penny

To penny a fuse box is to defeat electrical safety by jamming a coin, historically a penny, into a fuse socket to complete a circuit that would otherwise be broken by a blown fuse. The coin conducts current across the socket terminals, allowing current flow to continue even when the circuit is overloaded or faulted. This kills the protection mechanism that the fuse was designed to provide.

The practice emerged in the early twentieth century when fuses were the standard overcurrent protection in residential and light industrial wiring. A blown fuse meant the power went out; rather than replace it, someone would wedge in a penny or other metal coin as a mechanical shortcut. The coin's conductivity matched or exceeded the fuse's design, so it allowed the circuit to run indefinitely. The term persists even though modern breaker panels use automatic switches instead of replaceable fuses.

Pennying a fuse is genuinely dangerous. A fuse is rated to melt and open the circuit at a specific current, typically 15 or 20 amperes in household circuits. When that current is reached, the metal element inside the fuse body heats up and fractures, stopping current flow and preventing the wiring itself from overheating. Insert a coin and that interruption never happens. The wire in the wall continues to heat; insulation breaks down; fire starts. The coin itself will also heat and can weld to the socket contacts.

Why the term stuck

Pennies were small, abundant, and had just the right thickness to fit old Edison-base fuse sockets. Other coins worked too, but penny became the colloquial name for the entire unsafe shortcut. A 1946 electrical code amendment in the United States formally banned the practice in new installations, yet documented cases of pennied fuses appeared in fire investigation reports for decades afterward, often in older rental properties or industrial buildings where maintenance was deferred.

Modern breaker panels make pennying impossible mechanically: circuit breakers trip and reset; they cannot be jammed open with a foreign object. But the term remains in the vocabulary of electricians and fire inspectors as a reference to any dangerous workaround that defeats a safety device. It is also a diagnostic red flag: the presence of a penny in a fuse socket is evidence of prior electrical mismanagement and grounds for immediate condemnation of that portion of the wiring.