How to Keep Your Home's Knob and Tube Wiring Safe

What is Knob and Tube Wiring?

Knob and tube (K&T) wiring was an early standardized method of electrical wiring used in buildings in the United States from about 1880 to the 1940s. It consists of 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.

K&T wiring has the following characteristics:

• Wires are separate, with no ground wire. The hot and neutral wires are run through porcelain insulating tubes or knobs.
• There are no cables, just individual wires. This exposes the wires to contact with any insulation added later.
• Wires are run through open air, not inside cables, and are held apart from framing.
• It relies on air as an insulator between hot and neutral wires. Air is a poor insulator compared to modern insulated wiring.
• Older K&T wiring used cloth insulation on wires that can deteriorate over time. Newer K&T wiring used rubber insulation.

Dangers of Knob and Tube Wiring

K&T wiring can pose the following safety risks:

• Fire hazard - The insulation on old wires becomes brittle and cracked over time, leading to exposed conductors. This can cause arcing and sparks.
• Shock hazard - Without a ground wire, there is no path to safely divert current in case of a short circuit.
• Overload - K&T is designed for light electrical loads. Today's high-wattage appliances can overload the system.
• Insulation issues - Insulation added later may envelop K&T wires, causing them to overheat.

Tips to Keep K&T Wiring Safe

Here are some tips to keep aging K&T wiring safe in your home:

Inspect Annually

• Carefully inspect all accessible K&T wiring once a year. Look for cracked or damaged insulation, loose connections, or signs of overheating.
• Check that knob and tube insulators are intact and wires are supported properly.

Limit Loading

• Use light bulbs of 60 watts or less.
• Only use newer, energy efficient appliances. Avoid large, high-wattage appliances.
• Keep extension cords and power strips to a minimum. They can easily overload old wiring.

Don't Enclose Wires

• Don't install insulation over K&T wires. It could cause them to overheat.
• Don't install new wiring too close. Maintain at least 6 inches of separation.

Make Strategic Upgrades

• Upgrade high-use and high-risk circuits to modern wiring. Prioritize kitchens, bathrooms, and laundry.
• Install GFCI outlets on remaining K&T circuits to add protection from shocks.

Consider Full Replacement

• If the K&T wiring is extensive and deteriorating, full replacement may be warranted. Consult an electrician.

Final Thoughts

Knob and tube wiring that is in good condition can be relatively safe. But it requires diligent inspection and limiting electrical loads. Strategic upgrades are wise. If the old wiring is compromised, replacement should be considered. With proper care, K&T wiring can be maintained safely. But always have a qualified electrician assess any concerns.