Badger HomeownerWisconsin home guide

Knob-and-Tube and Other Old Wiring

Most old Madison-area houses have been rewired at least once, but pieces of the original system often survive in attics and walls. Here is how to recognize each era's wiring, what Wisconsin's code says about leaving it, and what has to change when you remodel.

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Knob and Tube

  • What it is. The first wiring in most historic Wisconsin houses. Two separate wires, hot and neutral, run through porcelain tubes in holes in the joists and studs and wrap around porcelain knobs where they turn. There's no ground wire.(Wisconsin Historical Society)
  • When. Virtually all urban Wisconsin houses had electricity, or could be wired for it, by the 1920s.(Wisconsin Historical Society) One city building department dates knob and tube to buildings built until the 1940s.(City of Menlo Park, CA) Check the year your house was built with the era lookup.
  • Why it's a concern. No ground means a shock hazard. Some early wire was coated with cloth soaked in a flammable asphalt solution, and coatings turn brittle with age.(Wisconsin Historical Society) Wisconsin's code notes that overloading wiring never sized for modern use is a common cause of hazards.(SPS 316)
  • Where to look. Unfinished attics and basements, where you can see the knobs and tubes on the framing.

Insulating Over It

  • The national electrical code bars concealed knob-and-tube in wall, ceiling and attic cavities filled with loose, rolled or foamed-in insulation that surrounds the wires (NEC 394.12).(NEC 394.12, as printed by Menlo Park, CA)
  • Wisconsin's weatherization program doesn't insulate wall cavities that contain live knob-and-tube. Where attic knob-and-tube isn't being rewired, crews build a barricade so insulation stays at least 3 inches from the wires.(Wisconsin Weatherization Field Guide)
  • The federal weatherization spec also requires 3 inches of clearance and caution tape visible from 5 feet.(DOE Standard Work Specifications)
  • Insulation and rebates go together, so sort out the wiring first. Energy rebates lists the 2026 Focus on Energy insulation incentives.

Fuses and Small Services

  • Early houses usually got 30 to 60 amps, without the 220-volt capacity modern dryers, ranges and central air need. Glass screw-in fuse panels still exist in some houses.(Wisconsin Historical Society)
  • CPSC: a panel with four fuses means a 60-amp service, and most of its fuses should be 15 or at most 20 amps.(CPSC)
  • A bigger fuse in place of one that keeps blowing defeats the protection. CPSC recommends type-S inserts in Edison-base sockets so only the right size fits.(CPSC) Wisconsin's weatherization crews match fuses to wire: at most 15 amps on 14-gauge wire, 20 amps on 12-gauge.(Wisconsin Weatherization Field Guide)
  • The Wisconsin Historical Society gives 100 amps at 220 volts as today's minimum house service.(Wisconsin Historical Society)
  • Upgrading in Madison. Only MGE employees set or pull meters, and MGE needs the city inspector's certificate before connecting a new or changed service. Old A-base meter sockets must be replaced when the service is upgraded.(MGE Contractors Handbook) On the Isthmus, an electrical remodel that replaces the service entrance must leave it ready for underground service.(MGO 19.13)

Two-Prong Outlets and GFCIs

  • A GFCI protects against shock even on an old two-slot circuit with no ground.(CPSC)
  • The code started requiring GFCIs outdoors in 1973, in bathrooms in 1975, garages in 1978, kitchens in 1987 and crawl spaces and unfinished basements in 1990, so older houses often have none.(CPSC)
  • Replacing an ungrounded outlet: another two-prong outlet, a GFCI outlet marked "No Equipment Ground," or three-prong outlets fed through a GFCI and marked "GFCI Protected" and "No Equipment Ground."(Leviton)
  • Never break off a plug's ground pin or defeat an adapter's ground.(CPSC) Test GFCIs monthly.(CPSC)

Aluminum Wiring

  • Installed in homes, additions and rewiring from about 1965 to the mid-1970s. Look for "AL" or "Aluminum" printed on the cable jacket in an unfinished basement, attic or garage.(CPSC)
  • A CPSC-sponsored survey found pre-1972 aluminum-wired homes 55 times more likely than copper-wired homes to have outlet connections reach fire-hazard conditions, often with no warning signs.(CPSC)
  • CPSC recognizes three permanent repairs: rewiring with copper, the COPALUM crimp, and the AlumiConn connector. Twist-on pigtails and CO/ALR devices are temporary fixes only.(CPSC)
  • This applies to branch circuits; CPSC's guidance doesn't cover the large aluminum wires used for the service, dryer and range.(CPSC)

