Own a Home Built Before 1980? Your Electrical System Might Need Some Attention

8/4/2020

Chris Powicki didn’t pay much mind to a loud pop he heard from his woodstove’s fire one cold November morning. But he snapped to attention five minutes later when the acrid aroma of roofing tar greeted him as he stepped outside. “I looked up and saw flames about six inches high right along the roof’s ridgeline,” he says. “I ran in and yelled to my wife, ‘Elizabeth, the house is on fire — get the cats!’”

Chris Powicki didn’t pay much mind to a loud pop he heard from his woodstove’s fire one cold November morning. But he snapped to attention five minutes later when the acrid aroma of roofing tar greeted him as he stepped outside.

“I looked up and saw flames about six inches high right along the roof’s ridgeline,” he says. “I ran in and yelled to my wife, ‘Elizabeth, the house is on fire — get the cats!’”

Fortunately, their wood-frame house in Brewster, Massachusetts — which dates to the 17th century — escaped largely unharmed. The fire itself  was restricted to a small piece of the roof, where a spark from the woodstove had landed. The most serious damage was actually caused by water sprayed to douse the blaze, which destroyed plaster in several rooms.

As it turned out, the small disaster was a stroke of luck. Repairing the damaged plaster exposed a potentially bigger fire hazard: deteriorating knob-and-tube electrical wiring — a technology U.S. electrical contractors haven’t used since the 1940s. 

WHAT IS KNOB-AND-TUBE WIRING?

Look at a lightweight extension cord. It appears to be two wires separated — and joined — by plastic insulation. Modern branch-circuit wiring — the wiring that runs from a breaker panel through the rest of the house — is a heavier duty version of the same thing. The two wires (known as conductors) are individually insulated, then wrapped together into a single cable.

In knob-and-tube systems, the two wires were individually insulated and installed, and supported by porcelain knobs or tubes. (Porcelain is a low heat conductor and doesn’t conduct electricity.) Porcelain knobs supported wires along their path inside walls. When wiring had to pass through floor joists or wall studs, drill holes were protected by porcelain tubes. 

modern versus knob and tube wiring schematic

Keep reading if your home is more than 40 years old

Knob-and-tube wiring is just one of the electrical issues buyers and owners of older homes might have to address. Some of these issues might be obvious during a first visit or inspection — an old-school fuse box instead of a circuit-breaker panel or too few outlets. Others, like knob-and-tube wiring behind plaster, might not be revealed until walls come down during a renovation.

A lot of homeowners could be living with these issues today. More than half of all U.S. homes were built before 1980, and more than 40 percent before 1970, according to 2017 U.S. Census Bureau figures. These dates are significant, according to Joseph Wages, technical adviser for education, codes and standards for the International Association of Electrical Inspectors, because this is around the time the National Electrical Code (NEC) started paying more attention to grounding and outlet placement. The N E C is referenced by most local building codes across the United States.

Electrical Wiring Knob, H. W. Lawrence. US Patent 907,251, December 22, 1908.

Wages’ rule of thumb: Homes more than 40 years old — built in the late 1970s or earlier — may have wiring that needs updating. People looking to buy older homes should pay special attention to electrical systems in pre-purchase inspections, he recommends.

“I think it goes back to the initial installation and whether [the electrical system] was installed by an electrical contractor,” Wages says, emphasizing that many decades-old electrical systems remain safe and functional. “It’s like building a house — if you don’t get the foundation right, you could end up having problems throughout the entire life of the home.”

Vintage appeal — and a mostly hidden web of wiring

For Powicki and his wife, Elizabeth Hopper, character — not wiring — was the most important quality when they first spotted their home a decade ago. Although they knew that outlet placement was spotty, they assumed the electrical service was in order because the home had a circuit-breaker panel with 200-amp service, the modern standard.

But a few years ago, the couple had an energy audit completed to qualify for insulation incentives offered by their electric utility. Auditors discovered heat signals characteristic of knob-and-tube wiring. Powicki, who’s an energy consultant and advocate by profession, went into research mode, reading papers back to the 1980s.

Outlet Box, H. G. Knoderer. U.S. Patent 1,527,655, December 31, 1923.

They thought the old wiring was limited to a few first-floor outlets, he says. “My questions were geared to, ‘Should we take it out?’ and the papers said, ‘No, it’s perfectly safe — it might be frayed, but if you don’t touch it, it’s fine.’”

After the roof fire, removing the damaged plaster revealed the true extent of the old wiring. An electrician traced the wires back to a first-floor junction box. Modern wiring had been brought from the breaker panel to that point and simply connected to the old network.

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