How to Identify and Replace Obsolete Knob and Tube Wiring

What is Knob and Tube Wiring?

Knob and tube wiring was an early standardized method of electrical wiring used in buildings in the United States from about 1880 to the 1940s. It consisted of single insulated copper conductors run within wall or ceiling cavities, passing through joist and stud drill-holes via protective porcelain insulating tubes, and supported on nailed-down porcelain knob insulators.

This old form of wiring has largely been replaced with modern sheathed cables and non-metallic conduit or armored cable systems. However, some older homes may still contain intact, operational knob and tube circuits.

Key Characteristics of Knob and Tube Wiring:

Two separate wires - The hot and neutral wires are separate with air as the insulator. There is no ground wire.
Ceramic knobs - Wires are supported by ceramic knobs nailed or strapped to framing members.
Porcelain tubes - Where wires enter a metal box or pass through wood framing, they are protected by a porcelain tube.
Cloth insulation - Early versions used cloth-covered wiring. Rubber insulation came later.
Exposed splices - Connections are wrapped with cloth friction tape, not inside junction boxes.

Why Replace Knob and Tube Wiring?

While knob and tube wiring may still be functional, there are some important reasons to consider upgrading to modern wiring:

Fire hazard - The old, dried out insulation can crack or split, exposing live wires to combustible materials. Thiscreates a serious fire risk.
Insulation deterioration - Cloth or rubber insulation becomes brittle with age, leading to cracks and exposure.
Lack of grounding - There is no equipment grounding conductor, which is essential for safety with modern appliances.
Unsafe modifications - Do-it-yourself repairs over the years may have compromised the original installation.
Inadequate for modern loads - Heavy use of electricity in modern homes can overload the limited capacity of knob-and-tube wiring.
Insurance issues - Many insurers refuse to cover homes with knob-and-tube wiring or charge substantially higher premiums.

How to Identify Knob and Tube Wiring

Here are the telltale signs that reveal knob and tube wiring:

Ceramic knobs and tubes are visible in unfinished basements, crawlspaces, attics.
Wiring enters metal boxes through porcelain tubes rather than via modern cable clamps.
Wiring along joists or studs lacks modern outer sheathing.
Junction boxes contain loose splices wrapped with tape, rather than secured wire connectors.
Two-prong outlets indicate no grounding conductors.
Fuse boxes (not circuit breakers) are present.
Wiring gauges appear relatively small compared to modern wiring.

If you see these signs, but are uncertain, an electrician can confirm through a more thorough inspection.

Replacement Options

Typically, it is advisable to completely remove knob and tube wiring when discovered and replace it with updated wiring throughout the house. There are a few approaches to consider:

Full rewire - Replace all wiring in the house with modern cables and new service panel/breakers. Most extensive but may be required by local codes.
Targeted replacement - Only replace specific runs of knob-and-tube wiring and splice to existing wiring. Partial upgrade costs less but doesn't address all risks.
Sheath overlay - Install new wires alongside old and splice into circuits. Less destructive to home but still leaves old wiring in place.
Pigtailing - Splice modern wiring into outlets/fixtures served by old wiring to add grounding. Cheaper but doesn't replace actual knob-and-tube runs.

Ideally, a full rewire is recommended whenever feasible, for maximum safety and full modernization. Some homeowners do opt for more limited targeted replacements if full rewiring cost is prohibitive.

Hiring an Electrician

Replacing knob-and-tube wiring is not a DIY project for the average homeowner. Hiring a professional electrician is strongly advised:

They can assess your specific needs and match a solution to your home's situation.
They have the expertise to carry out removal and replacement work in a way that meets current codes.
Professionals have the specialized tools and safety equipment to perform the work properly.
Experienced electricians can typically work more efficiently and avoid ad hoc issues that can arise with old wiring systems.
Good electricians will obtain necessary permits and arrange inspection of the completed work.

I recommend getting quotes from at least 3 local licensed electricians before selecting one to handle your knob-and-tube wiring replacement project.

Conclusion

While still functional, knob-and-tube wiring lacks key safety features of modern wiring. Substantial risks exist if left in place over time. Hiring a professional electrician to replace old wiring provides great peace of mind by eliminating fire and shock hazards. Homeowners can then enjoy fully updated electrical systems to meet all their modern needs.