How To Look At A House
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Why is undersize electric wiring in a house dangerous?
Monday, July 9, 2018
You might be scared of electrocution from faulty wiring, but the big hazard is actually fire. The first edition of the National Electric Code (NEC) was published in 1897 by the National Fire Protection Association (NFPA), a group of insurance firms, in response to the numerous house fires caused by the evolving technology of electricity. Today it is the most widely used and accepted code for electrical installations, and addresses the many ways that electricity can start a fire, one of which is undersize wiring.
When a wire is too small for the amount of current flowing through it, the resistance to current flow generates heat. The wire begins to imitate the heating element in a toaster as the current flow increases. First the wire insulations melts, after which it can get hot enough to ignite any nearby flammable materials.
The safety device that prevents this happening is officially called an OCPD for Over Current Protection Device. Fuses were an early OCPD, but now we mostly use mechanical circuit breakers as shown in the photo at the top of the page.
Because a circuit breaker or fuse that is the correct size (amperage rating) will shut off the current before wiring overheats, the only way a wire can be undersize is if the breaker that protects it is a higher amperage than what the wire is rated for. There are also other reasons why undersize wiring is not a good idea. Even if it does not start a fire, the electrical resistance in wire will cause voltage drop to the appliances it serves, some electric motors, pumps, or compressors can malfunction, and the heat generated in the wires is wasted energy that increases your electric bill.
Also see our blog posts What does over-fused mean? and What causes flickering or blinking lights in a house?
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