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 passing through holes in joists and studs and supported by ceramic knobs attached to the structural members. The knob served to keep the wire away from the lumber and to provide air space for cooling. Where wires entered a wiring device, such as a lamp or switch, or were pulled into a wall, they were protected by flexible cloth or rubber insulation called loom.
The two or more conductors were separated from physical contact with each other by approximately one inch of air space. This air gap helped to dissipate heat generated by the conductors, reducing the risk of fire. Since air was free to circulate over the wires, smaller conductors could be used than required in cables. By inserting porcelain tubes or "bushings" through joists, wires could pass without coming into contact with the lumber. Loops allowed for expansion and contraction of conductors due to temperature changes.
Key characteristics:
- Uses individual insulated copper conductors, run separately
- Conductors are supported along their length by ceramic knobs
- Air space between conductors serves as insulation
- Open wiring through walls and ceilings with porcelain tubes at joists
Is Knob and Tube Wiring Dangerous?
Knob and tube wiring that is in good condition and has not been altered or modified can be safe to use. However, there are some potential concerns, including:
- Overloaded circuits - K&T was designed for lower power consumption than today's homes. Connecting multiple devices can overload the wires.
- Insulation breakdown - The old cloth or rubber insulation can become brittle and crack over time, exposing bare wires.
- Unsafe modifications - Improper splicing, taps, or wires added without overcurrent protection can be fire hazards.
- Exposed wiring - Damage to plaster and lath exposes bare conductors, risking shocks or shorts.
- Lack of grounding - K&T has no equipment grounding conductor, increasing risk of shocks.
So while K&T can still function safely in some cases, deterioration or improper alterations over the decades can make it potentially hazardous. Any homes with knob and tube wiring should have it thoroughly inspected by an electrician before renovation.
Why Rewire Homes With Knob and Tube Wiring?
There are several compelling reasons you may want to rewire a home rather than leaving old knob and tube wiring in place:
- Safety - Upgrading provides modern safety features like grounding and arc fault/GFCI breakers.
- Reliability - New wiring eliminates worries about overloaded, damaged, or spliced K&T.
- Expandability - It allows adding circuits without overloading existing ones. New wiring methods make extending circuits easier.
- Efficiency - Improved wiring enables using modern high efficiency appliances and electronics without fire risk or power issues.
- Insurance - Some insurers won't cover homes with K&T or charge substantially higher premiums. Rewiring may qualify for lower rates.
- Resale value - Rewiring makes a home more attractive to buyers. Many won't consider purchasing a home with K&T due to safety concerns.
In summary, rewiring eliminates the fire, shock, and power reliability risks of old knob and tube wiring. The peace of mind and improved home function are worth the investment.
How to Rewire a House With Knob and Tube
Rewiring a house replaces old dangerous wiring with safe, reliable, modern wire meeting electrical code. The basic process includes:
Preparation
- Have an electrician inspect the existing wiring to identify all circuits, devices, and any issues.
- Create a rewiring plan - which parts of the house to rewire in what stages. Prioritize the most urgent electrical needs first.
- Obtain necessary permits from the building department.
- Purchase the new wiring and electrical hardware - breakers, receptacles, switches, etc.
Wiring Installation
- Run new cables through walls and ceilings between the main panel, devices, and fixtures. Use non-metallic sheathed cable (NM, aka Romex).
- Remove K&T wiring from overloaded or dangerous circuits first. Temporarily wire critical circuits like furnace or fridge to new wiring.
- Remove old wiring from walls and ceilings as new wiring makes circuits obsolete. Cap abandoned wires properly.
- Install updated load center breaker panel sized for home's needs. Create separate circuits for bedrooms, bathrooms, kitchen, laundry, A/C units, etc.
- Connect devices and fixtures to new wiring and confirm proper function.
Finishing Touches
- Update outlets and switches. Install tamper resistant receptacles.
- Install hardwired smoke and CO detectors.
- Have work inspected by the local building department. Correct any issues.
- Remove abandoned knob and tube wiring that is now unused.
- Add GFCI protection to receptacles near water sources.
- Ensure all accessible old wiring is made safe by removal or capping ends.
Safety should be the top priority during a knob and tube rewiring project. Always turn off power at the main breaker panel before working. Follow all electrical codes and permit requirements. Consider having an electrician do the more complex aspects of the rewiring.
Typical Cost to Rewire an Old House
The cost to fully rewire an older home with knob and tube wiring averages $8,000 to $15,000. However, costs can vary widely based on factors like:
- Home size - More square footage means more wiring required
- Home layout complexity - Simple designs are less labor intensive
- Accessibility - Ease of running new wires through walls, attic, basement
- Geographic location - Labor and material costs vary regionally
- Scope of rewiring - Just critical circuits vs whole home rewire
- Level of finish detail - Extra costs for fully concealing new wires
- DIY vs hiring an electrician - Contractor costs average $65-$100 per hour
Rewiring a 1000 sq ft house yourself might cost $2,000-$4,000 for materials like breakers, receptacles, and NM cables. Hiring an electrician could run $6,000-$10,000 including labor for a full rewire.
Critical needs rewiring of the most essential electrical circuits could potentially cost $2,000-$6,000, but leaves hazards. A whole home rewire is recommended for safety, function, and value.
Get quotes from at least 3 local electricians to find the best deal. Many provide free estimates to evaluate your specific needs.
Tips for Rewiring an Old House With Knob and Tube
- Label each circuit at the panel and all devices so you know what's connected where
- Take photos of existing wiring before removing to reference later
- Only open walls/ceilings where essential to limit repairs needed
- Fish wires or use existing holes rather than cutting unnecessary access holes
- Use step drill bits to bore holes through joists and studs
- Run all home runs (hot wires) through a new main circuit breaker panel
- Don't put more than 8 outlet circuits on a 15A breaker to avoid overloads
- Use junction boxes to split circuits as needed, not random splices
- Seal around all wire penetrations to prevent fire spread and drafts
- Use nail guards to protect cable near framing members
- Connect only one wire per screw terminal and pre-twist ends
- Group neutral wires from separate circuits when splicing in boxes
- Confirm proper polarity of hots, neutrals, and grounds everywhere
Work safely and get all required permits and inspections. The effort to rewire an old knob and tube house is worth it for safety and peace of mind.