About Knob and Tube Wiring

Submitted by Ray Thornburg on Tue, 03/26/2013 - 14:05


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About Knob and Tube Wiring

     Knob and Tube wiring is a method of wiring that was popular from about 1890 to 1940. Characterized by the ceramic knobs that held the conductors away from the joists and ceramic tubes that the conductors ran through when it had to pass through a joist or wall plate. While it is generally believed that knob and tube systems were safe and reliable when they were built it is likely to have been altered or damaged over time. In addition to this some consideration must be made as to the continued use of a system designed in the late 19th century here in the early 21st Century. For this reason home inspectors almost certainly will recommend the system be evaluated by a qualified electrical contractor. Below are some things to consider if k&t has been noted on your home.

Key Points

·        Knob and tube wiring was designed to be air cooled. So covering with insulation or combustibles is not allowed.

·        Splices to other wiring methods must be in boxes. The conductors of the knob and tube should enter through opposite side of the box.

·        Although as originally installed, splices were allowed in the open air however; they were soldered and taped in a special way.  A splice added at a later date is unlikely to have been done correctly so it should be in a box.

·        K&T wiring is subject to physical damage. Nails holding the knobs will rust through leaving the conductors hanging or lying on combustibles. Sometimes knobs are broken off for a variety of reasons.

·        The rubber insulation on the wire has a tendency to become brittle and crack over the course of time creating a safety hazard.

·        K&T systems do not meet modern electrical demands and needs. Many rooms may only have one outlet. Overloading this one circuit may tax the system.

·        K&T systems do not have a grounding conductor. This means it is a two wire system. Modern homes have an additional grounding conductor for safety. This grounding conductor improves safety and helps protect modern electronic devices.

·        A Ground Fault Circuit Interrupter (GFCI) can be used to protect people against electric shock in a two wire system. However it will not have a grounding conductor. Outlets without a grounding conductor must be labeled “No equipment grounding conductor” if they are of the three prong type.

·        Many systems services have been upgraded from fuses to circuit breakers which is a good thing.  Sometimes however this only gives the appearance of an upgraded system.  An untrained individual may think the whole system has been upgraded when actually only the main panel has been upgraded.  In this case a junction box is used to splice the old k&t circuits to the new breakers.

·        The System service although upgraded may still be rather small by today’s standards.

·        Outlets for k&t systems can lose their tension plug pull strength (not be able to hold a plug snugly) due to their age or be painted over. When an outlet gets loose and cannot hold a plug it should be replaced for safety.

·        K&T should not cross each other without an insulator.  Wiring should clear the framing materials by at least an inch. There should be 3” of separation for parallel conductors. Knobs should be no farther than 6” from a splice.

·        Insurance companies are sometimes hesitant to insure homes with k&t.

·        K&T systems were likely to have been professionally installed. Problems are usually with alterations, proper maintenance.

·        Because K&T wiring was designed to be air cooled; the homes it was installed in are unlikely to have insulated walls.  Homes of this era were often balloon framed which actually will make it easier to upgrade (i.e. it’s easier to pull the wires through the wall).

·        Remember that if more modern insulation requirements are desired then the knob and tube often must be removed because of the separation requirements.

  • Most knob and tube systems were rated at and utilized 25 amp fuses ( modern branch circuits are usually 15 or 20 amp by comparison). When installed correctly and not incorrectly modified or damaged it is considered safe.

Many homes in the Charleston area still have this type of wiring  If you're buying an older home be sure to have it inspected. Blue Palmetto Home Inspection serves the entire Charleston lowcountry area!

Now let's look at some Charleston home inspection photos....

knob and tube wiring sagging noted on Charleston home inspection


 The fasteners can rust through with the progression of time allowing the conductors to be improperly supported. Shown here on this Charleston home is a good example of this. The only support here seems to be the ductwork which may have a conductive surface. 



knob and tube buried in insulation

     Sometimes conductors are buried in insulation which is unsafe for this type of wiring. Originally very few homes of this era had insulation.





knob and tube spliced

     In this picture you can see how knob and tube systems are likely to have been altered. Splices with different wiring systems should be in a box. Shown are modern 12 -3 NM (yellow), older NM (red) and knob and tube (blue).





knob and tube broken insulator

      Here is an example of mechanical damage. A broken knob is allowing the conductor to lie unsupported.








knob and tube unsupported

     Here you can see how the fasteners that held the knobs to the bottom of the floor joists simply rusted completely through leaving the conductors unsupported.





active knob and tube wiring

     Even in houses that have been upgraded there is still likely to be some active knob and tube wiring remaining especially under the house. Often only one or two circuits will remain for some reason. The arrows show how another wiring method was incorrectly spliced into the existing knob and tube.






Submitted by Ray Thornburg on Tue, 03/26/2013 - 14:05