Knob & Tubing Wiring
Safety Tips from Breton Electric
Over the past few years, we've
received an increasing number of calls from owners of older homes who have
questions about knob & tube wiring. While we have always had calls from
people concerned about the safety of the wiring or about upgrading the
electrical systems in their older homes, many of the calls are now from
prospective buyers or their real estate agents.
the interest these days? It is because folks are discovering that an
increasing number of homeowners insurance underwriters are refusing to cover
homes that have existing knob & tube wiring, or will do so
at a significantly higher rate.
Obviously, the insurance
industry is concerned about the safety of knob & tube wiring. But, is it
really so dangerous? And, what is knob & tube wiring, anyway? Home Inspector
William Kibbel III presents a good explanation on The Old House Web (www.oldhouseweb.com),
from which the following is adapted, with some additions of more accurate
Basically, knob & tube is a wiring system, popular through the mid 20th
century, that uses porcelain insulators (knobs) for running wires through
unobstructed spaces and porcelain tubes to protect wires running through
studs and joists. While safe if properly installed and maintained, there are
two main drawbacks to knob & tube systems. 1) They don't have an equipment
grounding conductor, thus grounded (3-prong) outlets cannot be installed and
2) fuses and switches were often placed on the neutral wire. This means that
removing a fuse or shutting off a switch does not turn off the voltage
throughout the circuit.
There are also a number of
factors due to the age of these systems, some over 80 years old, that can
cause serious safety hazards.
As household power needs grew
over the years, alterations and additions to original systems by
well-meaning but unqualified people overloaded the original fuses. The easy
solution was to install larger fuses. Unfortunately, larger fuses allow more
current to flow through the system than was originally intended, leading to
more heat in the conductors. This heat causes the insulation on the wire to
become brittle and eventually disintegrate.
Heat directly above ceiling
lights and in unvented attics can also degrade the wire insulation. In
addition, some types of insulation used on knob & tube wiring seem to be a
“delicacy” for the critters that find their way into old homes. They can
make short work of the insulation covering the wires.
concern is insulation on top of knob & tube wiring. This is a major fire
hazard. One of the safety features of knob & tube is that the porcelain
knobs suspend the wire in open air to dissipate heat. Loose and
rolled insulation counteracts the original “open air” installation of knob &
The bottom line is this: If
you have an older home and are not sure if the wiring is safe, have it
checked by a licensed electrician, whether or not you are planning to sell
or change your homeowners insurance company. Any necessary rewiring could
entail a major investment, but an electrician with in-depth knowledge of old
homes can determine the most cost efficient means of ensuring the safety of
you and your family and maintaining the insurability of your home.
Breton Electric is an electrical service
and contract company based in Wakefield MA. For additional safety tips,
information about reducing energy costs, how to recycle mercury-containing
bulbs, and more, see
or call 781-245-0787.