I recently purchased a historic home built in the 1920s. Upon inspection, I discovered that parts of the electrical system still relied on an antiquated wiring method known as knob-and-tube. At first, I was wary of this old technology. But further research revealed that proper knob-and-tube installations can be perfectly safe and that replacing it can cost thousands of dollars. Here's what I learned about this forgotten wiring method and why it might be best to leave it alone.
What is Knob-and-Tube Wiring?
Knob-and-tube (K&T) refers to an early standardized method of electrical wiring used from about 1880 to the 1940s. It consists of insulated copper conductors run through ceramic knobs mounted to framing members and threaded through porcelain tubes to provide protection and structural support.
K&T has some key features:
- Two separate wires, one for the hot and one for the neutral rather than a bundled cable
- Open air between wires rather than grouping in a sheath
- Wires attached to insulators spaced apart from framing
- No ground wire
This method dominated building wiring for decades before newer types of insulated cables became prevalent.
Why Knob-and-Tube Was Used Historically
K&T wiring arose as a safer, more reliable, and standardized alternative to risky early electrical installations. It separated wires to reduce fire hazards and insulation issues. Mounting wires on insulators and open air circulation dissipated heat.
The spaced out wiring also made running circuits through finished walls and ceilings easier. Builders adopted K&T as the preferred wiring technique from the 1890s through the 1940s. Even modern wiring methods like Romex cables derived some principles from knob-and-tube.
Is Knob-and-Tube Dangerous?
Many people assume K&T wiring is obsolete and dangerous. In truth, it can be entirely safe when properly maintained. The National Electrical Code still permits existing installations under many conditions.
However, K&T does have some drawbacks compared to modern wiring:
- No ground - K&T lacks a grounding conductor to prevent shocks.
- Insulation concerns - Old brittle insulation is more likely to fail.
- Overloading - Light duty wire gauges can overheat from excessive current.
- Exposed wires - Damage is possible without conduit protection.
These factors require careful inspection and maintenance. But well-kept K&T with adequate capacity has proven itself reliable for over a century in millions of buildings.
Upgrading Can Be Very Expensive
Many electricians recommend fully replacing K&T wiring upon discovery. However, I learned this “upgrade” can come at an extreme cost:
- Labor intensive - Fishing new wires through finished walls is difficult and time consuming.
- Materials add up - Lots of cable, conduit, boxes, and fittings are required.
- Plaster repair - Wall and ceiling repairs from extensive holes can be massive.
- Equipment changes - New grounding may require electrical panel and appliance upgrades.
All this can easily cost tens of thousands of dollars. I got quotes as high as $25,000 to rewire my small home!
Maintaining Knob-and-Tube Can Save Money
Considering the high cost, I explored maintaining the existing K&T instead. With proper inspection, maintenance, and common sense, problems can be avoided. Here are some tips:
- Inspect annually - Check for damaged wires or connections. Verify insulation integrity.
- Splice in grounds - Adding a ground wire reduces shock risks for older systems.
- Avoid overloads - Use appropriate wattage bulbs and avoid overloading circuits.
- Keep cool - Don't cover K&T or install insulation that could cause overheating.
- Label live wires - Mark all known live K&T to prevent unsafe disturbance.
- Use GFCIs - Install ground fault circuit interrupters for added protection on older circuits.
- Limit alterations - Leave K&T intact as much as possible during any renovations.
With reasonable precautions like these, K&T can continue working safely for decades to come. The savings compared to a full replacement are enormous.
Weighing Your Options
Knob-and-tube wiring is outdated but not necessarily dangerous. Removing it wholesale can cost tens of thousands in repairs that may not even be necessary. In many cases, keeping the existing K&T functioning through inspection and maintenance makes better financial sense.
Of course, if the old wiring is badly deteriorated or insufficient for your needs, replacement may be warranted. But consider your options carefully, get professional assessments, and weigh the benefits versus the tremendous costs. With proper diligence, the old knob-and-tube method could save you a small fortune.