Knob and tube wiring gets its name from the ceramic "knobs" used to keep the wires isolated from objects and the wood
joists the wires run along, and the ceramic "tubes" that were used to isolate the wires from the wood joists that they ran
through. Knob and tube wiring was installed from the early 1900's until the late 1940's and in some instances into the
In a "typical" residential house circuit (general usage receptacles and lighting), the difference between knob and
tube wiring and the "modern" wiring used since the early 1960's, is that knob and tube wiring consisted of a black (hot) and
white (neutral) wire. Both wires ran separately to receptacles, switches and fixtures whereas the wiring used today
consists of a black (hot) and white (neutral) wire plus a ground wire, all contained inside an outer sheathing. Since
1979, and some incarnations before, the sheathing has been made of plastic. Today's wiring is mostly gauge 14 wire that is
to be protected by a 15 amp fuse or breaker, whereas knob and tube wiring could be a mixture of gauge 14 wire or
gauge 12 wire (capable of handling 20 amps) but is to be protected by a 15 amp fuse or breaker, and while being separated
and run as single wires actually run cooler than today's wiring.
In some of the homes built in the early 1900's there are no outlet boxes at fixture locations. Outlet boxes can
be added with minimal damage to plaster.
Due to the usage of high wattage light bulbs in fixtures that were designed only to handle 25-40 watt light bulbs, the
heat from the light bulbs could make the insulation on the knob and tube wiring become brittle and in some cases fail, leaving
bare wires at the fixture. This could result in a fixture becoming "live" or the two wires touching each other causing
sparks and therefore a blown circuit, or much worse a fire. When this occurs it is usually at an totally enclosed fixture
that has a very low profile. This conditions is seldom evident at a ceiling location where the fixtures hung down
from the ceiling. This condition is seldom at wall sconce locations either. Whereas wiring manufactured since
1986 is rated at 90 degrees Celsius, knob and tube wiring never had a heat rating. Wiring through the late 1950's to
the late 1970's had heat ratings of 60 degrees Celsius and wiring from the late 1970's up to 1986 had heat ratings of
75 degrees Celsius. It is possible to put new insulation on the knob and tube wires and then cover them with a
thermoplastic covering rated at 90 to 110 degrees Celsius. At some attic locations we are able to remove the wiring
back to where there is no damage, install a proper junction box and refeed the fixture with new wiring.
Knob and tube wiring is ungrounded and is connected to recptacles that have only two slots or openings.
Today you can only purchase receptacles that have a third hole. that being for the ground pin on a grounded appliance
or tool. It is illegal to connect a grounding type receptacle onto ungrounded wiring (even if the ungrounded wiring
is not knob and tube). Rule #26-700(8) in the Ontario Electrical Code allows the installation of a grounding type receptacle
that has an integral Ground Fault Circuit Interrupter (GFCI) onto ungrounded wiring. This type of receptacle will shut
down if it detects a leakage of current to the wire connected to the ground hole.
**Never cut off a ground pin from the cord of any appliance or tool. Not only is it unsafe and can cause
a serious shock hazard but the appliance or tool has been altered and is no longer CSA approved.**
Whereas most knob and tube wiring that runs through covered walls, ceilings and floors, and in those that have not been
tampered with, the wiring is more likely to be as good as the day it was installed. The area's that the wiring was accessible
to the "do it yourselfer or amateur" is in attics and basements. Because of this, we see lots of cases of incorrect
and often dangerous alterations or additions. Qualified electricians can make the necessary repairs, corrections or
replacements (if deemed necessary) to bring the wiring back to a safe condition.
The one thing we can't know is if there is any dubious wiring or connections inside of walls that have been altered.
Houses that have knob and tube wiring also have those wires protected by fuses (unless the service panel has already
been replaced with a breaker panel), thus being vulnerable to over fusing, which makes the wires run "hot" and make connections
along the system susceptible to breakdown, or worse a fire. The elimination of the fuses and installation of breakers
can help solve the problem.
What do you do?
More and more insurance companies are refusing to insure homes with knob and tube wiring. The first calls we received
in regards to this was as far back as 1993.
The Electrical Safety Authority (formerly Ontario Hydro) will send an inspector out to your home and for the approximate
cost of $125.00 to perform a Non Hazardous General Inspection for Insurance Purposes. If your house passes inspection
you will receive a certificate. If not, they will produce a list of defects that you will have to have corrected, or
additions that they deem necessary. Once these corrections have been made by a qualified electrician the inspector will
come back and re-inspect. Upon approval you will receive a certificate stating that all safety hazards have been removed.
The costs associated with these types of repairs are determined by the size of your house and naturally the condition the
wiring is in and how much updating has been previously done. Typically the costs run between $400.00 - $2000.00, plus
re-inspection fee and GST.
Unfortunately not all insurance companies are interested in an inspection, but rather insist on complete replacement
of the knob and tube wiring. Rewiring a house with today's standard (NMD) wiring is much a problem of access.
If your house has an unfinished basement we can rewire most of the main floor receptacles and feeds to the switches from below.
In a bungalow style house we can access the lighting fixtures and switches from above in the attic. If your house has
two or more stories the problem is that there is no access to the first floor light fixtures and switches without opening
the walls and ceilings. This is the same problem for any room with a finished floor or a flat roof above.
The costs associated with these types of repairs are determined by the size of your house and naturally how much updating
has been previously done. Typically the costs run in between $4500.00 - $20,000.00, plus re-inspection fee and GST.
For over fifty years Piper Electric has been installing and upgrading consumer's electrical systems in the Greater Toronto
area. We feel that with the additions of new wiring and extra outlets on new circuits to specific heavy duty appliances,
the load on the existing knob and tube wiring will be greatly diminished. Most electrical items that you have throughout
your home (table lamps, radios, clocks, stereos, cordless phones and TV's) are supplied by two prong cords and are not heavy
usage items. We believe a lot of the hysteria around knob and tube wiring happened with the change of the milennium.
Don't forget how all computers and electrically operated mechanical infrastructure was to stop working at the toll of
midnight December 31 1999.
Recently we have been receiving more calls from previous and potential customers who have already had the Electrical
Safety Authority out to their homes for their initial inspection. They had already been directed to the Electrical
Safety Authority by their own insurance company.
It is up to you and your insurance company as to how you will handle this dilemma that is in your home.
Which ever way you decide to address the wiring in your home, let the expertise of Piper Electric look after your project.
Please check the links below.
After viewing the link use the back button to come back to this site.