An Introduction to Knob and Tube Wiring
Knob and tube wiring was commonly used in homes built in the early 1900s up until the 1940s. This type of wiring consists of individual wires that are run through ceramic knobs and tubes, usually in between walls and ceilings. While knob and tube wiring was safe when it was first installed, it often does not meet today's electrical code requirements and can pose a safety hazard. However, many homeowners with vintage homes want to preserve the original knob and tube wiring for historical accuracy. This article will explore how I rewired my 1920s home while keeping the existing knob and tube wiring intact.
Reasons to Rewire With New Wiring
There are several important reasons to rewire a home with new wiring while preserving the original knob and tube wiring:
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Insufficient capacity - Knob and tube wiring often does not have enough capacity to handle the electrical load required by modern appliances and devices. This can lead to overloaded circuits, blown fuses, and the potential for fires.
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Unsafe materials - The old insulation on knob and tube wiring becomes brittle over time, leading to exposed live wires. The wiring itself is often made of materials that do not meet today's safety standards.
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No grounding - Knob and tube wiring systems generally do not include a ground wire, which protects you from shocks and allows modern GFCI outlets to work properly.
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Insurance difficulties - Many insurance companies will not insure a home with knob and tube wiring, so upgrading the wiring makes your home more insurable.
Pre-Rewire Inspection and Planning
Before starting any rewiring project, it is important to do thorough inspection and planning:
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Inspect the existing knob and tube wiring to see where it is run inside walls, ceilings, and floors. Look for hazards like damaged or exposed wiring.
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Consider the layout of your home and where you may want additional lighting, switches, or outlets. Plan the optimal wiring routes.
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Identify which circuits supply power to each part of the home so you can replicate the circuits with the new wiring. Label the circuits at the main panel.
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Take photos of the existing wiring to reference later during the rewiring.
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Obtain all necessary permits for the rewiring project.
Running New Wiring Next to Old Knob and Tube
The easiest way to rewire while preserving knob and tube is to run the new wiring alongside the old. Here is how to do it:
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Cut access holes between studs to route the new wiring through walls and ceilings. Keep these holes strategically placed near existing knob and tube to limit damage.
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When drilling into lath and plaster walls, use a special plaster bit to avoid excessive cracking and crumbling of plaster.
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Run flexible NM cables from the panel to each outlet and switch location, keeping the new cables grouped near the existing knob and tube. Use cable staples to secure.
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Use junction boxes to split off wire runs to multiple locations and to join existing and new wiring. Follow codes for size and placement.
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Connect the new wiring to the existing breakers or fuses using pigtails. Label each new circuit.
Repurposing Existing Knob and Tube Wiring
For certain short wiring runs, you may be able to carefully remove the old knob and tube wiring and install new wiring in its place:
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Verify the existing wiring is viable to reuse by checking insulation integrity and wire gauge size.
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Remove any asbestos on old wiring using proper hazardous material procedures.
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Run new NM cables through original porcelain knobs to maintain aesthetic while improving safety.
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Consider reusing original push-button switches and sockets with adapters so only the wiring itself is new.
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Take advantage of accessible runs through basements, crawlspaces, and attics where knob and tube replacement is easier.
Matching Old and New Wiring Aesthetically
To create a seamless look, choose new wiring components that match old ones in style:
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Seek period-accurate light switches, wall plates, and light fixtures, or recreate vintage pieces.
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Use cloth-wrapped wiring and ceramic wire nuts to mimic the look of old cloth wiring at junction points.
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Paint exposed conduit to match wall and ceiling colors for minimized visibility.
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Select reproduction or vintage-style receptacles and switchplates appropriate for the era of your home.
Maintaining Safety With Old Wiring
It is crucial to maintain safety when old wiring remains in place:
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Completely disconnect unused knob and tube circuits at the panel and cap wires.
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Label all live knob and tube wiring as "unsafe" at access points like the attic or basement.
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Use GFCI outlets or breakers anywhere old wiring connects to new, especially in kitchens, bathrooms and outdoors.
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Have wiring inspected by an electrician periodically to identify deterioration or hazards.
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Check that any remaining knob and tube circuits have proper overcurrent protection based on wire size.
When to Remove Knob and Tube Entirely
In some cases, completely removing all old wiring may be necessary:
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If the knob and tube wiring is so deteriorated or modified that it is unstable or hazardous.
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When doing extensive remodeling that requires removing walls and ceilings where wiring is run.
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If your local building codes do not allow any knob and tube to remain in place.
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For insurance purposes if your provider will not insure the home at all with any active knob and tube present.
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If you need to increase the home's electrical capacity - old wiring limits may be too small for today's needs.
Conclusion
With careful planning and execution, rewiring a 1920s home without harming original knob and tube wiring is definitely achievable. The most effective approach is running new wiring alongside the old, taking advantage of accessible spaces like basements and attics when possible. Maintaining safety with the existing wiring should always be the top priority. With some upgrades to electrical capacity and safety, while keeping beloved vintage wiring aesthetics intact, you can give your historical home the best of both worlds.