The holes in the flat metal prongs of standard North American electrical plugs are primarily manufacturing artifacts used to anchor the blades inside the injection mold during the plastic body overmolding process, though they secondarily serve as anchor points for industrial lockout/tagout (LOTO) safety locks. If you have ever inspected a standard 120V cord and wondered about the purpose of these voids, the engineering reality is rooted in factory assembly tolerances rather than receptacle mechanics.
The Manufacturing Reality: Injection Molding and Mold Pins
To understand the hole, you have to look at how a NEMA 5-15 plug is built. The metal prongs are not individually cast; they are stamped from a continuous coil of C26000 cartridge brass. This specific brass alloy is chosen because it offers the exact yield strength needed to survive both the manufacturing process and years of physical insertion into tight receptacles without bending.
During assembly, the stamped brass blades are placed into a steel injection mold. When the mold closes, molten thermoplastic (typically polycarbonate, nylon, or PVC) is injected into the cavity at pressures between 10,000 and 15,000 PSI. Without a physical anchor, this immense hydraulic force would push the 0.060-inch thick brass blades out of alignment. A misalignment of just 0.020 inches would cause the plug to bind, skew, or fail to insert into a standard receptacle. To prevent this, the mold features tiny steel retention pins that pass directly through the holes in the prongs, locking them in perfect parallel alignment while the plastic cools and hardens around the base.
| Parameter | Imperial | Metric | Engineering Notes |
|---|---|---|---|
| Blade Width (Line/Neutral) | 0.250 in | 6.35 mm | Strict tolerance to ensure proper wiper contact |
| Blade Thickness | 0.060 in | 1.52 mm | Thick enough to resist bending under insertion force |
| Hole Diameter | 0.100 - 0.125 in | 2.54 - 3.18 mm | Sized specifically for mold retention pins |
| Hole Centerline from Tip | 0.312 in | 7.92 mm | Placed far enough back to avoid insertion friction |
| Blade Material | C26000 Cartridge Brass | Often nickel-plated for corrosion resistance | |
Circuit Impact and the "Detent" Confusion
When evaluating what this feature changes in a real circuit or installation, the electrical answer is essentially nothing, while the mechanical answer is significant. Electrically, the hole removes a small fraction of the cross-sectional contact area. A standard NEMA 5-15 hot blade has a cross-sectional area of roughly 0.250 x 0.060 = 0.015 square inches. The 0.110-inch diameter hole removes about 0.0095 square inches of area at its widest point. However, a receptacle's internal brass wipers do not contact the tip of the plug; they grip the solid sides of the blade further back. Therefore, the current path bypasses the hole entirely, keeping contact resistance below 3 milliohms per the UL 498 standard, ensuring no localized heating occurs at 15A.
What people commonly confuse the holes with is a mechanical locking mechanism. A persistent myth suggests the holes are designed to "click" into a detent or bump inside the receptacle to hold the plug in place. Standard residential NEMA 5-15R receptacles do not have internal locking bumps. The retention force comes entirely from the spring tension of the brass wiper contacts squeezing the sides of the prongs. The holes play zero role in residential plug retention.
Where You Meet This in Practice: LOTO and Diagnostics
While the primary purpose of the hole is manufacturing, it finds a critical secondary life on the jobsite in the form of Lockout/Tagout (LOTO) procedures. Under OSHA standard 1910.147, workers servicing electrically powered machinery must isolate the energy source. While heavy-duty safety padlocks typically have 1/4-inch or 3/8-inch shackles that are too thick to fit through the 0.110-inch prong hole, specialized LOTO kits include small-diameter shackle locks (often 1/8-inch or 3.17mm) or specialized wire hasps that pass directly through the holes. This physically prevents the plug from being inserted into a receptacle while a machine is being serviced, providing a verifiable, physical barrier against accidental re-energization.
For electricians and maintenance technicians, the hole also serves as a rapid diagnostic indicator of mechanical abuse. If you are inspecting a plug and notice the hole is elongated, oval-shaped, or torn, it is a definitive sign that the plug was repeatedly yanked out of the receptacle by the cord rather than by gripping the molded body.
This specific type of mechanical stress work-hardens the brass at the base of the prong and compromises the structural integrity of the blade. An elongated hole means the prong has been subjected to severe bending moments, which can micro-fracture the brass or loosen the internal connection to the stranded copper wire. This increases the overall resistance of the assembly. On a continuous 12A to 15A load, that extra resistance translates directly into heat, frequently resulting in a melted plug face or a thermal trip at the breaker. If the hole is deformed, the plug must be cut off and replaced immediately.
Frequently Asked Questions
Do 240V or high-amperage plugs have holes?
Generally, no. Plugs like the NEMA 14-50 (used for EV chargers and ranges) or NEMA L-series twist-locks use much thicker, solid forged or heavy-gauge stamped prongs. Their manufacturing process relies on different mold retention methods, such as gripping the thicker base of the blade or using specialized internal armatures, eliminating the need for a pass-through hole.
Does the hole make the plug safer to touch?
No. The hole is entirely covered by the receptacle face when inserted, and when unplugged, the prong is de-energized (assuming the cord is not connected to a live source). It provides no dielectric or shock-prevention benefit to the end user.
