The holes in the flat blades of a typical North American 15-amp household plug were originally meant to help hold the plug in the outlet. Today they are optional in the plug design standard, a modern outlet does not need them to retain a plug, and they have no electrical or thermal role. The older detent purpose is the reason they exist, but it is not what keeps an ordinary plug in place now.
What the holes were originally for
The National Electrical Manufacturers Association (NEMA) addresses this directly in its WD 11-2023 wiring-device FAQ, section “Holes in Blades.” The statement reads: “The holes were originally intended to be a detent to help retain the plugs in the receptacle contacts.”
A detent is a small catch. In the older design, a matching nib inside some receptacle contacts could click into a hole in the blade, adding friction and a slight positive stop so the plug stayed seated. The hole, in other words, was the half of a simple mechanical latch that sat on the plug.
Why ordinary outlets no longer depend on the hole
NEMA says the straight-blade retention requirements were later revised so that receptacles and connectors could meet minimum retention without relying on those detents. The gauges used to test minimum retention for straight-blade devices have no holes in them. The same requirements also set a maximum insertion-retention limit, so a plug can still be pulled out by hand.
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That combination explains the modern design logic. A receptacle must grip a plug firmly enough that it does not fall out, yet not so tightly that the plug is hard to remove. A detent can add to the grip, but the standard no longer requires one to achieve it. NEMA does not publish the force values in the FAQ, so this article does not quote them.
The practical consequence is that a plug with no holes is not defective, and a plug with holes is not built to a different safety level. Both are judged against the same retention criteria.
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What the holes must meet when they are present
Holes are optional in the NEMA plug design standard for typical 15-amp household plugs. Where a manufacturer includes them, NEMA states that they must meet specified requirements for size, location, and a chamfered lead-in so that they work with older receptacles and connectors that still use the detent. A hole that is oversized, misplaced, or lacks the chamfer may not engage the detent as intended, which is a compatibility concern rather than a safety finding.
NEMA also says the hole has no other function. In its words: “The hole has no other mechanical, electrical, or thermal function.” Readers should therefore not assume the hole carries current, reduces heating, or indicates a fault. It is a retention feature and nothing more.
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Locking plugs are a separate case
NEMA notes that a related detent feature remains in NEMA locking products, where it helps retain the plug in its rotated or locked position. That is the context in which the detent still does active work today. The locking configuration is a different device family, so the straight-blade explanation above should not be extended to it without that distinction.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Japanese plugs use a similar idea with a different description
The Japanese Electrical Wiring Devices and Equipment Association (JEWA) publishes a Q&A on plugs at jewa.or.jp/qa/plug.html. It describes a Japanese 125 V, 15 A plug in which a spring protrusion on the receptacle engages a blade hole. According to the association, this helps indicate that the plug is fully inserted and prevents it from slipping out.
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The page states that plug-blade holes for this rating are specified in the applicable technical requirements. It does not give a publication date or a standard edition, so the clause should be read as JEWA’s own description rather than a confirmed current requirement. The table below keeps the two jurisdictions separate.
| Point of comparison | North American NEMA 15-amp household plug | Japanese 125 V, 15 A plug (JEWA description) |
|---|---|---|
| Source and date | NEMA WD 11-2023 FAQ, “Holes in Blades”; PDF accessed 8 October 2026 | JEWA Q&A page; no publication date or standard version stated |
| Documented purpose of the hole | Historical detent to help retain the plug in receptacle contacts | Helps indicate full insertion and prevents slipping |
| Receptacle feature that engages the hole | A matching nib in some older receptacle contacts | A spring protrusion in the receptacle |
| Status of the hole in the plug standard | Optional; if present, must meet specified size, location, and chamfered lead-in requirements | Specified in the applicable technical requirements; clause number not stated |
| Electrical or thermal function | None, according to NEMA | Not stated |
What the explanation does and does not cover
NEMA’s answer concerns the holes at the ends of typical NEMA 15-amp household plugs in North America. It does not establish that every plug worldwide has the same purpose for its holes, and it does not establish that every modern receptacle contains a mating bump. The JEWA description is a comparison tied to Japanese plugs, not a general statement about outlets elsewhere.
The NEMA FAQ refers to minimum and maximum retention requirements set by Underwriters Laboratories (UL) but does not reproduce the force values or current clause numbers, so this article does not state them.
If you are replacing a plug, check that the replacement is correctly rated for the circuit and carries the appropriate listing. Whether the blades have holes is not a measure of quality.
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