Active Knob-and-Tube Wiring Risk by County

Knob-and-tube wiring was standard in US homes built before roughly 1950. This early electrical distribution method relies on porcelain knobs for support and porcelain tubes for passing conductors through framing timbers. However, it has no ground conductor, its rubberized cloth insulation becomes brittle with age, and it is not rated to be buried in thermal insulation. When modern insulation is added over active circuits, heat cannot escape properly, creating a safety hazard.\n\nBecause of these characteristics, encountering energized knob-and-tube wiring during a real estate transaction is a significant concern for buyers. While many older properties have undergone partial or complete electrical upgrades over the decades, legacy circuits often remain hidden inside walls, attics, or ceiling bays. Understanding the era distribution of local housing stock helps evaluate how frequently homes from this era appear in a given market. For detailed steps on navigating a real estate transaction with legacy wiring, see Can I buy a house with knob-and-tube wiring?.

Why This Matters for Buyers

From an inspection and underwriting standpoint, energized legacy wiring creates an immediate friction point. The documented insurance impact is direct: knob-and-tube wiring — many insurance carriers will not write a policy while it is still energized. Without active homeowners insurance, mortgage lenders typically will not fund a loan. When evaluating an older home, ask your inspector to confirm in writing whether any knob-and-tube is still energized, then confirm insurability with your insurance agent before your option period ends. If active circuits are discovered, buyers often need to negotiate repairs or specialized coverage terms prior to closing. To understand coverage options and contractual obligations, review Can You Get Homeowners Insurance with Knob and Tube Wiring? and Does Knob and Tube Wiring Have to Be Replaced Before Closing?.

County Ranking

What This Data Shows

Comparing aggregate county data reveals how strongly early-20th-century urban expansion aligns with potential exposure to legacy electrical systems like knob-and-tube wiring. High-density East Coast and Midwestern urban hubs dominate the top of the ranking due to rapid residential development before 1950.\n\nKINGS County, NY leads the nation with 66.1% of its housing built in or before 1959. PHILADELPHIA County, PA follows closely at 65.6%, and QUEENS County, NY stands at 64.1%. Major industrial centers like WAYNE County, MI at 58.9% and NEW YORK County, NY at 58.2% also reflect housing inventories where well over half of all residential structures originated during eras where knob-and-tube wiring was standard practice. In these markets, older homes commonly feature a mix of updated panels and original, ungrounded circuits.\n\nBy contrast, fast-growing Sunbelt counties that expanded largely in the late 20th century exhibit very low proportions of pre-1960 housing stock. CLARK County, NV records the lowest concentration at 3.1%, followed by MARICOPA County, AZ at 7.1%, TRAVIS County, TX at 7.1%, PALM BEACH County, FL at 7.6%, and ORANGE County, FL at 9.4%. The spread of 63.0 percentage points between KINGS County, NY (66.1%) and CLARK County, NV (3.1%) illustrates how geographic development timelines directly dictate how often buyers encounter mid-century or early-century building defects across broad regional markets.

Methodology

This county ranking provides an estimate of historical housing exposure based on Census decade-bucket data for structures built in 1959 or earlier. Because Census data groups housing age into standard decade spans rather than exact year boundaries, this metric uses the earliest records–1959 decade-bucket span as an approximation for knob-and-tube wiring's primary era window (roughly 1800 to 1955). This percentage reflects broad housing stock age across a county aggregate and is not a precise measure of active, energized knob-and-tube wiring prevalence in individual properties.