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 consisted of single insulated copper conductors run within wall cavities, passing through joist and stud drill-holes via protective porcelain insulating tubes, and supported on nailed-down porcelain knob insulators.
This old wiring system has largely been replaced due to inadequacies compared to modern wiring methods. However, some homes still contain functional knob and tube wiring. This can present safety issues and difficulties when remodeling or upgrading electrical systems. Understanding how to evaluate and troubleshoot knob and tube wiring is important for homeowners and electricians dealing with older homes.
Hazards and Limitations of Knob and Tube Wiring
There are several hazards and limitations associated with knob and tube wiring that make it obsolete compared to modern electrical systems:
-
Fire hazard - insulation on wires often degrades over time, exposing bare conductors that can arc and spark. Knob and tube lacks grounding which can prevent faults from safely tripping breakers.
-
Insufficient for modern loads - designed for simple lighting and receptacle circuits. Cannot handle the power demands of modern appliances and devices.
-
Exposed splices - splicing of wires done openly rather than inside covered junction boxes. Exposed connections are vulnerable to damage and loose connections can overheat.
-
Inaccessible wiring - difficult to access inside walls and ceilings. Makes replacements, upgrades, and adding circuits extremely difficult.
-
No protection from overcurrent - lacks safety devices like fuses or circuit breakers to protect against excessive current flow. Risk of overloaded circuits overheating.
Evaluating and Troubleshooting Knob and Tube Wiring
When dealing with a home with knob and tube wiring, evaluation and troubleshooting is critical before undertaking any upgrades or changes. Key steps include:
Visual Inspection
-
Look for knob and tube wiring inside uninsulated walls, attics, basements. Identify locations and routes of wiring.
-
Check for improper splices, damaged wires, loose connections. These present fire and shock risks.
-
Evaluate condition of insulation. Cracked, damaged, or deteriorated insulation can expose bare conductors.
Check Circuits and Fuses
-
Test each circuit by turning on all loads separately. Determine which outlets/lights are on each circuit.
-
Check fuse/breaker panels. Note if non-standard or makeshift fusing has been added. This is a fire risk.
Load Testing
-
Plug high wattage appliances like heaters into each outlet and turn on. Check for dimming lights or warm outlets which indicate inadequate circuit capacity.
-
Test each circuit with outlet tester. Check for open neutral/hot, reverse polarity, or other improper wiring.
Check Grounds
-
Use a multimeter to check for presence of ground wires. Knob and tube has no grounding which is a safety hazard.
-
Test outlets for proper grounding. 3-prong outlets without grounds are unsafe.
Consider Repairs vs. Replacement
-
For minor issues, repairs may suffice, like replacing damaged wires or adding junction boxes.
-
However, for major hazards or insufficient capacity, complete rewiring may be required. Get quotes from electricians.
-
Replacing knob and tube with modern wiring (e.g. Romex) will provide safest and most robust electrical system.
Safe Approaches to Upgrading Electrical with Knob and Tube
When upgrading electrical service in a home with knob and tube wiring, extra care must be taken:
Never Overload Existing Circuits
- Never plug high wattage appliances like stoves, heaters, air conditioners into existing outlets. This risks overheating wires.
Do Not Conceal Existing Wiring
- Do not cover knob and tube wires inside insulation or walls. Keep wiring accessible and prevent damage.
Add New Circuits Properly
- When adding new wiring, keep it completely separated from existing knob and tube circuits. Do not tap into unsafe wiring.
Replace Dangerous Wiring Before Covering
- If replacing knob and tube wiring, remove all old wiring if possible before installing insulation or drywall. Eliminate fire risks.
Hire a Licensed Electrician
- Since knob and tube can present serious hazards if improperly rewired, hire a qualified electrician rather than doing DIY electrical work.
Options for Replacing Knob and Tube Wiring
Full Rewiring
- Most comprehensive option. All knob and tube wiring removed and replaced with new wiring (e.g. Romex). Allows for entirely new modern electrical system.
Partial Rewiring
- Only rewire some circuits, leaving others on knob and tube. Less expensive, but still leaves potential hazards.
Pigtailing
- Splice new grounded Romex wiring into existing wires. Grounds old system but doesn't replace all wiring.
Capping and Abandoning
- Disconnect knob and tube wiring at panels, cap ends, and leave unused in place. Wire home with entirely new wiring separate from old.
Replacing obsolete knob and tube wiring requires evaluating the existing system, addressing any hazards, and often rewiring all or part of the home to create a safer, more robust electrical system. Hiring a professional electrician is highly recommended. With the right approach, the risks of knob and tube wiring can be successfully eliminated.