An Introduction to Knob and Tube Wiring
Knob and tube wiring was commonly installed in homes built before 1950. This old electrical system uses ceramic knobs to hold wires apart and hollow tubes to protect wires through framing. While knob and tube can still function safely in old homes, it does not meet modern electrical codes and can be dangerous if modified or overloaded.
As an owner of an older home, you may discover knob and tube wiring hiding behind walls and ceilings. Leaving old wiring in place is not recommended, but a full rewire can be expensive. With proper precautions, you can revive and extend knob and tube wiring on a limited basis.
Dangers of Altering Knob and Tube Wiring
Knob and tube circuits typically provide only 30 amps to the entire house - barely enough for a few lights and appliances. The wiring lacks a ground wire and has no protection from overloads. Insulation is brittle or missing in spots.
Any changes to knob and tube can lead to fire or shock risks:
- Overloading circuits with too many appliances can overload the wires.
- Capping off wires improperly allows currents to flow on abandoned lines.
- Splicing into circuits can overheat existing wires.
- Running wiring too close to combustible materials creates fire hazards.
Because of these risks, I advise hiring a licensed electrician for any significant changes to knob and tube wiring.
Safe Options for Extending Knob and Tube Wiring
Use Extra Caution Near Combustible Materials
When tapping into old wiring, take extra care where wires pass through wood framing and insulation. Use metal conduit to protect wiring in these spots to prevent contact with combustible materials. Keep splices as short as possible and avoid routing new wiring above ceilings or under floors where fire risks are greater.
Add New Grounded Circuits
Rather than splicing into old wiring, run new cable from the main panel to outlets and lights. Install a new grounded circuit with ample capacity for modern loads. Use GFCI outlets or breakers for protection on ungrounded knob and tube lines.
Limit Lighting and Appliance Circuits
Avoid power-hungry devices like air conditioners on existing knob and tube circuits. Install new 20 amp circuits for high wattage lighting and outlets. Keep lighting and outlet loads under 1000 watts per circuit.
Service Equipment Cautiously
When working in the main panel, take care not to disconnect knob and tube lines or overwhelm their 30 amp capacity. Some upgrades like new panel or grounding require fully replacing old wiring.
Signs Your Knob and Tube Wiring Needs Replacement
While I don't recommend permanent use of knob and tube wiring, limited revivals can buy you time to save up for a rewire. However, the following issues indicate replacement is needed:
- Frequent tripped breakers or blown fuses from overloaded circuits
- Discolored or warm outlets and switches signaling overload
- Visible scorch marks or melting insulation around wires
- Numerous splices and alterations made over the years
- No ground wires present
If you experience flickering lights, shocks from appliances or outlets don't function properly, discontinue using those circuits. Hire an electrician immediately if you suspect a short or other hazard with old wiring.
Conclusion
Reviving knob and tube wiring in your vintage home can be done safely in limited cases by following proper precautions. However, I recommend fully replacing old wiring over time to prevent fire and shock risks. With careful planning, you can phase in a new electrical system wired to modern standards without burning down your historic home in the process.