Open knob-and-tube wiring was commonly installed in homes built before the 1930s. While it may seem antiquated, understanding this old electrical system can help solve hidden wiring issues in older homes.
What is Knob-and-Tube Wiring?
Knob-and-tube (K&T) wiring consists of individual wires running through ceramic knobs mounted to framing members and passing through porcelain tubes at joist crossings. It has:
- No outer sheathing or cable jacket
- Wires separated by air space rather than insulation
- Wires secured along their length
K&T wiring was the standard electrical method from about 1880-1930. Though no longer used in new construction, it's still functioning in many older homes.
Benefits of Knob-and-Tube Wiring
K&T wiring has some advantages over modern wiring methods:
- Air circulation - The air space between wires allows heat dissipation, reducing fire risk. Modern insulated wires retain heat.
- Sturdy mounting - The knobs and tubes firmly secure the wiring in place. Movement can lead to modern wire failure.
- Thicker wire gauges - K&T wires are often 10 or 12 gauge. Modern 14 gauge wire is more common. Thicker wire allows higher amp capacities.
Properly installed and maintained K&T wiring can be entirely safe and reliable.
Issues with Knob-and-Tube Wiring
However, K&T wiring does have some drawbacks:
- Lack of grounding - K&T systems have no equipment grounding conductor. This can be unsafe.
- Brittle insulation - The cloth wiring insulation becomes brittle with age, leading to cracks.
- Inflexible - K&T wiring cannot be easily rerouted or modified. New extensions require opening walls.
- Exposed splices - Spliced connections may be uncovered in ceilings or walls, leading to shorts.
- Overloading - Light 15A circuits were the norm with K&T. Modern loads can overload these wires.
Signs of Knob-and-Tube Wiring in a Home
How do you know if your older home still contains K&T wiring? Signs include:
- Ceramic knobs and tubes running through walls, ceilings and crawlspaces
- Wiring entering junction boxes through porcelain tubes
- Lack of ground wires on older receptacles and switches
- Blown fuses or tripped breakers only on some circuits
- Discolored or cracked outlet or switch faceplates
- Light fixture sockets hanging down from junction box
Having an electrician inspect is the best way to verify K&T wiring.
Dangers and Risks of Old Knob-and-Tube Wiring
While K&T wiring can seem harmless, deteriorated systems do pose serious safety risks such as:
- Shocks and electrocution - Lack of grounding can make fixtures electrified. Exposed splices also increase this risk.
- Fire - Overloaded, damaged or spliced wires can overheat, igniting nearby combustibles. This caused many historical fires.
- Damage - Rodents can chew on the old insulation, causing shorts. Vibration can also damage brittle insulation.
- Inadequate power - Heavy modern loads from appliances can overtax the small K&T wires, causing low voltage.
If your K&T wiring shows any signs of damage, upgrades should be made.
Upgrading from Knob-and-Tube Wiring
K&T wiring should always be evaluated by a licensed electrician familiar with its unique qualities. They may recommend:
- New circuits - For high load appliances like HVAC units or laundry, new properly grounded wiring circuits can be run.
- Jumper cables - Jumper cables can tie K&T circuits to new grounded wires for safety.
- Full rewire - For more peace of mind, many homeowners opt to completely rewire their home with modern cables. But this is expensive.
Upgrades can often be done incrementally over time to spread out costs. But portions of K&T wiring in good, safe condition can remain.
Working Safely with Knob-and-Tube Wiring
If your home still relies on aging K&T wiring, follow these tips for safety:
- Avoid overloads - Don't plug high-wattage appliances like space heaters into outlets on those circuits.
- Don't alter wiring - Never remove knobs or tubes, splice wires outside boxes, or make changes without an electrician.
- Check condition - Watch for cracking, exposed wires, warmth, or flickering lights which could indicate damage.
- Install GFCIs - GFCI outlets can provide protection on ungrounded circuits. But have an electrician verify safety first.
- Keep covered - Don't run wiring through walls or ceilings without proper drywall or insulation covering.
Is Knob-and-Tube Wiring Banned?
Many insurance companies and lenders will not cover homes with live K&T wiring due to the fire and shock risks. But K&T wiring itself is not banned. Some municipalities prohibit it in new construction only. Homeowners can choose to keep K&T wiring maintained and in good condition. But upgrades are often recommended and required by sellers.
Conclusion
Knob-and-tube wiring reminds us of the ingenuity of the past. While it's antiquated, K&T served homes faithfully for decades. Respecting its merits while recognizing the need for modern safety is key. With proper handling, even old knob-and-tube wiring can keep working for years to come. But upgrades should come before any wiring deterioration leads to a hidden disaster. For maintaining the wiring of the past safely, have an electrician check your classic K&T wiring today.