How to Work With Knob and Tube Wiring in Your Home

What is Knob and Tube Wiring?

Knob and tube (KT) wiring was an early standardized method of electrical wiring in buildings, in common use in North America 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.

KT wiring has a distinctive appearance:

• The wires are separate, not bundled together like modern electrical cable.
• White ceramic knobs hold the wires away from framing.
• Tubes protect wires through joist and stud drill-holes.

Advantages of knob and tube wiring included:

• Airflow in the wall cavities kept the wires cool.
• Wires were separated from potentially flammable framing.
• The open structure simplified making tap connections.

Disadvantages were:

• No ground wire, presenting a shock hazard.
• Insulation dried out over time, risking fire.
• Difficult to modify and expand.

While no longer used for new construction, millions of homes still have original knob and tube wiring.

Dangers and Limitations of Knob and Tube Wiring

Fire Risk

The insulation on KT wiring dries out over time, cracking and exposing the copper conductors. This creates a serious fire hazard. Damaged insulation can allow conductors to arc to grounded metal boxes or framing. KT wiring lacks a ground conductor, so faults may not trip breakers.

Frayed conductor insulation also risks arcing inside wall cavies. This can ignite flammable materials like wood framing, paneling, or insulation. Best practice is to keep insulation at least 1" away from KT wiring.

Insufficient for Modern Loads

KT wiring was designed for lower electrical loads, like simple lighting and motor circuits. Ordinary household wiring was just 30 amps. The wiring is insufficient and potentially dangerous with higher modern electric loads from large appliances.

No Equipment Ground

Without a ground wire, KT wiring presents a serious shock hazard. If an active conductor touches grounded metal, current can flow through anyone who touches it. GFCI outlets can help provide protection from shocks.

Difficult Repairs and Alterations

The wiring layout in KT systems is difficult to trace. Making alterations risks damaging brittle insulation. Installing grounds requires running new wires. These difficulties lead many homeowners to completely rewire rather than repair KT wiring.

Is Knob and Tube Wiring Safe?

Potentially, With Caution

Well-maintained KT wiring that hasn't been altered can remain safe to use. But it requires caution and awareness of fire risks. Homeowners should exercise care to keep insulation away from KT wiring and avoid overloading circuits.

Problems often arise when homeowners undertake renovations without understanding the risks and limitations of KT wiring. Even if problems aren't obvious, it's smart to have KT systems evaluated by an electrician.

Important Safety Precautions

• Use plastic guards to keep insulation at least 1" from KT wires.
• Do not overload circuits. Use heavy duty extension cords temporarily.
• Test for faults and hot spots using an infrared camera.
• Have an electrician install GFCI outlets for protection from shocks.
• Label KT circuits at the fuse box so they aren't overloaded.
• Consider replacing KT wiring that shows any deterioration.

Options for Replacing or Upgrading Knob and Tube Wiring

Full Rewire

A full rewire involves installing new grounded cable throughout the home, providing new properly sized circuits. This is the safest option but also the most disruptive and costly. Expect to pay $8,000-$15,000 for a full rewire of an average home.

Pigtailing

Individual ground wires are spliced onto existing KT wires, adding a safety ground without replacing the conductors. This preserves some fire risk from deteriorated insulation. Cost is around $2,000-$4,000.

Circuit Board Repairs

Damaged wiring is replaced with modern NM cable, while leaving other KT wiring in place. This is less disruptive but leaves much of the old wiring. Cost is $4,000-$8,000.

Additional GFCI Protection

Adding more GFCI outlets throughout the home adds shock protection. But it doesn't address fire risks. This is a lower cost option under $1,000 but not a full solution.

Maximizing Safety With Knob and Tube Wiring

Prioritize Repairs

Replace any visibly damaged or repaired KT wiring. Target high priority locations like kitchens and bathrooms. Get an electrician's guidance on the repairs needed to maximize safety.

Use Caution During Any Renovations

Avoid damaging fragile KT wiring. An electrician can isolate those circuits. Never bury KT wires behind new walls. Plan to replace KT wiring in any area being renovated.

Have an Electrician Evaluate Your System

Hire an experienced licensed electrician to thoroughly inspect your KT wiring and provide repair recommendations. Thermal imaging can check for hot spots. Assess both condition and capacity.

Add GFCI Protection

Installing GFCI outlets throughout the home provides important protection from shock hazards. Though it doesn't reduce fire risks, it's a quick affordable safety improvement.

Create a Safety Plan

Develop a plan for KT wiring repairs and upgrades you can accomplish over time. Priority should be areas of deterioration or high electrical loads. Getting grounded wiring in place now can save lives.

Key Takeaways About Knob and Tube Wiring

• KT wiring lacks a ground and has fire risks, but can be safe if in good condition.

• Never overload KT circuits or make alterations without caution.

• Insulation must be kept at least 1 inch away from KT wiring.

• Options exist ranging from full rewire to targeted repairs and improvements.

• An electrician should evaluate any KT wiring to determine recommended safety upgrades.

• GFCI outlets are crucial protection against shocks with ungrounded KT wiring.

• With careful precautions, KT wiring can be used safely as part of a long-term upgrade plan.