Knob-and-Tube Wiring

Houses wired from about 1880 into the 1940s were usually wired with knob-and-tube ¹. In a house from 1905, the original wiring may be long gone, or it may still be feeding a ceiling light upstairs, quietly hidden behind plaster and under the attic floorboards. Before you insulate, remodel, or buy insurance, find out.

What it is

Knob-and-tube uses single insulated copper wires rather than cables. The hot and neutral run separately, usually 4 to 6 inches apart, held off the framing by nailed-on porcelain knobs. Where they pass through a joist or stud, they go through porcelain tubes in drilled holes. Where they enter a box or a wall, they’re sleeved in flexible cloth or rubber tubing called loom ¹.

InterNACHI, the home inspectors’ association, makes a point that surprises people: it is not inherently dangerous. The separated wires shed heat into the air, so they can carry more current than a modern cable of the same gauge. They sit away from the wood, so they’re less likely to be pierced by a nail. The porcelain lasts almost forever, and the original work was often more careful than later cable jobs, because it took skill ¹.

Where it goes wrong

The problems are mostly about age and what people have done to it since ¹:

• The insulation on the wire. Early wire was insulated with varnished cloth and fiber that gets brittle. Bending old wire can crack the insulation right off. It isn’t rated for damp locations.
• No ground. There’s no equipment grounding conductor, so it can’t properly serve three-prong outlets.
• A century of modifications. Old systems are notorious for amateur work: modern cable spliced in without a box, splices wrapped in masking tape, and circuits overloaded as appliances multiplied.
• Oversized fuses. When old circuits kept blowing fuses, people put in bigger fuses so they’d stop. The wire then overheats before the fuse blows.

Insulation is the big one

Knob-and-tube depends on air around the wires to stay cool. Bury it in blown, rolled, or foamed insulation and the heat builds up ¹. The National Electrical Code therefore prohibits concealed knob-and-tube in hollow walls, ceilings, and attics insulated with material that envelops the wires ¹. Wisconsin’s electrical code adopts the 2023 NEC ².

Some places make an exception if a licensed electrician inspects the wiring and certifies it’s in good condition, properly fused, and free of bad splices; Berkeley, California, for example, requires a filed certification, noncombustible insulation, and warning signs at the attic hatch ³. Absent that kind of local rule, the practical sequence for an old house is: deal with the knob-and-tube first, then insulate.

Keep, retire, or replace?

No code requires ripping out all existing knob-and-tube, and rules differ from place to place, but it’s not allowed in new work ¹. Your options:

1. Keep it where it’s intact, unburied, properly fused, and lightly loaded, after a qualified electrician has evaluated it ¹. Don’t extend it.
2. Disconnect and abandon it circuit by circuit as you run new cable, leaving the dead wires in place where removing them would wreck plaster. Make sure each abandoned run is truly disconnected at both ends.
3. Replace it wherever wiring is brittle or cracked ¹, wherever you plan to insulate, and wherever you need grounded outlets.

Insurance can force the issue: many insurers won’t cover houses with knob-and-tube, or want an electrician’s report first ¹.

In Wisconsin, you’re allowed to do your own wiring in a home you own and live in, unless a local ordinance requires a licensed electrician ⁴. That’s permission, not a reason to skip the permit or the inspection.

Finding it

Look in the unfinished spaces first: the attic floor and the basement ceiling, where you can see white porcelain knobs and tubes and pairs of single wires. Old push-button switches and a fuse box may be clues too. At the panel, trace which circuits feed which rooms. Before touching anything, turn off the circuit and confirm it’s dead with a tester at the wires you’ll handle.

Try it

Go up into the attic with a flashlight (stay on the joists) and look along the tops of the walls and around ceiling-light locations. Any porcelain? Any wires disappearing under insulation? Make a sketch of what you find and which rooms it serves; it’s the first page of your rewiring plan.