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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Here’s links to a collection of our other blog posts about ELECTRICAL WIRING:

Which house appliances need a dedicated electrical circuit?

Can a short circuit cause a high electric bill?

What is the maximum spacing requirement for securing NM-cable (nonmetallic-sheathed cable)?

Is it alright to just put wire nuts on the end of unused or abandoned NM-cable or wiring?

What causes copper wires to turn green or black in an electric panel?  

What are typical aluminum service entrance wire/cable sizes for the electrical service to a house?

Why is it unsafe to bond neutral and ground wiring at subpanels?

Should I get a lightning rod system to protect my house?

Why is a strain relief clamp necessary for the cord connection to some electric appliances?  

Does a wire nut connection need to be wrapped with electrical tape?

What is the minimum clearance of overhead electric service drop wires above a house roof?

What are the requirements for NM-cables entering an electric panel box? 

What is the color code for NM cable (Romex®) sheathing?

Why are old electrical components not always "grandfathered" as acceptable by home inspectors?

How can I find out the size of the electric service to a house?

Can old electrical wiring go bad inside a wall? 

What is an open electrical splice?

What are the most common electrical defects found in a home inspection? 

What is the life expectancy of electrical wiring in a house? 

What is an "open junction box"? 

How dangerous is old electrical wiring? 

What is a ground wire? 

I heard that aluminum wiring is bad. How do you check for aluminum wiring?  

What is "knob and tube" wiring?  

What is the code requirement for receptacle outlets in a closet?

   Visit our ELECTRICAL page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles. 

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