Why Is Undersized Wiring Dangerous?
Electrical conductors must be sized for the circuit and the conditions in which they are installed. A conductor that is too small can create several problems:
- Overheating and fire risk: Every conductor has a permitted ampacity. If current exceeds what the conductor can safely carry for the installation conditions, the wire and its insulation can overheat.
- The breaker may not protect an undersized conductor: A standard breaker responds primarily to current, not directly to conductor temperature. If someone installs a breaker that is too large for the wire, the conductor can be overloaded and overheat while the breaker remains below its trip threshold.
- Excessive voltage drop: Smaller conductors have more resistance. On long runs or heavily loaded circuits, too-small conductors can cause excessive voltage drop, dim lighting, poor equipment performance, and additional heating in some loads.
- Equipment problems: Motors, compressors, contactors, and other equipment can operate poorly or overheat when voltage at the equipment is too low.
- Code and installation issues: Conductor sizing must account for more than the breaker handle rating. Load type, continuous loading, conductor material and insulation, terminal temperature ratings, ambient temperature, number of current-carrying conductors, and equipment-specific rules can all matter.
Why Does a Breaker Not Simply Trip?
This is the part people often get backward. A smaller wire does not automatically "force" a breaker to trip. If a 15-amp-rated conductor is incorrectly protected by a much larger breaker, for example, the breaker has no way to know that the conductor itself is too small. It only sees circuit current. That mismatch is exactly why oversized overcurrent protection can be dangerous.
What About Conduit Fill?
Conduit fill and conductor ampacity are related design concerns, but they are not the same calculation. The physical percentage of a raceway that conductors occupy is a conduit-fill issue. Separately, having multiple current-carrying conductors together in a raceway or cable can require ampacity adjustment. Both need to be checked when they apply.
How Is Wire Size Verified?
An electrician looks at the actual conductor size and material, breaker or fuse rating, terminal ratings, connected load, circuit length, wiring method, temperature conditions, number of current-carrying conductors, and any equipment-specific requirements. The correct fix may be a smaller overcurrent device, larger conductors, a different circuit arrangement, or another change depending on what is actually wrong.
Related Electrical Issues
See also breakers that keep tripping and open electrical splices.