What is Knob and Tube?
K&T (Knob and Tube) wiring was the first widely-installed type of electrical wiring for residential homes. It was used until 1950. Insurance companies consider K&T to be high-risk for a number of reasons.
Why Do Insurers Consider it High-risk?
Knob and Tube is considered obsolete. Here are a few reasons why K&T systems may be unsafe:
Age
Sheathing and connections may deteriorate over time making them more vulnerable to arcing, shock, and damage.
Insulation
K&T was never designed to be covered by insulation. The tubes were designed to be in the open air to dissipate heat. Covering them with insulation increases the risk of overheating and fire.
No ground wire
K&T has no ground wire. Because of this, it is more susceptible to shock in the event of a ground fault, electrical fire, and it cannot service any 3-prong receptacles.
Modified
K&T required more skill to install or update correctly than modern electrical wiring. Since it has not been used for a long time, most electricians have limited or no experience working with it.
Added onto
Homeowners sometimes improperly splice new wires onto an existing K&T system. For many reasons, modern wires and K&T installations are not compatible, and existing K&T wiring should not be added onto.
Inadequate service size
60A service was common during this era, which is often too small for modern needs. K&T systems may not be large enough to run modern appliances, and were sometimes over-fused by frustrated homeowners trying to avoid nuisance fuse-blowing.
Do I Have it in my Home?
If your house was built before 1950, it is possible that K&T is still in service in the home. Often, accessible parts of the system (in attics or unfinished ceilings in basements) were upgraded over time, while inaccessible parts (inside walls) were not. The most common clue that the house had K&T is the presence of the old porcelain knobs, which were often left in place when the system was upgraded.
Original porcelain knobs left in place. They look kind of like marshmallows!
What Can I Do?
If you suspect (or know) you might have Knob and Tube wiring, engage an electrician to assess the system. K&T might be an insurance issue, so you should check with your insurer about whether they will issue coverage for your system. Do not add onto or modify existing wires, or cover the wiring with insulation.
Key Points
Knob and Tube was installed until 1950.
It is considered obsolete.
It can be unsafe, and may be an insurance issue.
If you have it, get the system assessed by an electrician.
Do not add onto or modify any K&T wiring you find.