Knob and tube wiring was the standard method of electrical wiring in homes and other buildings in North America from about 1880 to the 1940s. Although obsolete today, this little-known system powered residences and businesses for many decades.

What Is Knob and Tube Wiring?

Knob and tube (K&T) is an early electrical wiring system that consists of insulated copper conductors passing through ceramic knobs and tubes as they run through structural framing.

This method leaves space between wires and building materials to prevent contact that could lead to fires. It also allows air circulation to dissipate heat.

K&T wires do not sit inside metal conduits or flexible sheathing like more modern wiring systems. The wiring is open to the air, similar to a modern surface-mounted wiring system.

When Was Knob and Tube Wiring Used?

Knob and tube wiring was prevalent from about 1880 to the 1940s. It was one of the earliest standardized methods of electrical wiring used in buildings.

However, some homes today still contain legacy knob and tube wiring over 100 years old. This can pose safety hazards due to the wiring's age and obsolete design.

How Does Knob and Tube Wiring Work?

The knob and tube system contains only two main components - the ceramic knobs and tubes:

Wires are either single copper conductors with rubber insulation or "twinned" pairs wrapped in cloth insulation. They connect back to porcelain knob-mounted fuse boxes.

Since wires are not grounded, knob and tube relies on air space for insulation. Wires also can't contact metal framing, which acts as a heat sink.

Why Was Knob and Tube Wiring So Widely Used?

Knob and tube wiring had certain advantages that made it the standard for decades:

So while very outdated today, knob and tube served a valuable role in bringing electricity into North American homes before newer wiring standards evolved.

Why Is Knob and Tube Wiring Problematic Today?

While knob and tube was sufficient for early 20th century homes, it can be dangerous in modern households:

For these reasons, knob and tube wiring requires very careful upgrades or complete removal in modernized homes. Never take risks with outdated electrical systems.

Signs Your Home Still Has Knob and Tube Wiring

If your home is over 50 years old, there's a chance it still contains knob and tube wiring. Signs include:

If in doubt, have an electrician inspect the property and advise you on any needed repairs or replacements. Never attempt DIY electrical work on a knob and tube system.

Is Knob and Tube Wiring Grounds for Rejecting a Home Purchase?

The discovery of knob and tube wiring is often enough to make buyers walk away from an older home purchase. K&T can be prohibitively expensive to replace. Other key risks include:

However, minor knob and tube wiring may be acceptable if an insurer is willing to cover it. Consider getting quotes before canceling a purchase. Also have an electrician evaluate required updates to see if they are manageable.

How Is Knob and Tube Wiring Upgraded or Replaced?

K&T wiring should eventually be completely removed and replaced in most homes. Upgrades may involve:

This can be an extensive, labor-intensive process requiring access to all structural bays. Complete replacement often costs $8,000-$15,000.

Some electricians may use workarounds like running modern NM cables next to old wires inside tubes. But a full overhaul is recommended for safety. Never take shortcuts.

Can I Do Anything to Make My Knob and Tube Wiring Safer?

If knob and tube wiring removal is not immediately possible, some precautions can help minimize risks:

However, these are temporary measures only. Replacement should still occur as soon as feasible.


While antiquated and even dangerous today, knob and tube wiring powered American homes for over 50 years. Millions of structures still contain remnants of this early system. Homeowners and buyers should be aware of risks posed by K&T and have it thoroughly inspected and upgraded or replaced. With proper maintenance, knob and tube can be made reasonably safe, but complete overhaul provides maximum safety.