Knob and Tube Wiring in Older Philadelphia Homes: Risks, Insurance & What Rewiring Costs
A lot of older homes across Philadelphia and Montgomery and Bucks Counties still have some knob and tube wiring. Here’s how to spot it, why it’s a risk today, why insurers ask about it, and what decides the cost of replacing it.
Philadelphia is full of homes built long before modern wiring: rowhomes and twins in the city, and stone and Victorian houses in Jenkintown, Abington, Elkins Park and Glenside. Many of them still have original knob and tube wiring somewhere in the walls, attic or basement, often spliced into newer wiring from later renovations. It isn’t automatically an emergency, but if you own an older home it’s worth understanding. 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 into the 1940s. 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 knobs and tubes kept the insulated conductors suspended in air so heat could escape. That’s the key detail: K&T was designed to be surrounded by air, not modern insulation, and it has no ground wire. Both facts are why it causes problems in a modern home.
Ceramic tube protects the wire where it passes through a joist or stud.
Ceramic knob holds the wire away from the wood along its run.
Hot and neutral run separately, inches apart, so heat escapes into open air.
No ground wire. Modern cable has a third conductor; K&T never did.
How to Tell If Your Home Has Knob and Tube Wiring
You can often spot the signs yourself, but only an electrician can tell you how much is still live. Look for:
Ceramic knobs and tubes
White porcelain fittings in an unfinished basement or attic, with single wires running between them a few inches apart.
Cloth- or rubber-covered wire
Fabric or rubber insulation instead of the plastic-sheathed cable used today.
Two-prong outlets
Not proof of K&T on their own, but they mean those circuits have no ground.
A fuse box
Screw-in fuses instead of breakers, or an old panel with only a few circuits feeding the original house.
Built before about 1950
Homes from that era that have never been fully rewired are the likeliest to have some.
An inspection report
A home inspector noted knob and tube, or said some wiring couldn’t be traced.
Don’t pull at old wiring to check it
Wiring that looks abandoned is sometimes still energized, and brittle insulation can crumble when it’s disturbed. An electrician can test each circuit and map which ones are still K&T.
When Was Knob and Tube Wiring Discontinued?
K&T was phased out through the 1940s and 1950s as sheathed, grounded cable (the ancestor of today’s Romex) became standard. So a home built before about 1950 that hasn’t been fully rewired may still have some in place.
Why Knob and Tube Wiring Is a Risk Today
- 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 doesn’t mix with insulation. When insulation is blown into attics and walls, it traps heat around wiring that was designed to breathe. That is a genuine fire risk.
- Age and brittleness. Insulation on the conductors that is many decades old can crack and fall away, leaving bare wire.
- Overloading. K&T was installed for a few lights and small appliances, not space heaters, window AC units and today’s kitchens. A common problem is someone fitting a larger fuse so it stops blowing, which lets the wire carry more current than it was meant to.
- Amateur modifications. After 80-plus years, K&T has often been spliced into by past owners, and those connections are where problems tend to start.
Knob and Tube Wiring and Home Insurance
Insurance is often what makes people deal with K&T. Some insurers ask about it when you apply for or renew a homeowners policy. Depending on the company, active knob and tube may mean a higher premium, a request for an electrician’s inspection, a requirement to replace it, or a declined policy. Rules vary from insurer to insurer, so the only reliable answer for your home comes from your own insurer or agent.
It also comes up when you buy or sell. Home inspectors routinely note K&T, and buyers sometimes find out about it when they try to insure the house. If your insurer has asked about it, a licensed electrician can assess the wiring and document any replacement work.
Questions to ask your insurer before work starts
- Do you need all active knob and tube removed, or only certain circuits?
- What paperwork do you need: an electrician’s letter, the permit, the final inspection?
- Is there a deadline for the work to be finished?
This is general information, not insurance advice.
Partial Replacement or a Whole-Home Rewire?
Replacing knob and tube doesn’t always mean rewiring the whole house. There are two broad approaches:
Partial replacement
Find the remaining K&T circuits and replace just those.
- Earlier renovations already replaced most of the wiring
- Only a few circuits are left
- The rest of the system is in good shape
Whole-home rewire
Replace everything in one planned whole-home rewire.
- K&T still feeds much of the house
- There’s a mix of old wiring types
- You’re renovating and the walls are already open
Either way, check the panel too. Older homes with K&T often have a fuse box or an undersized panel, and new circuits may need a service panel upgrade to go with them. If your insurer has asked for K&T to be removed, find out whether they need all of it gone before you choose a partial job.
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
Find which circuits are still live K&T and which were already replaced.
Run new wiring
Fish grounded cable through walls and ceilings to limit damage.
Update devices
Fit grounded, code-compliant outlets, switches and boxes.
Retire the old
Disconnect the K&T safely so nothing is left energized.
Permit and inspect
The work is permitted and inspected, so it’s documented.
How Much Does It Cost to Rewire a House?
There’s no honest flat price for rewiring a house, because the cost depends on the house. These are the things that move the number most:
| Cost factor | Keeps the cost lower | Pushes the cost higher |
|---|---|---|
| How much K&T is left | A few circuits in one area | Most of the house still on original wiring |
| Size and layout | Smaller home, fewer floors | More rooms, floors and circuits |
| Access | Open basement and attic | Finished plaster-and-lath walls |
| The panel | Modern panel with room to spare | Fuse box or full panel that needs upgrading |
| What turns up | Clean, traceable runs | Hidden splices or buried junction boxes |
Two more things to ask about when you compare quotes. Wall repairs: small openings are usually needed to fish new wire, so check whether patching is included or left for your contractor. Permits and inspection: proper work is permitted and inspected, and that is part of the job. If the panel needs replacing too, our guide to electrical panel upgrade cost explains what drives that part.
The only reliable figure is one given after someone has looked at your house. Wescott Electrical offers a free consultation and quote for knob and tube removal across Montgomery County, Bucks County and Philadelphia. We’ll tell you how much K&T you have and what it will take to replace it.
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 work, but it has no ground, becomes brittle with age, and is a fire risk when covered by insulation, overloaded or modified badly. Most electricians recommend replacing any K&T that is still live.
You can’t tell by looking. Some K&T that appears abandoned is still connected. A licensed electrician can test each circuit and map what is still in use.
Not necessarily. An electrician can replace the remaining K&T circuits first. But some insurers want all active K&T removed, so check what your insurer needs before deciding.
It depends on the insurer. Some will ask for an inspection, charge more or require replacement, and some may decline coverage. Ask your insurer or agent what they need. A licensed electrician can assess the wiring and document any replacement.
It depends on how much old wiring is left, the size of the home, how easy it is to reach the walls and ceilings, and whether the panel needs upgrading. A walkthrough is the only way to get an accurate quote.
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.