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ElectricianJanesville, WI
Porcelain knobs and cloth-covered wires running across the wooden joists of an old attic

How Old Is the Wiring in Your Rock County House?

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Short answer

Start from when the house was built. In Janesville 4,173 homes date from 1939 or earlier, the knob-and-tube era, and 9,594 from 1960 through 1979, which spans the years the CPSC says aluminum branch wiring went in. Beloit is older: 55.4% of its homes were built before 1960.

Part of Knob-and-Tube Wiring.

The numbers by city

The U.S. Census Bureau's American Community Survey asks when each home was built. The chart shows the share built in each wiring era for the five cities on this site and the county (ACS 2020-2024, table B25034).

Share of homes built before 1940, 1940 to 1959 and 1960 to 1979, by cityJanesville: 14.2% built 1939 or earlier, 15.8% built 1940 to 1959, 32.7% built 1960 to 1979; Beloit: 26.9% built 1939 or earlier, 28.5% built 1940 to 1959, 22.5% built 1960 to 1979; Milton: 24.2% built 1939 or earlier, 10.4% built 1940 to 1959, 22.5% built 1960 to 1979; Edgerton: 23.5% built 1939 or earlier, 15.4% built 1940 to 1959, 25.7% built 1960 to 1979; Evansville: 25.4% built 1939 or earlier, 10.3% built 1940 to 1959, 11.7% built 1960 to 1979; Rock County: 19.2% built 1939 or earlier, 17.9% built 1940 to 1959, 27.7% built 1960 to 1979Janesville30% before 1960Beloit55.4% before 1960Milton34.6% before 1960Edgerton38.9% before 1960Evansville35.6% before 1960Rock County37.1% before 19601939 or earlier1940 to 19591960 to 1979
Share of all housing units in each band. The Census records when a house was built, not what wiring is in it. Source: U.S. Census Bureau, ACS 2020-2024 5-year, table B25034.
CityHomes1939 or earlierBefore 19601960 to 1979Median year
Janesville29,3304,173 (14.2%)30%9,594 (32.7%)1972
Beloit14,5313,915 (26.9%)55.4%3,270 (22.5%)1957
Milton2,533613 (24.2%)34.6%569 (22.5%)1974
Edgerton2,517592 (23.5%)38.9%647 (25.7%)1969
Evansville2,607661 (25.4%)35.6%306 (11.7%)1984
Rock County71,10913,653 (19.2%)37.1%19,718 (27.7%)1970

What each era usually means

  • 1939 or earlier. Knob-and-tube was in common use from about 1880 into the 1940s (InterNACHI). Many of these houses have been partly or fully rewired since; some still have live circuits in the attic or behind plaster. See knob-and-tube wiring.
  • 1940s and 1950s. Small services by today's standards, built before central air, electric dryers in every house, or cars that plug in. The usual job is a panel or service upgrade.
  • 1965 to the mid 1970s. The CPSC says cable from these years may have aluminum branch wiring, and houses built before 1965 are unlikely to (CPSC). See aluminum wiring.
  • Newer. Copper wiring and larger services are the norm; the common jobs are an EV charger circuit, a generator or added circuits.

What the numbers cannot tell you

The Census counts when a house was built, not what is in its walls, and nobody publishes how many Rock County homes still have either kind of wiring. The way to know is to look at the cable in an unfinished basement or attic, or to have an electrical inspection, which runs about $150 to $350 (see the cost page).

Sources: U.S. Census Bureau, ACS 2020-2024 5-year, table B25034 (year built), via Census Reporter; InterNACHI, knob-and-tube wiring; U.S. CPSC, safety recommendations for aluminum wiring in homes (1974)

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Quick answers

How do I know how old my house's wiring is?

Start with the year the house was built, then look at the wiring in an unfinished attic or basement: porcelain knobs and tubes mean knob-and-tube; cable jacket markings show aluminum. An electrician's inspection maps the rest.

How many homes in Janesville were built before 1940?

4,173 of 29,330 housing units, per the Census ACS 2020-2024 5-year estimates. The median Janesville home was built in 1972.

When was aluminum wiring used in houses?

The CPSC says cable installed from 1965 to the mid 1970s may contain aluminum branch wiring, and homes built before 1965 are unlikely to have it.

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