What should I know about knob and tube wiring in homes?

Maybe I am simple, but the wonder of electricity has never been lost on me. I am fascinated by the amazing things we have been able to do with the movement of electrons through a wire from one end to the other. I know it’s not likely anything most folks think about but take a second to consider how magical it must have been the first time a home could be lit up at night by turning a switch if you had never seen such a thing before. The earliest form of harnessing electricity and making it do what you want was using a system called knob and tube. There are several homes in both urban and rural contexts that still have this installed and it bears a basic understanding of the issues. Although I am not an electrician, let’s consider knob and tube as it relates to facts and practice.

History 

The early days of electricity were primarily expositions and scientific displays, where large crowds gathered to see instant lights spring to life simultaneously to gasps and applause. It didn’t take long for governments to realize they would need a network of wires, today this network of wires is referred to as an electrical grid to service businesses and homes. The earliest of these installations were called knob and tube where porcelain “knobs” carried a single wire along wooden joists, and “tubes” carried the wire through the joist where necessary. Because two wires are required to make a circuit, the knobs and tubes would generally run parallel separated by four to six inches so they could not touch.

Due to this being the system used by early electrical pioneers, it was the standard in building wiring from the early 1900s when homes first started getting electrical wiring, until the late 1940s! The expert who harnessed and installed this new, mysterious, and at the time scary system was called an electrical technician, shortened to the term electrician, used today to refer to those qualified and licensed to install and maintain electrical systems.

The Problem

Actually, the strange part about the knob and tube system is not that it is dangerous in itself and is even today included in the Canadian Electrical code as acceptable when left alone, but obviously not for new installations. The issue that has arisen for knob and tube is partially because of its longevity, as the wires themselves were coated with a rubber and tar insulation with a braided cloth jacket that gets brittle as it ages. Once brittle, any movement of the wire will cause the insulation to crack and flake off leaving bare wires exposed and open to a possible short or electrical shock when accessible.

To compound the issue, a home built over 100 years ago is unlikely to remain without any upgrades which means as new electrical circuits are added, the original knob and tube can be disturbed causing the insulated casing to flake off. Also, with upgraded insulation in the walls or ceiling, often times knob and tube wires can be covered and hidden. Any accidental touching of knob and tube wiring can be an electrifying experience and because of the likelihood of handyman alterations over 100 years, most homeowner insurance policies will not cover homes that have knob and tube wiring anymore unless a plan is made for immediate replacement.

The Solution 

Modern wiring solutions function in the same way as knob and tube, except the wires are brought close together and encased in permanently flexible plastic insulation, including also a third ground wire since about 1966. Circuits must be adequately sized to ensure the wires don’t overheat before tripping a breaker, and by binding the wires in a single cable they have superior strength against breaking and stretching. For this reason, the only true solution is to run new wires and circuits to replace old knob and tube circuits. Fortunately, original knob and tube circuits can usually be abandoned at the source and do not need to be removed, simply staying in the wall or joist space while a new wire is pulled. Unfortunately, however, this will usually also require a new breaker panel, plugs, switches, and fixtures, plus more times than not the exterior service from the municipality is undersized for modern needs and should also be upgraded at the same time.

How to tell if you have knob and tube wiring

Well, this is a tricky situation. Most often a home inspector will see old knob and tube circuits in the joists of an unfinished basement or in the attic space which are a clear sign that there is likely knob and tube in the walls. Additionally, a giveaway could be old plugs with only two prongs and no third ground hole since the knob and tube system doesn’t use ground wires, although two prong plugs endured into the 1960s. As a final thought, if the property is built between 1900 and 1950, it is a good idea to bring in an electrician to give you advice on the system as part of your buyer’s conditions.

If you made it through the whole article, you should have a feel for the pitfalls of knob and tube wiring but know enough to identify it if you see it and know to contact a licensed electrician.

As mentioned, most insurance policies for homeowners will not cover for knob and tube wiring so it is a problem that once identified needs to be brought to a licensed electrician for advice on remediation. To be fair, it was used for over 50 years in application, so a few extra weeks getting quotes and arranging remediation shouldn’t cause any panic or overreaction on anyone’s part.