Why do most NEMA 5-15P (North America 125V 15A) line and neutral blades have holes in them? I've been wondering this for quite some time and haven't yet found a satisfactory answer.

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    \$\begingroup\$ I've always wondered this, too, thinking maybe it's a way to allow wires to be attached directly to the blades. \$\endgroup\$
    – endolith
    Commented Mar 15, 2011 at 16:15
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    \$\begingroup\$ 5 Mpix image for that!? I reduced it to 1/3 Mpix with no loss of information, and made the dark areas easier to see while I was at it. Now why couldn't you have done that!? Please be more considerate next time. \$\endgroup\$ Commented Oct 26, 2012 at 15:49
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    \$\begingroup\$ I wasn't as experienced a used back in March 2011 when this was originally posted. My bad. (Not that you accept any excuses...) \$\endgroup\$ Commented Oct 28, 2012 at 18:36
  • \$\begingroup\$ someone should call these guys and ask them signalandpower.com/Power-Cords/… \$\endgroup\$ Commented Oct 31, 2012 at 21:51
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    \$\begingroup\$ Someone is gonna have to pay for the wasted electrons on @Olin's computer, processing all those extra pixels. :) \$\endgroup\$
    – JYelton
    Commented Aug 21, 2013 at 17:25

7 Answers 7


According to page 17 of ANSI/NEMA WD 6-2002, "The hole in flat blade is optional, and it is intended for manufacturing purposes only. However if used it must be located as per dimensions shown above."

That doesn't really answer your question though. Since they have specific dimensions, I'm guessing it's a hold over from a deprecated feature. I'm about to replace some 60 year old outlets today, so I'll have to tear one apart and look for captive ball bearings, bumps on the wipers, etc.

Here is an example of a receptacle that locks using the blade holes. http://fam-oud.nl/~plugsocket/NorthAm1L.html

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    \$\begingroup\$ This is the first spec citation anyone has been able to provide me in 20 years of asking this question. Wow! \$\endgroup\$ Commented Oct 26, 2012 at 20:15

Almost a century ago, back in 1915, John J. Kenney filed his idea of an electrical connection system and connectors that "prevent accidental separation" for a patent that was granted to him on 25 May 1920: US patent 1341468.


A small quote: "When the terminals (4) and (5) reach final position these projections spring into openings (25) in said terminals and positively lock the cap and shell and prevent accidental separation thereof."

Another slightly younger 14 September 1920 US patent 1352817 shows the holes without extensive comment/description: as if they were already routine by that date.


Even relevant modern day patents like Mark Kenny's US Patent 6595810 from 2003 still reference this John J. Kenney patent and clearly state it's use.

As Cubiclegnome & Olin Lathrop already said: there were even physical locks in production, besides the known "out-dents" on the "blade-engaging members" in the receptacles.

I think this 'origin' out-dates ANSI/NEMA WD 6-2002 regulation, that also had to regulate what was already in large scale use.
Just like the wide-spread use of non-fixed lightning armatures (like desk-lamps and such) and house-hold appliances like boilers (whose chassis was 'grounded' to 'neutral' instead of 'earth', which was actually legal North America until 1996!!) lead to the (invention and eventually mandatory) use of polarized wall-outlets.

But who knows, there might even be an older origin/patent (but I can't find any reference to it).

I was once told (what I believe is an urban myth since the dates and proof that I presented above are prior to WWII) that the holes are a left-over from World War II and a result of the War Powers Act to maximize the quantity of basic material for the war effort. Thus the SEAMA (Small Electric Appliance Manufacturers of America) was able to save tons of brass that was more "Efficiently Allocated" :P .

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    \$\begingroup\$ Nice find! This question is becoming quite a treasure hunt. \$\endgroup\$ Commented Oct 29, 2012 at 3:00
  • \$\begingroup\$ I just updated the answer so it reads easier and has the relevant images embedded. Also added the second pdf-link :P I read your profile and if my answer helps in any way, might we please have your power-supply expertize on my 'friends' ac reversed polarity question? \$\endgroup\$
    – GitaarLAB
    Commented Oct 29, 2012 at 3:56

Oddly enough, the holes in the blades are for locking out the plug. A padlock or a tag (saying "factory sealed", or a warning, or an instruction) can be threaded through the holes. With the tag, the plug wouldn't fit into the socket, so the user has to remove the tag, thus acknowledge the instructions.

Better retention in the socket too.

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    \$\begingroup\$ This is intuitive, but is there some official source somewhere that agrees your theory? I'm looking for an authoritative answer - something from a spec ideally. \$\endgroup\$ Commented Oct 26, 2012 at 12:10
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    \$\begingroup\$ It’s not intuitive, it’s done in practice. I’ve seen this sort of thing numerous times. But if you (still) require an official explanation, look into Lockout-Tagout. \$\endgroup\$
    – Synetech
    Commented Jun 22, 2017 at 5:23

I was told a long time ago that the holes are for optional locking purposes. Modern outlets don't tend to use them, but in the past there were outlets that had a spring-like arrangement that used the holes so that extra force would be needed to pull the plug out.

