This battle is over a century in the making, literally. Both kinds of antique wiring have been in homes for OVER 100 YEARS at this point. Customers ask us which is better all the time, or more commonly, which is worse to have in your home? Let’s dive in and check out the differences between these and I’ll share my opinion on which one I’d rather have in my own house.
For those who don’t know at this point, knob and tube wiring is the first generation of wiring used in homes. It was used from the 1880’s until the 1930’s (in Massachusetts). In other parts of the country, particularly the west coast, it was used into the late 1940’s / early 1950’s.
BX wiring is the second generation of wire used in homes, being used from ~1903 until the late 1940’s / early 1950’s as well. Given these two wiring types overlapped with each other, there were many homes that have a combination of the two wiring types.
Both wiring types have their downsides / issues. Let’s see how they stack up!
Can’t Install Insulation Around It?
Knob and tube wiring: This is correct. This wiring was run in open air to dissipate heat. Once insulation is installed around it, it can no longer do this becoming a potential fire hazard. If you are having your home insulated, the knob and tube wiring must be removed / deactivated prior to installation.
BX wiring: No issue with insulation. This type of wire has an exterior sheathing (metal) that encompasses the two conductors within it
Winner: Easy one, BX wiring!
Ungrounded Wiring Style?
Knob and Tube Wiring: This wiring style has NO ground. If you have ever seen knob and tube wiring (see the picture at the top of this blog), you will notice the knob and tube wiring has individual conductors. Each circuit consists of a hot and neutral wire, no ground.
BX Wiring: This is true for BX wiring as well, although it’s a bit more complicated. The metal exterior sheathing on ALL early BX wiring is not a ground. I don’t care that it’s metal and when tested may act as a ground. The fact is it is NOT a ground. The scary thing about this from an electrician’s point of view (or anyone into safety for that matter) is many electricians treat the metal sheathing as a ground. They bond the metal receptacle box to the sheathing, then put a three-prong receptacle in. If there is a short circuit to ground, this ground path back to the panel must be strong (low resistance). If it is not, the circuit breaker will not trip, and the metal sheathing will become a resistor and carry current back to the panel never tripping the circuit breaker. Can you imagine having live wires running in your walls / basement with current (electricity) flowing on the OUTSIDE of the wire? Really scary stuff!
What I described above is more common than you may think. For the metal sheathing to have any chance of being an effective ground and allowing the circuit breaker to trip, all the connections between the BX wire and boxes need to be tight. Imagine in your mind, the run of a circuit from your panel to your receptacles / switches / lights. The wire may leave your panel, hit a box in a basement light, then a box on your dining room wall, then a light in the center of the room, then branch down to several receptacles. The connectors at all these (we just count out 7 different boxes including the panel) boxes must be tight for it to work at all. One hundred years later, electricians or handymen (sometimes homeowners!) may have manipulated the wiring, changing receptacles, switches etc, and may have inadvertently loosened the clamps / connectors that are in these boxes.
Winner: Can they both lose? There is no winner here, it comes down to the best of the worst. I suppose I would consider the K+T the winner in this case as there no chance for what I described with the BX sheathing happening. The sheathing of the BX wiring carrying current back to the panel and not tripping a circuit breaker is absolutely frightening.
Keep in mind the ground wire is in modern circuits to protect personal (you!).
Brittle Insulation?
Knob and Tube Wiring: Knob and tube uses a rubber-based insulation around its conductors. When exposed to heat, usually from light fixtures that have too high of wattage light bulbs installed, the insulation can become very brittle. So brittle in fact, that when you touch it the insulation crumbles right off exposing the bare conductor. Every electrician has experienced this where they pull a light down / remove the wire nuts, where the insulation falls off and turns to dust. Definitely not fun!
BX wiring: BX wiring uses a similar rubber insulation so it’s susceptible to the same insulation break down as knob and tube wiring. I will say that with the way BX wire is run, light boxes often have multiple wires in them. You can often see 4 different BX wires in one box, which is 8 conductor’s total. Knob and tube light boxes usually have 2 to 4 conductors. There is nothing quite like removing a light fixture and finding 4 BX wires with all the insulation cracking off.
Winner: I’ll go with knob and tube wiring on this one because there are usually fewer wires in light boxes.
Can’t be Insured by Homeowners’ Insurance?
Knob and Tube Wiring: There are some insurance companies that will insure it still. The ones that do, will charge a premium for it. Many insurance companies won’t insure it (at least here is Massachusetts) and require it to be removed or the insurance will be cancelled. We do many knob and tube replacements for customers for this very reason.
BX wiring: I’m not aware of any issues with insurance companies whatsoever.
Winner: Easy one, BX wiring!
Switched Neutrals?
Knob and Tube Wiring: It is not uncommon to find wall switches “switching” the neutral instead of the hot wire. It was a very odd practice used by early electricians and I’m not sure why they did it this way. The light would still go on / off like normal, but instead of the switch opening (switching off) the hot wire, it would open the neutral wire. This could be an issue for inexperienced electricians or homeowners working on their own home expecting there to be no power at the light with the switch off. With a switched neutral, the light will still have live power regardless of the switch position.
BX wiring: I have seen switched neutrals with BX wiring as well many times.
Winner: We have a tie!
Mice / Squirrels and Other Critters Like to Eat the Insulation?
Knob and tube wiring: Oh yes, a favorite snack amongst uninvited attic dwellers. I have seen full runs of knob and tube wiring in attics with ZERO, I mean, none, nada, no insulation around the conductors at all. So yeah, imagine having live wires with no protection running around your attic!
BX wiring: If the critters could get through the metal sheathing I’m sure they would love to munch away. Good thing here is the metal sheathing is really solid and there is no way a mouse is getting through it – at least I’ve never seen it happen in my career of attic crawling!
Winner: BX wire!
Can’t Extend New Wiring Off It?
Knob and tube wiring: Because knob and tube wiring is an ungrounded wiring style, new wiring cannot be extended off of it. A good example of this would be if you wanted to add a receptacle in a bedroom and there is a nearby K+T receptacle. An electrician cannot take power from that K+T receptacle and run a wire to your new receptacle.
BX wiring: The same is true for BX wiring as it’s also an ungrounded wiring style.
Winner: Tie. If you are a homeowner and an electrician is offering to take power from a BX or K+T circuit to add a new device or light, be wary!
And The Winner Is?
BX wiring!
As you can see, they both have their serious disadvantages. If I had to choose between the two for my own house, I would pick BX wiring. My reasoning here is that in most cases it’s not quite as old as knob and tube and given the system was not manipulated, it works quite well for what it was designed to do. Also, the insulation piece is a deal breaker – not being able to insulate your home with knob and tube. Or even worse, your house was insulated years ago before it was a known issue to have insulation around knob and tube – This could be a ticking time bomb.
