How to Check Your Home's 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 cavities, passing through joists and studs.

The wiring was supported along its length by knob insulators made of porcelain or wood attached to the structural framing. Where wires entered a wiring device such as a lamp or switch, or were pulled into a wall, they were protected by flexible cloth tubing called "loom".

Key Features of Knob and Tube Wiring:

• Single conductor wires - The hot and neutral wires were separate wires, unlike modern Romex cable which has both together.

• Open air installation - The wires were run through open spaces within walls and ceilings, and were not enclosed within conduit.

• Insulators - Knobs held the wires away from framing. Tubes protected wires through joist and stud holes.

• Cloth insulation - Early versions had cloth insulation, later upgraded to rubber insulation.

• Loom tubing - Flexible tubing protected wires entering switches and devices.

Dangers and Issues with Knob and Tube Wiring

While knob and tube wiring was suitable for the low voltage lighting and few appliances used when initially introduced, it can be problematic and dangerous by modern standards. Some key concerns include:

• Fire hazard - The old cloth and rubber insulation can become brittle and cracked over time, exposing the copper wiring to potential short circuits and arcing faults.

• Lack of grounding - Knob and tube systems typically have no equipment grounding conductor, increasing the risk of shocks.

• Insufficient capacity - These wiring systems often cannot handle the power needs of modern appliances and electronics. Circuits can be easily overloaded.

• Inaccessible locations - Buried within walls and difficult to reach, problems with old wiring can go unnoticed. Failures and hot spots go unseen.

• Insurance issues - Many insurance companies will not issue policies on homes with knob and tube wiring due to the risk it poses.

How to Check for Knob and Tube Wiring in Your Home

If your home dates to the 1940s or earlier, checking for the presence knob and tube wiring should be a priority. Here are the main ways to check:

Examine the Electrical Panel

• Look for screw-in fuses rather than modern circuit breakers. This can indicate an old system.

• Check for two-wire circuits sharing a neutral. This was common with knob and tube.

• See if any circuits are labeled with wire insulation types like BX, CL2, or SR. This can indicate some updated wiring has been done.

Inspect the Attic and Basement

• Carefully look for the distinctive porcelain knobs and tubes running across joists and studs.

• Verify that wiring is single conductor, with no modern sheathed cable.

• Try to follow runs as far as possible to determine how much old wiring remains.

Check Accessible Outlets and Fixtures

• Remove covers of accessible outlets and switches to look for cloth-wrapped wires entering boxes.

• Check light fixtures to see if they are wired with modern cables.

• Look for junction boxes with old spliced connections between knob and tube and modern wiring.

Consider Hiring an Electrician

For a thorough inspection, it may be wise to hire a qualified electrician. They can check wires in walls, perform electrical load calculations, and provide documentation of any issues. This may be necessary for insurance purposes as well.

Options for Remediating Knob and Tube Wiring

If your home does still contain knob and tube wiring, some options to consider are:

• Full rewire - Replacing all wiring with modern cables. This ensures safety but is expensive.

• Targeted replacement - Strategically rewiring high risk and high load areas first. Does not eliminate all risks.

• ** Pigtailing ** - Attaching modern wiring to old circuits to extend them. Considered a temporary solution by many.

• Circuit breaker panel upgrade - Does not change wiring but adds circuit breakers for overcurrent protection.

• Additional GFCI/AFCI protection - Adding ground fault and arc fault outlets and breakers help protect from some risks.

• Insurance inspection - Getting an assessment and documentation from your insurance company before planning repairs.

Ultimately, the safest option is fully rewiring a home to remove all old knob and tube wiring. But targeted replacements of dangerous or overloaded circuits can be a more affordable intermediate step. Consulting professionals is highly recommended before doing any work.