Some parts of an old house can be admired for their craftsmanship of an earlier era and patina of age, but the electrical system is not one of them.  While home inspectors do not expect a mid-20th century or earlier home to be compliant with the current electrical codes, they do examine an older electrical system for safety issues. Aging home electrical systems are also much more likely to start a fire, according to the National Fire Prevention Association (NFPA), whose statistics indicate that 95% of electrical house fires occur in homes more than 10-years old.

    Like roofing and plumbing, electrical components deteriorate over time and eventually need to be replaced. Also, some components that were once acceptable are now known to be unsafe. So everything is not “grandfathered in” just because it passed a municipal building department inspection when the home was first built.


    Another factor is that insurance companies require a satisfactory four-point inspection report in order to insure an older home. A “4-point” is filled out by the inspector on a standardized form that is completely separate from a home inspection. The form asks questions that probe for deteriorated or defective components that are likely to fail and cause a future insurance claim. The electrical system is one of the points, and the others are roof, plumbing, and heating/air conditioning. Because a home has to be insurable in order to be purchased, we call out electrical defects that would cause a house to fail a 4-point inspection.

    Here’s eight examples of original electrical equipment that is not acceptable today:

1) Knob-and-tube wiring - This system, shown below, uses the air space around the wires as an insulator, and porcelain knobs and tubes to turn corners and pass through wall framing. It was abandoned after the mid-1940s. New knob-and-tube wiring is no longer allowed by National Electric Code (NEC). Although the NEC does not require old knob-and-tube to be removed, 4-point inspection forms ask if there is any active knob-and-tube wiring in the home, and insurance companies will not insure the property if there is. You can find out more about this old wiring type at our blog post What is "knob and tube" wiring?  

2) Screw-in fuse panels - An example is shown at the top of the page and these panels are also not acceptable to insurance companies. They are now over 60-years old and many of them allow a fuse to be inserted that exceeds the capacity of the wiring.  See our blog post What are the code requirements for an old fuse panel/box?

 
3) Aluminum wiring - Aluminum wiring in small conductor sizes (#12 and #10 AWG) was installed in millions of home from the mid-1960s to mid-1970s. Unfortunately, the high rate of expansion and contraction of the metal with temperature changes caused wiring connections to work loose, and numerous electrical house fires resulted. Although special breakers and other fittings were created to solve the problem, many insurance companies will not insure homes with this wiring. We only see it as original wiring in homes from that era, since it is no longer available.    Multi-strand aluminum wire, however, is approved and still in use for service cables and larger amperage circuits. Built-in twisting of the strand bundles along the cable absorbs the expansion and contraction, and a different aluminum alloy is now standard. See our blog post I heard that aluminum wiring is bad. How do you check for aluminum wiring? to learn more.


4) Deteriorated cloth-and-rubber wire insulation - A gum-rubber composition with cloth embedded around it was used for wire insulation up until the mid-1950s, when it was replaced by a thermoplastic material. Over time, and especially in the heat of an attic, the rubber cracks and the cloth deteriorates. Small pieces begin to break away, and exposed electrical wires are both a safety and fire hazard. Visit our blog post How dangerous is old electrical wiring? for more on this. 

5) Federal Pacific Stab-Lok electric panel - Another product that proved to be faulty over time and is not accepted by many insurance companies, Federal Pacific breakers had an excessive rate of failure to trip when current exceeded the breaker rating, and the company was also found to have submitted fraudulent data to obtain UL-approval of the equipment. To learn more about Stab-Lok panels, go to our blog Who is the manufacturer of those "bad" electric panels?

6) Three-slot receptacles in two-wire system - Prior to the early 1960s, most house receptacles and lighting was “two wire” and the receptacles had two slots. The 1962 edition of the NEC required all house receptacles to be three slot. The third, rounded slot provided grounding, which greatly reduced the risk of electrical shock from appliances. Because the new appliances that followed had cords with three prongs that would not fit into a two-slot receptacle, some homeowners and handymen changed out the receptacles on two-wire circuits to three-slot even though the third slot was not connected to anything. The required safety benefit of grounding was voided.

    Existing two-slot receptacles in older homes are still acceptable but, when a home has no grounding receptacles, a new problem is created. How do you plug in all those three-prong cords? “Cheater plugs,” like the one shown below, are an easy solution available at any hardware store, but the little clip on them that is supposed to fit under the set-screw of a receptacle cover plate only creates the illusion of grounding, and most people don’t even bother to connect it anyway. We see them often in older homes with not enough three-slot receptacles. See our blog post How can I tell if the electric receptacle outlets are grounded? 
  
7) Undersize electric service - An ampere is a unit of measurement of electrical capacity, and it is usually shortened to simply “amp” in common usage. Early electric panels were 30-amps and we haven’t seen one of them in a long time. 60-amp panels followed as electric usage grew. We rarely encounter them anymore, and a 60-amp panel is woefully inadequate for the level of electric usage today. Many 100-amp panels from the 1950s onward are still in use and may be satisfactory, especially for a small home or apartment, or one with a gas water heater and range. But sometimes an older home grows with additions over the years to 2,000 square feet or more, with air conditioning, electric clothes dryer, range, and dishwasher. The original panel is undersize in both electrical capacity and space inside the box to fit all the wiring. To find out the rating of your service, see our blog post How can I find out the size of the electric service to a house?
 

8) Deteriorated receptacles and switches - Old electric receptacles that are used frequently lose the ability to grab and hold the prongs of a cord. They sit loosely in the slot, making a poor connection that allows arcing that causes burn marks around the slots, like in the photo below, which is so old that it is unpolarized. Aging wall switches often have arcing that can be heard when you throw the switch and may function erratically. Both are electrical fire hazards and need to be replaced. See our blog post When should I replace electric receptacle outlets? for more info.

    Another problem we find specifically in homes from the 1930s and earlier, when they have not had an electrical upgrade, is too few wall receptacles for the level of usage today. Only one receptacle per room was required at the time. While we do not call it out for repair, because the receptacles themselves are not defective, it is brought to the attention of the homebuyer. Homes from this era usually have extension cords snaking around the base of the walls behind the furniture because of the lack of sufficient outlets.

    The principle behind all of this is simple: a grandfathered hazard is still a hazard. Although the National Electrical Code (NEC) is a standard for new construction and repairs, and is not applicable to existing electrical systems, the NFPA has a separate standard for evaluating the safety of older wiring and electrical components. It’s called NFPA-73 Standard for Electrical Inspection for Existing Dwellings. See our blog post What are the code requirements for an old fuse panel/box?

  
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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?  

• How can I tell the age of a Square D electric panel from the serial number?

• 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 is undersize electric wiring in a house dangerous? 

• What causes flickering or blinking lights in a house?

• 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?    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.