A long time ago only once I saw a wall outlet that had a lever at the top. You plugged the socket into the hole and flipped the lever. Spring-loaded pins would then go thru the holes to lock the plug into the outlet. The pins were spring loaded so that if a plug didn't have the holes there would be extra friction but not positive locking.

I just looked around, and can't find any product that is like that now. I guess there wasn't enough of a market for it.



I have seen those holes used to retain the plug in the socket. Imagine a spring loaded ball that nestles into the hole when the plug is inserted. The ball is larger than the hole and when engaged with the holes on the blades provide a retention force. In picture blade is removed by pulling to the right. enter image description here


Well, this is rather embarrassing. I totally misread your question.

What's worse is that I don't actually know why the blades have holes in them. I suspect it's to help dump crud that wipes off when the blades are inserted into sockets, but that's just a guess. It could just be a result of the manufacturing process.

Edit: actually, from what I can tell, there are bumps on the wipers inside of some receptacles. The bumps fit into the holes, which makes the plug stay in the socket more securely. But, I did a little destructive testing just now on an extra receptacle in my junk drawer, and it definitely had no bumps.

Anyway, here's an irrelevant explanation of why the holes in NEMA 5-15P receptacles are different sizes.

One of those holes will shock you; the other won't. Normally, this doesn't matter, because you don't stick your finger in either hole.

When you wire a light fixture in a lamp, you put the switch on the hot side of the circuit. That way, when you replace the bulb, if you accidentally stick your finger in the bulb socket, you won't get shocked if the lamp is switched off. If you could reverse the blades, this wouldn't be true-- you could get shocked even with the lamp off, because the switch would be on the neutral side, while the hot wire would be unbroken.

More generally, electricians try to minimize the length of wiring that is hot so there's less damage done when something goes wrong. From a functional perspective, it makes no difference, but when we have actual humans interacting with circuits, faulty hardware, and bad luck, it's nice to be able to keep track of which wire is hot.

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    \$\begingroup\$ The holes in the blades are just over 3mm in diameter. There's no way you can put your finger in either! \$\endgroup\$ Commented Mar 15, 2011 at 15:12
  • \$\begingroup\$ Some other speculation: straightdope.com/columns/read/579/… \$\endgroup\$
    – pingswept
    Commented Mar 15, 2011 at 15:39
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    \$\begingroup\$ I don't see how adding holes to anything improves the manufacturing process. I see it as being an extra step or an extra tooling requirement. Unfortunately, that article is mostly hearsay. I'm hoping for an authoritative answer. \$\endgroup\$ Commented Mar 15, 2011 at 15:45
  • \$\begingroup\$ Yeah, agreed. It's at least true, judging by the outlets I've checked, there are no longer bumps interfacing with the holes, if ever there were. \$\endgroup\$
    – pingswept
    Commented Mar 15, 2011 at 22:00
  • \$\begingroup\$ The holes keep the blade from slipping out of alignment when some stress is applied on the crimp attached cord to keep it straight when the rubber molding is applied. \$\endgroup\$
    – Skaperen
    Commented Oct 26, 2012 at 3:39

Opinion only:

I believe the hole serves to provide a higher pressure wiping action for low insertion force in a hostile environment. The result is longevity, lower resistance, more reliable connection, lower temperature contact from lower resistance, more reliable connection for household frequent cycling, which is not necessary for low cycle appliances of higher ratings.

There may be other valid historical reasons for the creation, like being able to remove dry bubble gum stuck on the plug tips by prankish little kids. In any case the hole serves to reduce surface area for more pressure while wiping the surface area of the recepacle and visa versa. Good receptacles have a 3 point contact per pin with several different patented designs. (See Hubbell) The object is to keep the mating surfaces as clean as possible while providing low insertion force and improved retention force.

Smooth blades are poor for mating relaibility in my experience with older sockets. Reducing the contact area of the blade which mates with pretensioned spring brass contacts in the receptacle give higher contact PSI and permits lower contact resistance and temperature rise without excessive insertion force.

Hubbell Corp the pioneer of receptacle design have several designs for Nema 5-15p plug enter image description here enter image description here

You can see the contacts do not provide a spherical mate with the hole but rather wipe each other to allow dirt to escape thru the hole and allow the hole to wipe the mating contacts.

It seems the larger rating plugs for 30A do not have holes and require much more exertion to install by design to provide the contact pressure for low resistance and harder for many people to use. They are intended for industrial equipment or fixed appliances so that should not be a serious impediment.

These are only my opinions.


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