Knob and tube wiring was the original internal wiring system installed in homes across North America.
The following information will provide
insight into:
- · The Main Components
- · Buyer Considerations
- · Knob and Tube Advice
Knob and Tube (K&T) Wiring can be
found in buildings built in the late 1800’s and first half of the 1900’s. It is
sometimes referred to as “Open Wiring” because it consists of two wires, live
and neutral, suspended throughout the structure with the use of insulating
“knobs” for surface mounting and “tubes” for passing through wooden beams and
other flammable barriers. The copper
strands are insulated with rubberized or asphalt saturated cloth. These systems
were designed for a much smaller electrical draw (ampacity) than our usage
requirements are today. The original fuse
type panel boxes were typically rated for 60 amps or less.
Some
potential problems occur when the system is improperly tapped into with
additional circuits, the cloth wrapping material around the wires deteriorates
or is broken, or walls and ceilings are insulated covering the components
thereby trapping excessive heat in the wires causing damage.
Knob and tube wiring was eventually displaced
from interior wiring systems because of the high cost of installation compared
with multi-strand power cables, which combined both power conductors of a
circuit in one run (and which later included grounding conductors). With most
conversions, all traces of the K&T wiring is removed. Sometimes however,
the old system is disconnected and left in place. This can be deceptive and
should be tested for live current.
Considerations
for buildings with K&T wiring
As existing
K&T wiring gets older, insurance companies may deny coverage due to a
perception of increased fire risk. Several
companies will not write new homeowners policies at all unless all K&T wiring is replaced, or an
electrician certifies that the wiring is in good condition. Also, many
institutional lenders are unwilling to finance a home with limited ampacity
i.e. 60 amp service (which, as noted above, often goes hand-in-hand with
K&T wiring), unless the electrical service is upgraded.
- Have
the system evaluated by a qualified electrician. Only an licensed
professional should confirm that the system was installed and modified
correctly.
- Do not
run an excessive amount of appliances in the home, as this can cause a
fire.
- Where
the wiring is brittle or cracked, it should be replaced. Proper
maintenance is crucial.
- K&T
wiring should not be used in kitchens, bathrooms, laundry rooms or
outdoors. Wiring must be grounded in order to be used safely in these
locations.
- Rewiring
a house can take weeks and cost thousands of dollars, but unsafe wiring
can cause fires, complicate estate transactions, and make insurers
skittish.
- Homeowners
should carefully consider their renovation options before deciding whether
to rewire their house.
- The
homeowner or an electrician should carefully remove any blown-in
insulation that is found surrounding K&T wires.
- Prospective
home buyers should get an estimate of the cost of replacing K&T
wiring. They can use this amount to negotiate a cheaper price for the
house.
Hopefully this information will help you and your clients better
understand some of the factors involved with buildings where Knob and Tube
wiring is present. If you suspect that a prospective property has live Knob and
Tube wiring, have your home inspector verify its presence during the Home
Inspection and discuss recommended next steps.