Fuses larger than the circuit conductors warrant are found in an older fuse panel. What is this called?

Overcurrent protection works by being the weakest deliberate link: the fuse or breaker is sized to open before the conductor it protects can overheat. Install a larger fuse and that relationship inverts — the conductor becomes the weakest link, and it can carry current well beyond its rating without the fuse ever noticing. The wire then heats inside walls, ceilings and insulation where nothing is visible until something ignites. Fuse panels are especially exposed to this because the fuseholders accept any Edison-base fuse regardless of rating, so a homeowner who tires of replacing a 15-amp fuse can install a 30-amp one in seconds. The recognized remedy is a rejection-base adapter that physically prevents oversized fuses, or replacement of the panel, and the inspector reports the condition as a fire hazard.

Overfusing means the protective device is rated above the conductor it protects, so the wire can be loaded past its capacity without the fuse opening. The conductor overheats inside concealed spaces, which is a fire hazard.

Overcurrent devices must be sized to the conductor's ampacity. Edison-base fuseholders accept any rating, making overfusing easy; Type S rejection bases restrict each holder to a single rating and are the standard corrective measure.

The Fuse Should Die First. Make it bigger than the wire and the wire becomes the fuse — inside the wall, where you cannot see it.

Look for the tell-tale signs alongside the fuse rating: multiple fuses of mixed ratings, coins or foil behind a fuse, or scorching at the fuseholder.

• •Sizing the fuse to the appliance rather than to the conductor
• •Confusing overfusing with load balancing or bonding issues
• •Reporting the fuse rating without checking the conductor gauge