How to Splice Your Home's Knob and Tube Wiring

What is Knob and Tube Wiring?

Knob and tube wiring was an early method of electrical wiring used in homes in the United States from about 1880 to the 1940s. It consists of separate hot and neutral wires that are not bundled together like modern wiring. The hot and neutral wires are supported on porcelain knobs and run through porcelain tubes across open spaces.

Some key characteristics of knob and tube wiring:

• Not grounded - There is no ground wire, which can be a safety hazard.

• Insulated with cloth/rubber - The wiring insulation tends to become brittle and cracked over time.

• Open air - The wires are separated and suspended, not bundled. This allows air to circulate and prevent overheating, but leaves the wiring more exposed.

• Higher amperage - Knob and tube wiring typically uses thicker gauge wires that can carry more current than modern wires do.

Dangers and Issues with Knob and Tube Wiring

There are several important problems that can occur with old knob and tube wiring:

• Fire hazard - The outdated, cracked insulation can expose bare wires, allowing wires to arc, spark, and overheat. This is a serious fire danger.

• Shocks/electrocution - With no ground wire, risk of electrical shocks is increased. Faults in the wiring can energize metal case appliances and fixtures.

• Lack of capacity - Heavily spliced wiring and undersized wire gauges can limit the electrical capacity, causing breakers to trip or fuses to blow with higher loads.

• Unsafe splices - Haphazard and uncovered splices were often used to tap power off of knob and tube wires. These create additional failure points.

• Insurance issues - Many insurance providers will not cover houses with knob and tube wiring due to the risks.

Options for Replacing Knob and Tube Wiring

If your home has knob and tube wiring, one of the following options should be considered:

• Full rewire - Replacing all of the knob and tube wiring with new wiring throughout the house. This is the best but most invasive and expensive option.

• Targeted replacement - Only replacing the knob and tube wiring in high risk or high load areas, leaving the rest as-is. This is lower cost but still leaves some risk.

• Pigtailing - Attaching new grounded wires to tap power off the old wiring. The old wires stay in place while new wiring is added for outlets.

• Circuit breakers - Installing GFCI and AFCI circuit breakers to help protect from shocks and arc faults. Breakers don't fix old wires but reduce risk.

How to Splice into Knob and Tube Wiring

If you need to splice into existing knob and tube wiring to add connections or tap power, follow these important steps:

• Turn off power at the main breaker panel before working on any wiring. Verify power is off!

• Carefully access the existing knob and tube wiring you want to splice into. Remove any old insulation to expose several inches of clean wire.

• Use porcelain wire nuts designed for splicing old wiring. Do not use regular plastic wire nuts - the wires may pull out.

• Connect any new wiring by twisting wires together securely with wire nuts. Match the wire size and use good technique for a clean mechanical joint.

• Apply electrical tape over any splices for additional insulation. The joints should be secure with no exposed copper.

• Consider running any new wiring through non-metallic flexible conduit for protection. Conduit also allows easy wire removal if needed.

• Attach additional wire supports if necessary. Use porcelain knobs or staples rated for the wire size. Do not let wires rest or droop on insulation.

• Label and document all new splices. Drawing a simple diagram is recommended to keep track of new connections.

• Restore power and test operation of newly connected lights, outlets, or appliances. Check for secure splices.

Be very careful working around old wiring! Consider having an electrician handle any major new wiring.