The holes in standard North American plug prongs exist primarily to anchor the metal blades inside the plastic plug mold during manufacturing and to allow safety lockout padlocks to pass through for industrial de-energization. If you have ever stared at a NEMA 1-15 or NEMA 5-15 plug and wondered about the purpose of those small circular cutouts near the tip of the blades, you are not alone. It is one of the most common questions asked by DIYers and junior electricians. The internet is full of plausible-sounding but entirely incorrect theories about these holes. Before we break down the exact mechanical and safety engineering behind them, we need to clear up the most pervasive myth in home electrical work.
The One-Sentence Definition and the Great Wiper Myth
The most common misconception is that the internal wiper contacts inside a wall receptacle have small bumps or dimples that 'snap' into these holes to hold the plug in place. This is entirely false. Standard North American receptacles do not have detents, bumps, or pins designed to engage the prong holes. The retention force of a plug comes entirely from the spring tension of the brass or phosphor-bronze wiper contacts squeezing against the flat, smooth sides of the plug blades. When a plug feels 'loose' and falls out of an outlet, it is because the internal wipers have lost their spring tension from heat cycling and mechanical fatigue, not because the holes are failing to 'grab' anything.
What people commonly confuse this with is the design of specialized twist-lock connectors (like NEMA L5-20), which do use curved, hooked blades that mechanically lock into the receptacle. But for standard straight-blade plugs, the holes are strictly for the factory floor and the safety padlock.
What the Hole Changes in a Real Circuit (Numeric Breakdown)
In a real circuit, the hole changes absolutely nothing electrically. It does not increase resistance in any meaningful way, nor does it alter the ampacity of the plug. To prove this, we can look at the exact dimensional requirements set by the NEMA WD-6 Wiring Devices standard.
Width: 0.250 inches (6.35 mm)
Thickness: 0.060 inches (1.52 mm)
Hole Diameter: ~0.156 inches (3.96 mm)
Let us run a quick cross-sectional area (CSA) calculation to see how much conductive material the hole actually removes. The total cross-sectional area of the solid brass blade is width multiplied by thickness: 0.250 × 0.060 = 0.015 square inches (9.67 mm²). This is roughly equivalent to the cross-section of an 8 AWG copper wire, which is massive overkill for a 15-amp circuit.
Now, look at the narrowest point of the blade where the 0.156-inch hole is punched through. The hole removes 0.156 inches from the 0.250-inch width, leaving two 'bridges' of brass on either side that total 0.094 inches of width. The cross-sectional area at this exact narrow point is 0.094 × 0.060 = 0.00564 square inches (3.64 mm²). This is still larger than the cross-section of a 12 AWG wire. Furthermore, this narrowed section is only about 4 millimeters long. The micro-ohm resistance added by this tiny bottleneck is so small that it cannot be measured by a standard bench multimeter and generates zero measurable voltage drop or heat under a continuous 15A load. Electrically, the hole is a non-entity.
Where You Meet This in Practice
You will encounter the functional reality of these holes in two specific environments: the manufacturing floor and industrial job sites requiring Lockout/Tagout (LOTO) compliance.
1. Injection Molding Manufacturing
When a factory mass-produces replacement plugs or molded power cords (like the ones on your laptop charger or a shop vacuum), the brass blades are placed into an injection mold. The mold features small steel pins that pass through the holes in the prongs. These pins hold the heavy brass blades perfectly suspended in the center of the mold cavity while molten plastic is injected at high pressure. Without the holes, the blades would shift, warp, or sink to the bottom of the mold, resulting in defective plugs with exposed metal or uneven insulation.
2. OSHA Lockout/Tagout (LOTO) Procedures
On a jobsite, if a technician is servicing a hardwired machine or a heavy power tool plugged into a wall, simply unplugging it is not enough; someone could accidentally plug it back in. Under the OSHA 1910.147 Lockout/Tagout standard, the energy source must be physically secured. For cord-and-plug connected equipment, a specialized plug lockout device is clamped over the prongs, and a padlock shackle is passed directly through the prong holes. This makes it physically impossible to insert the plug into a receptacle until the authorized worker removes their lock.
Troubleshooting Decision Tree: Loose Plugs and Prong Holes
Because people misunderstand the purpose of the holes, they often misdiagnose loose plug issues. Use this decision path to solve retention and safety problems correctly.
| Symptom / Observation | Root Cause | Concrete Fix / Part Pick |
|---|---|---|
| Plug falls out of the wall easily; you suspect the holes aren't 'grabbing'. | The receptacle's internal wiper contacts have lost spring tension due to heat cycling or cheap residential manufacturing. | Replace the receptacle with a commercial-grade unit. Pick: Leviton 5262 (15A Commercial Duplex). The phosphor-bronze wipers will grip the flat sides of the blade securely. |
| Plug prongs are physically bent, and the holes look stretched or deformed. | The plug was used as a handle to yank the cord from the wall, or it was dropped on the prongs. | Do not bend them back. The brass work-hardens and will snap. Cut off the molded plug and wire on a replacement heavy-duty plug. Pick: Leviton 5266-C (15A Rubber Straight Blade Plug). |
| You need to lock out a corded tool for maintenance, but the plug has no holes (e.g., a foreign adapter or older molded cord). | Non-NEMA standard plug or specialized molded strain relief that omitted the tooling hole. | Use a universal plug lockout that clamps the entire cord/plug body rather than relying on the prong holes. Pick: Brady 146756 Universal Plug Lockout Device. |
| Plug feels tight, but the cord gets warm near the prongs under a 12A continuous load. | Internal crimp/solder joint inside the molded plug is failing, unrelated to the prong holes. | Replace the entire cord assembly. The hole has nothing to do with this thermal failure. |
Frequently Asked Questions
Do all plugs have holes in the prongs?
No. While standard NEMA 1-15 (ungrounded) and NEMA 5-15 (grounded) plugs almost always feature them due to standardized manufacturing molds, many international plug types (like the UK BS 1363 or European Schuko) do not have holes in their pins. Additionally, some heavy-duty 20-amp or 30-amp straight-blade plugs omit the holes because their thicker blades are held in the mold by different clamping mechanisms.
Can I drill a hole in a solid prong to use a LOTO padlock?
Absolutely not. Drilling into a plug blade alters its cross-sectional area, creates stress risers that can cause the blade to snap inside a receptacle, and violates the UL listing of the device. If your plug lacks a hole and you need to lock it out, use a universal clamp-style lockout device that secures the cord and plug body without modifying the metal blades.
Why do some new outlets have small plastic shutters that seem to block the holes?
Those are Tamper-Resistant (TR) shutters, mandated by the NEC for most residential receptacles. The shutters are designed to block foreign objects (like a child's hairpin) from entering the hot slots. They are spring-loaded and open only when equal, simultaneous pressure is applied to both the hot and neutral slots by the solid tips of the plug blades. The shutters interact with the solid tip of the prong, well before the hole, and do not interfere with the prong hole in any way.
