Knob and Tube Wiring: What It Is & Why It Matters
If you own an older home in the Jenkintown area, there’s a real chance some of it is still wired with knob and tube. Here’s what it is, why insurers and inspectors flag it, and how it’s safely replaced.
The greater Philadelphia area is full of beautiful older homes — and many of them still have original knob and tube wiring hiding in the walls, attic or basement. It isn’t automatically an emergency, but it is something every owner of an older home should understand. This guide answers the questions we hear most.
What Is Knob and Tube Wiring?
Knob and tube (K&T) is the wiring method used in North American homes from roughly the 1880s through the 1940s. It’s easy to recognize: individual copper conductors run through the framing, held away from the wood by white ceramic knobs, and protected by ceramic tubes where they pass through joists and studs. The hot and neutral wires run separately, spaced apart in open air.
How Does Knob and Tube Wiring Work?
In its day it was a sound system. The ceramic knobs and tubes kept the insulated conductors suspended in air, which let heat dissipate. That’s the key detail — K&T was designed to be surrounded by air, not modern insulation, and it has no ground wire. Both of those facts are why it causes problems in a modern home.
When Was Knob and Tube Wiring Discontinued?
K&T was gradually phased out through the 1940s and 1950s as newer sheathed, grounded cable (the ancestor of today’s Romex) became standard. So if your home was built before about 1950 and hasn’t been fully rewired, it may still have some K&T in place — often mixed in with newer wiring from past renovations.
Why Knob and Tube Wiring Is a Concern
- No ground. K&T has no grounding conductor, so it can’t safely serve modern three-prong appliances and offers less protection against shock.
- It hates insulation. When later owners blow insulation into attics and walls, it traps heat around wiring that was designed to breathe — a genuine fire risk.
- Age and brittleness. Decades-old insulation on the conductors can crack and crumble, exposing bare wire.
- Amateur modifications. Over 80+ years, K&T is often spliced into improperly, which compounds the risk.
- Insurance and sales. Many insurers won’t write or renew a policy on a home with active K&T, and it routinely comes up in home inspections.
How Is Knob and Tube Wiring Replaced?
“Replacing” K&T means running new grounded cable to the affected circuits and retiring the old wiring. In practice, a licensed electrician will:
- Assess and map which circuits are still live K&T versus already replaced.
- Run new grounded wiring to outlets, switches and fixtures, fishing it through walls and ceilings to limit damage.
- Update devices and boxes to grounded, code-compliant outlets and junction boxes.
- Disconnect and abandon the old K&T safely so nothing remains energized.
- Permit and inspect the work so it’s documented and code-compliant.
How disruptive it is depends on access. Homes with an open basement and attic are easier; fully finished walls take more care. A good electrician plans the runs to keep drywall repair to a minimum.
How Much Does It Cost to Replace Knob and Tube Wiring?
Cost depends almost entirely on how much K&T is left and how accessible it is. Replacing a couple of circuits in an open basement is a modest job; rewiring a whole finished home is a larger project closer to a whole-home rewire. Because it’s so dependent on your specific house, the only honest cost is one given after a walkthrough — which is exactly what we provide. If you’re ready, Wescott Electrical also handles knob and tube removal across the area.
FAQs
Knob and tube was phased out through the 1940s and 1950s as grounded sheathed cable became standard. Homes built before about 1950 that were never fully rewired may still have some in place.
K&T that is undisturbed, in good condition and not buried in insulation can still function, but it has no ground, becomes brittle with age, and is a fire risk when covered by insulation or modified improperly. Most experts and insurers recommend replacing active K&T.
Not necessarily. An electrician can prioritize the highest-risk circuits first. That said, insurers often want active K&T fully removed, so it’s worth understanding your goal before deciding.
Many will. A number of insurers won’t issue or renew coverage on a home with active knob and tube, or will require it be replaced. If your insurer has flagged it, a licensed electrician can document the replacement.
No — K&T was installed without a grounding conductor, and you can’t safely add a ground to it. The correct fix is to replace the circuit with modern grounded wiring.