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#71371 - 10/27/06 10:32 AM NEMA 2-15/2-20 plugs and receptacles  
Dawg  Offline
Joined: Jan 2005
Posts: 59
Hey....I'm doing some research on how electiricty was used back in the old days....say from it's induction around 1890 up to around 1960.

One of the things I'm still leaning is about the NEMA 2-15/2-20 receptacles and plugs...for those that don't know the NEMA 2-15 receptacle is better known as the "t-slot looks sort of like the typical 5-20 receptacle you can buy at the hardware store except this is an ungrounded receptacle with T-slots on both the neutral and the hot.

So far from what I have gathered the "dual T slot receptacle" could be wired for either 120 volts or 240 volts. The slots look like this: -II- and the tandem slots, or the -- slots were used for the 240 volts while the standard II slots were used for 120.

I'm curious as to what ran off of a 240 volt 10 amp circuit in the old days?

Also I'm curious if anyone knows when these receptacles were made?

Work Gear for Electricians and the Trades

#71372 - 10/27/06 11:23 PM Re: NEMA 2-15/2-20 plugs and receptacles  
Beachboy  Offline
Joined: Sep 2006
Posts: 26
Wamego, Kansas USA
Dawg, I've been around more years than I want to admit, but I also hung around my grandparents houses a lot. As I was told, especially in rural areas, only 120 volt service was available to most homes pre WWII. Only after WWII were the generating capacity and transmission lines available for 220 volt service. Back when my grandparents first got 220V service, somewhere around 1950 or so, the first major items were electric hot water heaters and electric stoves, which obviously would have never used that type of outlet. Then in the mid '50's, window airconditoners came into popularity, but every window AC I've seen (and my parents had one from '55) had a much larger 220V plug on them. So basically, I don't think there was ANY 10 amp 220V service in use in most of the USA in the old days. Any of you other guys got any thoughts on this?

#71373 - 10/28/06 12:30 AM Re: NEMA 2-15/2-20 plugs and receptacles  
Dawg  Offline
Joined: Jan 2005
Posts: 59 240 v until after WWII eh? I was thinking I read somewhere they had 240 v. as early as '41?

Well here's a pic of the receptacle in question:

[Linked Image]

If you notice stamped on it it reads "15 amp/125 volts" and "10 amp/250 volts".

I asked an individual that has worked in the hardware buisness since the early 60's...he recalls they could be wired for 120 v. (standard type plug) or 240 v. (tandem blade plug). This was before NEMA stepped in and standardized the plugs and receptacles.

I believe this is the receptacle that is known as a NEMA 2-15. And yes obviously if this were set up for 240 v. it was something that I'd guess probably didn't draw a whole lot of current.

Beachboy do you have any pics of the a/c receptacle that your parent's window a/c they had in 1955 looked like? Most window a/c's I've seen run on a 15 amp/125 volt or 15 amp 250 volt set up. The window a/c we had in our house (I believe it was from 1966) was a 20 amp/250 volt set up.

[This message has been edited by Dawg (edited 10-28-2006).]

#71374 - 10/28/06 05:20 AM Re: NEMA 2-15/2-20 plugs and receptacles  
e57  Offline
Joined: May 2003
Posts: 2,876
I have worked on homes here in San Fran that were wired for 240 during the reconstruction after the quake of 1906. Some even earlier.... 1880's! Some with reminants of DC! Though most of those homes had combination gas/electric lighting - they thought electricity was a fad..... "It'll never get off the ground better keep the gas flowing." There are a few homes that still have operational gas lighting!

My home was built in 1934 and had a 240v 30A service, as most of the housing stock was originally built with. As was the same for all the homes built in the 20's - but most of those were larger and from 50A to 200A services. (much larger homes - early McMansion) Not long ago I removed a 200A service done in 1923, lead sheathed/cambric and cloth UG conductors to a CT cabinet and massive fused disconnect. This was for a single family dwelling... Large architechual lighting. And was high-tech for its day - electric buzzers for the staff... [Linked Image] Another home near this one has an original 240 delta service from 1918, due to its ancient elevator and original Otis controls. PG&E rep told me the distibtion in that area was designed around that house to keep the delta.

We had some early facination with electric lighting here as you can see in this site:

[This message has been edited by e57 (edited 10-28-2006).]

Mark Heller
"Well - I oughta....." -Jackie Gleason

#71375 - 10/28/06 08:07 AM Re: NEMA 2-15/2-20 plugs and receptacles  
pauluk  Offline
Joined: Aug 2001
Posts: 7,520
Norfolk, England
Have a good look through some of the pictures in the Electrical Nostalgia area. We've had quite a number of discussions about receptacle configurations of old (I too find the development of them quite fascinating).

