How to Replace Old Knob and Tube Wiring

What is Knob and Tube Wiring?

Knob and tube wiring was an early standardized method of electrical wiring used in buildings in the United States from about 1880 to the 1940s. It consists of single insulated copper conductors run within wall cavities, passing through joist and stud drill-holes via protective porcelain insulating tubes, and supported on nailed-down porcelain knob insulators.

This old type of wiring has some major drawbacks compared to modern electrical systems:

Fire hazard - the wiring is not protected from physical damage and the insulation can become brittle over time. This leaves it prone to short circuits and overheating.
Insufficient for modern loads - knob and tube was not designed to handle the amount of electricity used by modern appliances and devices. It poses a fire and shock risk if overloaded.
Difficult to upgrade - the wiring is not easily accessible and upgrading or repairing it is labor intensive. Adding grounded outlets or additional circuits is extremely difficult.

For these reasons, knob and tube wiring is considered obsolete and dangerous by modern standards. Most electrical codes require it to be replaced in any homes where it still exists.

Why Replace Knob and Tube Wiring?

There are several important reasons to replace knob and tube wiring in your home:

Safety - As mentioned above, knob and tube poses serious fire and shock hazards due to deterioration and insufficient capacity. Replacing it eliminates these risks.
Reliability - The outdated wiring can cause flickering lights, poor receptacle connections, and other annoying issues. Upgrading provides steady, dependable power throughout your home.
Flexibility - New wiring allows you to conveniently add electrical circuits and outlets wherever you need them. With knob and tube, this is difficult or impossible.
Resale value - Homes with modern wiring and electrical panels are much more attractive to buyers. Removing old systems like knob and tube improves salability.
Insurance - Many insurance companies won't issue policies for homes with knob and tube due to the fire risk. Upgrading is often required to get coverage.
Peace of mind - Eliminating the safety hazards of very old electrical work lets you rest easy knowing your home's systems are up-to-date and secure.

How to Find Knob and Tube Wiring in Your Home

If your home is over 50 years old, it may contain some original knob and tube wiring. Here are some ways to check if it's present:

Examine the electrical panel - Clear signs include fuse boxes,minimal circuits, old-fashioned glass or porcelain fuses, and two-prong ungrounded outlets.
Look in the attic or basement - You may see the distinctive porcelain knobs, tubes, and unraveling black wire running across joists.
Check for outlets on the ceiling or high on walls - These were needed to provide lighting power before wiring reached all areas.
Consider two-prong outlets - While not definitive, these often indicate knob and tube branches.
Review your home inspection report if available - This should note any obsolete electrical systems.
Hire an electrician to thoroughly inspect - They can methodically locate and assess all old wiring.

When to Call a Professional

I highly recommend having a licensed electrician inspect your home for knob and tube if there's any chance it exists. They can safely confirm if and where it's present.

Once found, always hire an electrician to perform the replacement work. The risks of DIY are too high with outdated electrical wiring. The project also often involves significant changes to wires within walls and the main electrical panel. Professionals have the expertise to handle this delicate process.

How Knob and Tube Wiring is Replaced

Replacing knob and tube wiring completely revamps the electrical system with modern wires, outlets, switches, and safety features. The general process involves:

1. Full Inspection and Planning

Electrician inspects attic, basement, walls for all K&T wiring.
Maps out new circuit layouts and runs needed.
Plans approval process and required permits/inspections.

2. Install New Electrical Panel

Removes old fuse box and installs modern breaker panel.
Often increase amperage and number of breaker circuits.

3. Run New Wiring and Outlets

Fishes modern NM cables through walls and ceilings.
Removes old knobs and tubes, replaces with new wires.
Adds grounded outlets and switches throughout home.

4. Connect Devices and Fixtures

Links all lights, appliances, devices to new wires and breakers.
Installs safety features like AFCIs and GFCIs.
Verifies functionality of all electrical items.

5. Remove Old Components

Disconnects and removes old knob and tube wires.
Takes down any obsolete distribution boxes.
Patches all holes/openings.

Cost to Replace Knob and Tube Wiring

The cost to replace knob and tube wiring in a home generally ranges from $8,000 to $15,000. However, prices can vary dramatically based on:

Home size - More wiring required in larger houses.
Accessibility - Easy attic access lowers costs vs difficult crawlspaces.
Wiring scope - Whole home replacements cost more than partial upgrades.
Electrical panel - Upgrading an old fuse box adds expense.
Conduit needs - Running new conduit for wiring adds labor costs.
Permit fees - These vary by municipality.
Electrician rates - Pricing and work time affect overall costs.

To determine the investment for your specific project, get quotes from licensed electricians. Be sure to get at least 3 bids for the best deal.

Is Knob and Tube Wiring Dangerous?

The short answer is yes, knob and tube wiring is considered dangerous by today's electrical standards for several reasons:

No grounding - Lack of ground wires or grounded outlets raises shock risk.
Overloading - Can't safely handle electricity demands of modern devices and appliances.
Exposure - Open wiring in walls and attics risks damage from humidity, pests, friction.
Loose connections - Outlets and light fixtures may heat up and start fires over time.
Old insulation - Cracked, brittle covering on wires poses serious fire hazard.

While historic, knob and tube does not provide the safety, capacity, or reliability of modern electrical systems. Homeowners should have it inspected and replaced as soon as possible.

Electrical Code on Knob and Tube Wiring

Most modern electrical codes consider knob and tube wiring unacceptable and require upgrades. For example:

The National Electrical Code prohibits any modifications or extensions to existing K&T. Any alterations require full replacement.
HUD mandates K&T replacement in order to qualify for government insured loans.
Many municipalities do not allow K&T and require upgrades upon sale of a property.
Most insurance companies refuse policies on homes with K&T wiring due to fire risk.

Always check your local electrical code for the exact rules in your area. But in general, regulators and officials consider knob and tube wiring obsolete and hazardous due to its limitations.

Is It OK to Leave Some Knob and Tube Wiring?

Electricians do not recommend leaving any original knob and tube wiring in place. Some reasons it should always be fully replaced:

Remaining K&T branches can overload when other wiring gets upgraded.
It's impossible to selectively upgrade parts of a circuits.
Leaving any in walls risks exposure damage and deterioration over time.
Partial upgrades cause inconsistencies with modern wires and breakers.
Can still fail inspection and prevent insurance coverage.
Creates liability if problems occur down the road after upgrade.

While full K&T replacement is expensive, experts strongly advise against any shortcuts. Removing all the outdated wiring is by far the safest option.

In Conclusion

Knob and tube wiring is too dangerous and insufficient for modern electrical loads. Homeowners should have their system evaluated by a professional electrician. In most cases, completely replacing K&T with updated wiring, outlets, and an electrical panel is required for safety. Though not a cheap project, the investment protects your home and provides peace of mind.