If your house was built before about 1940, there is a fair chance it was wired with knob and tube. Thousands of homes across Detroit, Grand Rapids, the Upper Peninsula mining towns and small towns statewide still have some of it in service.
What it is
Knob and tube wiring runs individual conductors through the framing, supported on porcelain knobs and passed through joists in porcelain tubes. The wires are insulated with rubber and cloth. There is no ground wire.
How to spot it
- White porcelain knobs and tubes on joists in the basement or attic.
- Single cloth-covered wires running separately instead of in a cable.
- Two-prong outlets throughout the house.
- Push-button light switches.
Why it is a concern
- The insulation becomes brittle with age and falls away, leaving bare conductors.
- It has no grounding, so three-prong equipment and surge protection do not work as intended.
- It was designed to shed heat into open air. Covered with attic insulation, it can overheat.
- Decades of amateur splices and added circuits are common.
- Many insurance companies will not write or renew a policy on a home with active knob and tube.
What replacement involves
The old circuits are disconnected and new grounded cable is fished through walls, floors and ceilings, generally from the attic and basement. A careful electrician keeps holes in plaster to a minimum. The work can be done all at once or in phases, starting with the highest-risk areas. It is often paired with an electrical panel upgrade.
Next steps
Start with an electrical inspection to learn how much knob and tube is still live. Then ask for a written plan. Our whole house rewiring page explains the process in more detail.