Knob-and-Tube Wiring

Many homes in and around greater Cincinnati have knob-and-tube wiring.  Buyers usually check the property off knob and tubetheir list as soon as they see the electric box….but knowledge provides buying power.

Kim Hunter, Loan Officer (Extraordinaire) is always passing along great information and the article below, compliments of Coldwell Banker Home Loans, provides a detailed information about knob-and tube wiring.

In a past column, we discussed the benefits and challenges of buying “fixer uppers.” When you are walking clients through an older house, you can add great value by knowing the ins and outs of some of the challenges that older houses present. In particular, there is one that stands out for its down-the-road cost potential, both in terms of remediating before “something happens” and, most expensively and frustratingly, AFTER something happens: knob-and-tube wiring.

Knob-and-tube wiring – the basics?
According to the International Association of Certified Home Inspectors (InterNACHI), knob-and-tube wiring (K&T) was the accepted means of running electrical wiring in buildings in North America between roughly 1880 and the 1940s. Quite simply, the knobs are porcelain support points along the wiring “grid,” nailed to the ceiling of the basement or attic, and the tubes are just that – porcelain tubes that are driven into drill-holes in the house’s lumber framing through which the wiring is run. The wires themselves were of insulated copper, wrapped in either a cloth or rubber insulation material where they ran into a fuse box or through walls and into switches, plates, etc.

While many agree that in some respects K&T was superior to today’s wiring systems, it has design drawbacks that impact today’s electricity-dependent homeowner – and, again, time and human interference have taken their toll on almost all active systems. And insurance companies have taken notice: some will refuse to insure a house that has active K&T unless it is changed at worst and require higher premiums at best.

Assigning “fault”
Over time, the insulation around the wiring can decay exposing the live wires beneath – and it just isn’t time that can eat away at the insulation, but any kind of animal that sees it as a tasty snack. But the “critter” that can do the most damage has two legs and is armed with wire, tools and an inexact understanding of K&T.

Remember what we were saying about electricity dependency? Today’s homeowner uses many more appliances, devices, etc., than K&T was designed to support – the answer was/is to splice branches into the system to feed more outlets and switches. Much active K&T that you will see will have more branches than a 60-foot pine tree – and like too much snow weighing on said branches, too much electricity is going to cause something to “give.” In the case of K&T, that will be fuses that will blow when driven to overload. Often, larger fuses are added to compensate – but this is only adding fuel to the fire, because their addition increases electrical flow, which makes the wire too hot and the insulation more likely to grow brittle and deteriorate.

Another insulation problem isn’t in the wiring but in the house itself: adding thermal insulation over K&T that is nailed to attic framework can cause the wiring to heat up unsafely. The whole purpose behind the way K&T was run was to allow surrounding air flow to keep the wire cool. Thermal insulation not only defeats this purpose, but creates a fire hazard. Speaking of fire hazards, another issue with DIY splicing is that wire connections are not often made using the proper materials – no junction boxes, no electrical tape, etc. – creating another problem waiting in the rafters.

Getting amped up and staying grounded
Today’s electricity-hungry houses require additional amps to properly power everything that is in them. K&T is primarily 60-amp service – unlike today’s standard 100 or 200 amp – which means a few things. First, 60-amps simply aren’t sufficient to “keep the lights on” when you are simultaneously doing laundry, running the dishwasher, keeping cool with your central air and working on your desktop computer. Something is going to give – namely, your fuses. 60-amp service also offers fewer outlets, which invariably leads to some of those less-than-safe DIY measures of splicing and dicing to increase their number – many 60-amp houses have one outlet per room, which means a nest of snaking extension cords.

As for those outlets, without getting too technical, K&T only has two wires: a hot (black) and neutral (white) – those two-prong outlets with which you are familiar. Today’s standard is three, which includes a ground wire, connected to the breaker box, that dampens the electric current and protects against surges, etc. Many homeowners get around this by using plug adapters that convert a two prong to a three – but plugging in your computer or laptop into a non-grounded outlet can be the recipe for a very expensive doorstop if your house is struck by lightning.

“Watt” can a home buyer do?
When looking at a house with K&T, it is important to verify if it is active or inactive. Many homeowners have taken the step of re-wiring their houses with more up-to-date materials and systems leaving the inactive remnants – the porcelain knobs and tubes – behind. These are not a problem. Current and future concern comes from active K&T service, in terms of its state of deterioration, amperage and the condition of outlets and switches.

While it may not be a deal breaker for the intrepid home buyer, you can help them better understand the risks and make an informed decision to either continue the walk through – or walk away to the next prospective home.

Doug Bonderud, “Is Knob and Tube Electrical Wiring Safe?” Angie’s List, October 2014,
Nick Gromicko and Kenton Shepard, “Knob-and-Tube Wiring,” International Association of Certified Home Inspectors,
William Kibbel III, “Knob and Tube Wiring,” Old House Web,