Why Do Electrical Plug Prongs Have Holes? The Real Engineering Reasons

The holes at the tips of standard North American NEMA 1-15 and 5-15 plug prongs are primarily manufacturing artifacts used to pull brass stock through progressive stamping dies, which double as anchor points for safety lockout/tagout (LOTO) padlocks. In a real circuit, these holes change absolutely nothing electrically; they do not alter impedance, current capacity, or voltage drop, but they fundamentally change how the plug interacts with automated factory equipment and safety lockout hasps. Most people commonly confuse these holes with a mechanical retention feature, falsely believing they are designed to 'snap' over a bump inside the receptacle to hold the plug in place.

Manufacturing Stamping and Lockout/Tagout (LOTO) Mechanics

To understand the hole, you have to look at how the prong is made. Plug blades are not cast individually; they are stamped from long, continuous coils of brass or bronze alloy using a progressive die. Think of the brass strip like a movie film reel moving through a projector; the holes are the sprocket perforations that allow the machine's gears to pull the material forward with exact precision. The stamping press uses these holes to index the strip, punch the blade profile, and hold the metal securely in place while the plastic plug body is injection-molded directly over the rear of the blade. Once the plug is finished, the hole remains as a harmless byproduct of the manufacturing process.

However, that manufacturing byproduct found a critical second life in industrial safety. Under OSHA's Lockout/Tagout standard (1910.147), any equipment undergoing maintenance must be isolated from its energy source and physically locked. The prong hole provides a perfect, standardized anchor point for LOTO padlocks and plug lockout devices, preventing a plug from being inserted into a live receptacle while a technician is working on the downstream circuit.

NEMA Blade Profiles: Holes vs. Solid Prongs

Not all plug prongs have holes. As amperage and voltage requirements increase, the mechanical and manufacturing needs change. Here is how the presence of the hole correlates with different NEMA configurations.

Notice that higher-amperage plugs (like the 20A, 50A, and twist-lock variants) typically feature solid prongs. At 20 amps and above, the brass stock is thicker (often 0.080 inches or more), and the sheer mass of the plug requires heavier progressive die carriers that grip the edges of the strip rather than pulling through a central hole. Furthermore, twist-lock (L-series) plugs rely entirely on the curved geometry of the blade rotating inside the receptacle for retention, rendering a tip hole mechanically useless.

The Retention Myth vs. Receptacle Contact Physics

The most persistent myth in home electrical forums is that the holes in the prongs are meant to catch on a small 'bump' or detent inside the receptacle, creating a satisfying 'click' that holds the plug in place. While you might occasionally feel a slight bump when pulling a plug from a brand-new, tight receptacle, this is not the engineered retention mechanism, and the holes are not required for it.

Under UL 498 (the safety standard for attachment plugs and receptacles), mechanical retention is achieved through the spring tension and wiping action of the receptacle's internal contacts, not a snap-fit detent. The internal wipers in a quality receptacle are stamped from Beryllium Copper (BeCu) or high-grade phosphor bronze. These alloys are chosen specifically for their high yield strength and fatigue resistance.

When you push a NEMA 5-15 plug into a receptacle, the flat sides of the 0.060-inch thick brass blade force the BeCu wipers apart. The wipers exert a continuous, high-pressure lateral force against the flat sides of the blade. This lateral pressure does two things:

1. Wiping Action: It scrapes away microscopic oxidation or dust on the blade surface, ensuring a low-resistance electrical connection.
2. Frictional Retention: It generates the physical pull-force required to keep the plug seated. UL standards require a specific minimum withdrawal force (typically several pounds of pull) to ensure the plug doesn't fall out under the weight of a cord.

If a receptacle has a small internal bump that aligns with the prong hole, it is merely a byproduct of how the internal contact spring was stamped and formed, or a minor secondary feature to aid in initial alignment. If you cut the tips off your plug prongs (removing the holes entirely), the plug will still be held firmly in the wall by the lateral spring tension of the receptacle contacts—though you will have destroyed the plug's LOTO capability and likely voided its UL listing.

Where You Meet This in Practice

As a DIYer, maker, or electrician, understanding the true purpose of these holes changes how you interact with cords and safety protocols on the jobsite or at the workbench.

• Executing LOTO Procedures: If you are servicing a hardwired appliance that has been converted to a plug, or working on a shop tool, you don't always need a massive, expensive plug lockout clamp. If the tool uses a standard 15A cord, a simple 1/8-inch shackle padlock passed directly through the prong holes and locked to a hasp will physically prevent the plug from entering a receptacle.
• Inspecting Cord Damage: When buying used power tools or inspecting extension cords, look closely at the prong holes. If the brass around the hole is torn, elongated, or showing stress fractures, it means the plug was yanked from the wall by the cord repeatedly, or someone forced an oversized lock through it. Elongated holes indicate the brass has been work-hardened and fatigued; the entire plug head should be replaced, as the internal wire terminations are likely compromised as well.
• Choosing Heavy-Duty Cords: If you are building or buying heavy-duty 12 AWG extension cords for a 15A circuit, you will often find that premium contractor-grade plugs (like those from Hubbell or Leviton) feature solid prongs without holes. This is a sign of a heavier, thicker brass casting designed for maximum conductivity and durability, where the manufacturer opted for edge-grip stamping or solid forging rather than the standard hole-indexed progressive die.