How to Troubleshoot Old Knob and Tube Wiring
As a homeowner with an older house that still has original knob and tube wiring, I know how frustrating and potentially dangerous this outdated system can be. Properly troubleshooting and addressing issues with knob and tube wiring requires knowledge, patience, and care. In this guide, I will share what I've learned about troubleshooting common problems and determining the best solutions for updating old knob and tube wiring.
Understanding Knob and Tube Wiring
Knob and tube wiring was commonly installed in homes built before 1950. This type of wiring consists of:
- Knobs - ceramic spacers that separate and support the wires.
- Tubes - protective casings for wires that run through open spaces like walls and attics.
- Wires - usually rubber-wrapped in cloth or asbestos insulation.
The wiring is routed throughout the house with the knobs keeping the wires separated and the tubes protecting where wires pass through open stud bays.
While knob and tube was suitable for smaller electrical loads of the past, it can be problematic and unsafe for modern power usage. Key issues include:
- Insulation - old and cracked insulation leads to exposed live wires.
- Overloading - system not designed for high wattage appliances and electronics.
- Spliced wires - DIY connections and taps can loosen over time.
Now that we've reviewed the basics of knob and tube wiring, let's look at troubleshooting common issues.
Signs of Problems with Knob and Tube Wiring
There are several signs that can indicate issues with old knob and tube wiring:
- Blown fuses - fuses blow frequently when circuits are overloaded.
- Flickering lights - lights dim and brighten from faulty connections.
- Discolored outlets/switches - scorched or warm outlets indicate overload.
- Buzzing/humming noise - often from old transformers vibrating.
- Sparks/smoke - dangerous shorts from damaged wires.
- No power to outlets - opens in the wiring or bad connections.
- GFCIs tripping - ground faults detected due to lack of grounding.
Any of these signs mean it's time to thoroughly inspect the wiring and make needed repairs.
Testing Knob and Tube Wiring
Here are some steps I take to test and troubleshoot my knob and tube wiring:
- Carefully examine wiring in the attic, basement, and accessed walls.
- Look for cracked, frayed, or exposed wires.
- Check for loose connections, damaged insulation at joints.
- Verify wires are not in direct contact with combustible materials.
- Shut off power to the circuit at the main panel.
- Disconnect wires to devices and use a multimeter to test continuity.
- Probe different points along the length of the wires.
- Check for opens, shorts, and high resistance that indicates damage.
- With power on, plug high wattage appliances or equipment into outlets.
- Note any decrease in voltage, dimming lights, blown fuses.
- Confirm wires, splices and fuses can handle the amperage.
- Install GFCI outlets or breakers on ungrounded circuits.
- Press the "Test" button to verify protection from ground faults.
- If it trips with nothing plugged in, there's a ground fault from damaged wiring.
Once I've identified any problems through testing, I determine repair priorities as follows:
- Exposed wires - Very dangerous, wrap in electrical tape as a temporary fix.
- Shorts/sparks - Trace wires to find damaged insulation and splices causing shorts.
- Frequent blown fuses - Evaluate if smaller breakers can be installed to prevent future blowouts.
- No power - Trace opens in wiring and repair connections to restore power.
- Overloads - Note outlets used for large appliances, consider upgrading wiring.
I focus on any immediate fire and shock hazards first. Smaller issues may be managed by replacing fuses, adding circuits, or installing GFCIs until more extensive repairs can be made.
Upgrading from Knob and Tube Wiring
When issues become too persistent and repairs too costly, upgrading the wiring is the best solution. Here are my top options:
- Full rewire - Expensive but provides completely modern wiring. I'd hire an electrician.
- Run new wiring - Add grounded NM cables from panel to devices, leaving old wiring intact.
- Subpanel install - Less invasive way to distribute new circuits throughout the house.
Rewiring the whole home and removing all old knob and tube can be very costly. I got estimates from $8,000 to $15,000 for a full rewire of my 1,200 sq. ft. house. Running new wires or installing subpanels to add grounded circuits is a more affordable interim option.
Troubleshooting and repairing knob and tube wiring requires knowledge of the system, careful inspection, and testing. Prioritize immediate safety hazards first, then consider upgrades to provide modern, grounded wiring. With some effort and diligence, you can keep old wiring in reasonable shape and determine the best long-term solutions when problems persist. Hiring a professional electrician is advisable for major repairs or upgrades.