Federal Pacific and Other Panels

  • CPSC closed its investigation of Federal Pacific Electric Stab-Lok breakers in March 1983. Its own tests found the breakers failed some UL calibration tests, but CPSC couldn't link that to hazards in homes and made no safety determination either way.(CPSC)
  • We found no CPSC recall or statement on Zinsco, Sylvania or Pushmatic panels. Have an electrician look at any panel you're unsure of: CPSC says panel work is for a qualified electrician, since parts stay live with the main off.(CPSC)
  • CPSC: ordinary breakers don't respond to early arcing, and older homes may especially benefit from arc-fault (AFCI) protection.(CPSC)

What Wisconsin's Code Requires

  • Wisconsin's electrical code, SPS 316, adopts the 2023 National Electrical Code with state changes, effective September 1, 2026.(SPS 316)(DSPS)
  • Existing wiring and repairs must meet the code in force when they were installed. The state can order an upgrade only when the wiring is a hazard. Additions and alterations must meet the current code.(SPS 316.003)
  • Defective wiring that endangers life or property must be promptly repaired or disconnected.(SPS 316.021)
  • In Madison, inspectors can order dangerous wiring fixed and, if it isn't, order the service disconnected.(MGO 19.07)

Warning Signs

CPSC's list: fuses blowing or breakers tripping often, dim or flickering lights, sparks, sizzling or buzzing, warm or discolored outlet plates, a burning smell, loose plugs, damaged insulation, and any shock, even a tingle.(CPSC) Stop using a warm outlet or switch until an electrician checks it.(CPSC) For trouble with the wires coming into the house, don't touch them; call the electric utility.(CPSC)

USFA estimates 23,700 residential electrical malfunction fires in the U.S. in 2023, with 305 deaths.(U.S. Fire Administration) In its 2014-2016 data, wire and cable insulation was the item most often ignited first, and attics or crawl spaces were the second most common place these fires started.(U.S. Fire Administration)

Insurance

  • Wisconsin's Office of the Commissioner of Insurance says an insurer may refuse to renew a homeowners policy because of the home's condition, and some allow six months to a year to make repairs.(OCI) OCI's guides don't name wiring types, so ask your own insurer what it requires.
  • A nonrenewal needs written notice at least 60 days before the policy ends, with the reason, and must explain how to apply to the Wisconsin Insurance Plan.(OCI)(Wis. Stat. 631.36)
  • In the first 59 days of a new policy, an insurer can cancel without a reason on 10 days' notice.(OCI)

Permits and Electricians

  • Madison requires an electrical permit for everything except minor repair, which means replacing broken or defective switches, outlets and fixtures.(MGO 19.09) An alteration permit is $25 for the first 10 openings plus $1 for each one after; a service replacement is $50 per panel; fees double if work starts without a permit.(MGO 19.11)
  • Doing it yourself. State law lets an owner wire the home they own and live in, unless a local ordinance requires a license.(Wis. Stat. 101.862) Madison limits that to a single-family home the owner lives in, the owner must show competence, and the work still needs a permit and inspections.(MGO 19.08)(City of Madison)
  • Hiring. Here's what an electrician's license looks like and how to check it. The Wisconsin Historical Society suggests an electrician experienced with plaster walls and old houses.(Wisconsin Historical Society)

Badger Homeowner doesn't recommend contractors and takes no money from them.

Questions

Is knob-and-tube wiring safe?

The Wisconsin Historical Society says it should be replaced, since it has no ground wire and some old cloth-and-asphalt coatings turn brittle and flammable. Wisconsin's code doesn't force removal of wiring that met the code when installed, unless it's a hazard.

Can I insulate an attic with knob-and-tube wiring?

The electrical code bars burying concealed knob-and-tube in insulation that surrounds the wires. Wisconsin's weatherization program won't insulate wall cavities with live knob-and-tube, and in attics keeps insulation at least 3 inches from it behind a barrier.

Will my insurance cancel me for old wiring?

Wisconsin's insurance regulator says an insurer may decline to renew because of a home's condition, and some give six months to a year for repairs. A nonrenewal needs at least 60 days' written notice with the reason. OCI's guides don't single out wiring types.

Can I replace a two-prong outlet with a three-prong one?

Not as a plain swap on an ungrounded circuit. The usual options are another two-prong outlet, a GFCI outlet marked No Equipment Ground, or three-prong outlets protected by a GFCI and labeled GFCI Protected and No Equipment Ground. In Madison, replacing a broken device doesn't need a permit; other electrical work does.