill in 1998. Possibly out of print now.)

It has a nice brief history and then gets deeper into the individual changes/additions to the NEC over the years.

There are some nice (B&W) photos inside, as well as strategies for dealing with various scenarios one might encounter when facing a retrofit job. has some pages available for (free) preview...

Re: Old-style U.S. Plugs (compare to Paul’s UK Plugs) #10217 08/20/02 03:00 PM
Joined: Aug 2002
Posts: 1,081
ThinkGood Offline
BTW, Bjarney, thanks for the history for us "younger" readers.

I remember my father telling me [when I was the curious tot that I still seem to be...] that if I ever put something into a receptacle (thank Heaven for those receptacle guards) that I would get sucked up into the wall and wouldn't be able to get out.

I still get the chills when I have to plug something in today ;-)

Re: Old-style U.S. Plugs (compare to Paul’s UK Plugs) #10218 08/20/02 03:59 PM
Joined: Aug 2001
Posts: 7,520
pauluk Offline
I always feel in technical fields that a little historical background should form part of regular training, as it often explains some of the quirks and features that we still live with today.

Can anyone place just when polarized 1-15 plugs first appeared?

Re: Old-style U.S. Plugs (compare to Paul’s UK Plugs) #10219 08/20/02 09:12 PM
Joined: Feb 2002
Posts: 2,233
harold endean Offline

You talk about a "ring circuit", that is funny but the power co. around here NJ will sometimes do that to a town. The will connect a town on both ends so that in case of emergency, they can shut off 1 feeder into town and turn on the other side. Also it sort of makes a loop, so that they can "cut out" a bad section of poles/wires but still keep the majaority of the towns up and running.


Re: Old-style U.S. Plugs (compare to Paul’s UK Plugs) #10220 08/21/02 12:48 AM
Joined: Apr 2002
Posts: 2,527
Bjarney Offline OP
Harold — You bring up an interesting comparison. Off-topic war story — I used to work for a small utility-type operation that, partway during my stay, cut over from an older distribution-station of two semi-parallel feeders with crossarm-mounted air switches that allowed for limited sectionalizing for restoration and maintenance. To increase reliability and confidence in service continuity for some “picky” customers, a new station more centered in the area, the feeder count went from two to four, with some air switches left open and some ran normally closed. The pole-line feeders were laid out in roughly a figure-8 pattern.

The day after cutover me and an oldtimer lineman went around in a bucket truck to all the normally-open switches, and after proving that each side of the switch had the normally expected three ø-ø voltages we would test across each open switch pole to be sure we had zero volts, to minimize the chance of later fireworks and outages. It was a day of some nice, low-pressure overtime. One term for step-by-step planned switch operation is “sectionalizing.”

Switching aside, it’s the effectively same idea as 230V 2-wire ring circuits, except in the 3ø 5-35kV range.

[This message has been edited by Bjarney (edited 08-21-2002).]

Re: Old-style U.S. Plugs (compare to Paul’s UK Plugs) #10221 08/21/02 02:04 PM
Joined: Apr 2002
Posts: 2,527
Bjarney Offline OP
To attempt to address your question, Paul—I found something about the current state of 1-15 device polarization—sorry this is not a historic perspective, but does explain what has evolved a bit.

See: for some nema dimensioning. The 1-15R drawing calls out for a female polarized receptacle only, but there are two 1-15P male cord caps—one polarized and one non-polarized version. My guess is that for ungrounded, maybe double-insulated or unswitched appliances, there is accommodation for that in design flexibility.


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