A few examples:

[This message has been edited by pauluk (edited 10-28-2006).]

#71376 - 10/28/06 07:25 PM Re: NEMA 2-15/2-20 plugs and receptacles  
iwire  Offline
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
Some NEMA info here from Wikipedia.

Bob Badger
Construction & Maintenance Electrician

#213300 - 04/06/14 02:43 AM Re: NEMA 2-15/2-20 plugs and receptacles [Re: Dawg]  
76Dave  Offline
New Member
Joined: Apr 2014
Posts: 2
The NEMA 2-15 configuration refers to tandem plugs. The tandem plug, originally rated at 10A 250V, is Hubbell's first design for a blade-type connector, created in 1903, patented in 1904. His parallel blade connector would be patented a few years later. Until ~late 1920s, there was no standard for voltage in the United States, although most small-duty appliances were operating on 120V. The "powers to be," in the late 1920s, decided, for a number of reasons, to standardize on 120V for the nation's voltage.

Most small applications in the U.S. have always been 120V, but a few were 240V prior to the late 1920s. You will sometimes find old electrical fans, light bulbs, etc. that were not exports which operate on 240V. I have an 8" Westinghouse fan from the 1910s that operates on 240V, and I have seen 240V light bulbs made by Nalco. It seems that 240V, when found, was more likely to be in places such as apartment and office buildings in larger cities. But tandem OR parallel plugs may have been in use there. When the nation standardized on 120V, they made a compromise, so to speak: 120V for lighting and general duty appliances in the home, 240v for heavy duty appliances, such as ovens, stovetops, water heaters, large space heaters, and later air conditioners and clothes dryers.

Hubbell's plug inventions were all rated at 250V. I have yet to discover any evidence of his motive in designing more than one plug configuration (there were at least 4 of them before it was all said and done), but one can reasonably assume that his latent intent at least was to account for the different voltages in use at the time. There were, however, no official voltage associations with plug configurations. (125V was officially assigned to the parallel configuration in the 1930s, but 250V was not OFFICIALLY assigned to tandem until the 1950s--it came to be "understood" as being "for 250V" by default until it was made official.) So both parallel and tandem plugs were used interchangeably. This is why we have the existence of "t-slot" outlets and its predecessor ( And yes, these receptacles could be wired for either 120V or 240V. While almost all were wired for 120V, not all were, and you needed to be certain which voltage was present, as both parallel and tandem slots were energized at the same time! A "2-15r" receptacle does indeed exist, but there was never a NEMA configuration assigned to it. It seems that production of it had ceased by the 20s. You can see an example of it in one of my attached pics. It would have been rated at 10A, just like the plug. The NEMA 2-15p designation was assigned to the tandem plug in the 1950s, then the tandem plug was "banned" in the late 1960s, because it utilizes two "hot" wires and no ground. Fraidy cats, heh. T-slot receptacles were commonly made up to this time (and are still made today, believe it or not). I assume tandem plugs were also made into the 1960s.

In the 1950s, there was talk of converting the U.S. to 240V standard, but it was determined that it would be too costly and inconvenient to do so. 240V (@60Hz) is actually a more efficient system, as wire gauges used can be smaller because of, relatively speaking, the lower current drawn. And either voltage can kill. I think's it unfortunate we didn't standardize on 240V, to be honest.

Attached Files
hubbell plug + adaptor.jpg (78 downloads)
(l): Hubbell's original patent--tandem plug + Edison adaptor. (r): tandem plug + wall receptacle, patented a couple of years later

#213301 - 04/06/14 04:48 AM Re: NEMA 2-15/2-20 plugs and receptacles [Re: Dawg]  
Tesla  Offline
Joined: Jun 2004
Posts: 1,273
Sacramento, CA
What Hubbell left unsaid was that users found screwing cords in to be a hassle AND as time went by the cords deteriorated... a LOT.

The Edison scheme was dropped like a bad habit.

Beyond that, if you tripped on a cord connected to a Hubbell plug -- it would come out of the wall.

An Edison attached cord would pop at the terminals -- not infrequently triggering a fire right then and there!

Hubbell made a fortune with his 'better mouse trap.'


#213327 - 04/10/14 09:39 PM Re: NEMA 2-15/2-20 plugs and receptacles [Re: Dawg]  
76Dave  Offline
New Member
Joined: Apr 2014
Posts: 2
update: apparently the tandem plug is still being made (as a molded-on plug) by some manufacturer(s)--

Member Spotlight
CA, Inland Empire
Posts: 43
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