ELECTRICAL SERVICE AND CONSTRUCTION

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What's the deal with Knob and Tube wiring?

You've seen this stuff before; perhaps in your own home. Go into the attic or basement of any house built before 1950 and you will see this antique wiring system strung thru the joists. White porcelain two-piece knobs hold the conductor away from the structure, porcelain sleeves insulate it as it passes thru the joists. Can this type of wiring still be safe, you may ask? Maybe.

If properly installed the first time, knob and tube (cleverly named after the little porcelain insulators I previously described) really only becomes a problem when, A) people mistreat the wire, or B) people overload it. Prime examples of mistreating your wire would be things like, insulating the joist spaces it runs thru (good thing we don't need that stuff at the 49th parallel), piling boxes of christmas ornaments on top of it, or illegally splicing on to it so you can connect that attic fan. In almost 30 years of working on older homes, I've rarely seen a legal splice or tap made onto knob and tube. The two biggest reasons are, modern electricians don't posess the proper tools or materials to do it correctly, and most supply houses don't sell the stuff anyway.

The other problem I mentioned is people overload the wires with too much demand for current. As I stated in the article, "Is this home up to code?", when older homes were built there were not as many fancy appliances to plug in. What we end up seeing is people buying adapters at the hardware store that allow them to plug in ten cords into a receptacle designed for two items. The house may only have two circuits feeding all of its lights and receptacles, which simply is not enough to power kitchen and bathroom appliances, as well as the Christmas lights and the vacuum cleaner.

One thing I haven't mentioned yet about knob and tube is that it does not have a ground conductor accompanying it, so unless someone went to the unlikely trouble of adding one later, everything it feeds is ungrounded. "Why is this important?", I think I just heard you ask. Grounding is a whole 'nother topic for another day, but let's just say it was a great invention for those who use electricity, and a pain in the b*** for those who install it. The most common safety violation I see with knob and tube is when someone has replaced an old, worn-out, ungrounded, two-prong receptacle with a modern three-prong receptacle so they could plug in their shop vac, but failed to actually ground the third pin. Bad, bad! I another article I'll tell you how to upgrade a two-prong outlet legally.

So if your home has knob and tube, my recommendation is to have it inspected for illegal and unsafe taps, and have the load measured to see if it is within the safe zone. If given a clean bill of health, by all means leave it alone. If, however, it needs repair, make sure it is done by a licensed electrician who knows how to do it. And any time you need to add outlets or other circuits, make sure they originate at your circuit breaker panel and not from existing knob and tube. If your home has finished walls and ceilings on every level, and no unfinished crawlspaces or attics to run wires through, then your budget needs to go up to include a good plaster or sheetrock repairman. Otherwise, a good electrician can add modern wiring without hardly being noticed.

The author, Gary Darling is a licensed Journeyman Electrician, Electrical Administrator, and Contractor in Washington State.