Knob and tube wiring is not illegal, but it does ignite great shock and fire risk
You can identify K&T wiring by its white porcelain or ceramic knobs or tubes
Insurance companies may charge higher premiums or refuse to insure your home unless you have it rewired
Call a professional electrician to inspect your K&T system and suggest additional safety measures
If you own a classic home built between the late 1800s and mid-1900s, you may have a form of electrical wiring called knob and tube or K&T wiring. While the nature of the system itself is safe, it's essential to have your electrical system professionally inspected to ensure it's still in working order and meets local safety codes. The simple wear and tear of time, decades of questionable repairs, and our ever-growing need for more electricity may put too much pressure on these antique systems.
To ensure your K&T wiring is safe, be sure you understand knob and tube wiring dangers and how to keep your electrical system in working order.
What Is Knob and Tube Wiring?
Take a peek around your attic, basement, or under your floorboards, and you may spot the signs of an original knob and tube wiring system. Knob and tube wiring features insulated copper wiring that runs through porcelain or ceramic knobs or tubes—which offer an added layer of protection.
From 1880 to 1950—and as late as the 1970s—knob and tube was the go-to wiring system for homes powering the occasional toaster or living room lamp. The porcelain knobs kept the wiring several centimeters away from the wooden walls to prevent fire risk. For the most part, original K & T systems linked to 60-amp service boxes.
As electrical technology got smarter and we added things like televisions—and eventually air fryers—to our homes, the knob and tube wiring system became a thing of the past. Homes now typically use 100-amp breaker boxes, much higher than the original electricians installed the system to handle.
A great K & T system itself can be safe, but federal and local regulations made it illegal in new construction some time ago. And so, there's a good chance that modifications or decay put the system at risk.
According to the National Fire Protection Association, half of all home fires stem from issues with electrical wiring. Knob and tube wiring has a bad reputation for safety due to improper repairs and modifications made to the system. Old hardware and inadequate insulation also put this wiring at greater risk of causing issues. Here are some of the most common knob and tube wiring issues.
K&T Wiring Is Not Grounded
Knob and tube wiring systems don't have a ground wire—that third bottom hole that gives excess electrical charges a safe place to go. Most modern outlets have three holes, but knob and tube wiring systems only have two prongs that conduct electricity.
You can use prong adapters, but this won’t solve the problem entirely because it leaves your appliances or devices ungrounded, increasing the risk of shock. Plus, two-pronged outlets do not often meet local electrical codes and will eventually need to be replaced to reduce the risk of an electrical fire.
Imagine 30 years before you owned your home, a local handyman spliced your K&T system to work with modern wiring, a higher-capacity service box, or a powerful appliance. Without a thorough home inspection, no one spotted the dangerous setup until decades later. Overloading your electrical service box can damage your appliances and even lead to an electrical fire.
Older homes are the most common victims of quick and fast electrical work from an unlicensed electrician—a practice that became more common as regulations changed and required more effort for pros to meet code. It's important to have your knob and tube system inspected for these haphazard modifications.
Poor Wiring Insulation
Insulation around wiring keeps the electrical current concealed and protects against overheating. Knob and tube wiring is often insulated with rubber or cloth wiring—which pros now deem too thin and quite delicate to protect your home properly. Additionally, pests like rats and mice have been known to nibble at the insulation.
Damage can also occur to the ceramic insulator knobs in your K&T system. A shifting home, extreme weather, or shaking from construction can deteriorate the knobs over time.
Contact with Building Insulation
One of the biggest issues with knob and tube wiring is its proximity to your home’s insulation. The design of K&T wiring required ample space between your walls and the wire, leaving room for heat to release from the wires when conducting electricity. When you add a layer of foam insulation inside your walls, the heat becomes trapped, leading to a fire hazard.
For this reason, many states require a home inspection before installing or replacing insulation. The 2008 National Electrical Code specifies that professionals can not install wires in the "hollow spaces of walls, ceilings, and attics where such spaces are insulated by loose, rolled, or foamed-in-place insulating material that envelops the conductors."
You may also encounter issues insuring a home with a knob and tube wiring system. Due to their risks and expensive upkeep over time, companies may charge higher premiums or refuse to insure your home unless you have it rewired.
How to Identify Knob and Tube Wiring
Knob and tube wiring is easy to identify if you have the access to the exposed wires in your basement or attic. Knob and tube wiring will consist of insulated copper wires supported by bright white porcelain or ceramic knobs.
If you have wires that run through wood framing, look for porcelain or ceramic tubes nailed to exposed joints. If you find evidence of K&T, get a professional inspection to ensure it is safe.
What to Do About Knob and Tube Wiring in Your Home
The very existence of a knob and tube system in your home doesn't mean that you need to have it removed and replaced. However, it's very smart—and often required—that your knob and tube system undergoes an inspection to look for any hazards.
If the time has come to replace the old system, the cost to rewire a home is between $1,500 and $10,000, depending on the difficulty of rewiring and the size of the new service panel.
Always take your home’s electrical wiring safety seriously. Work with a local electrician near you if you want to have your knob and tube wiring system inspected or replaced. They’ll check for damaged or incorrectly-modified circuits and recommend added protection to reduce shock and fire risk.