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My American house was built in 1942, and much of the wiring is still knob-and-tube wiring (2-wire, no ground). Over the years new modern wiring has been added to the house, but we still have some KnT wiring for the ceiling lights, and a few branch circuits, etc. We are planning to replace the KnT wiring when we have the budget to do so. I have replaced many of the receptacles on the KnT circuits with GFCI receptacles to provide added protection, which is recommended.

I have seen AFCI and GFCI circuit breakers which plug into the service panel, and they are reasonably priced at $30-50 per breaker). Would these AFCI or GFCI circuit breakers provide any protection on old, knob-and-tube wiring?

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Arc-Fault Circuit Interrupter (AFCI)

An arc-fault circuit interruption device is designed to detect dangerous arcing within the protected circuit, and open (turn off) the circuit to prevent damage caused by the arcing. It does this using special circuitry to analyse the electrical characteristics of the circuit, looking for characteristics that match specific pre-programmed values. If the AFCI detects suspicious goings on, it opens the circuit.

AFCI breakers are typically combination devices, meaning they also provide similar overcurrent and short-circuit protection to a standard breaker.

Installing a combination AFCI breaker on a circuit containing knob and tube wiring would be a great idea, and could potentially prevent a fire.

Ground-Fault Circuit Interrupter (GFCI)

Ground-fault circuit interruption devices are designed to detect ground-faults, and open (turn off) the circuit when a ground-fault is detected. They work by using a current transformer (CT) to detect current imbalances between the ungrounded (hot), and grounded (neutral) conductors of a circuit. This blog entry might help you understand how GFCI devices work.

GFCI breakers are typically combination devices, meaning they also provide similar overcurrent and short-circuit protection to a standard breaker.

Installing a GFCI breaker on a circuit containing knob and tube wiring, probably won't provide any benefit. GFCI devices are designed to prevent electrocution, not to protect the wiring.

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Agree that a combination AFCI/GFCI breaker is a good idea, but disagree that GFCI won't provide any benefit. Since KnT is poorly insulated wiring, there is a high risk of accidental electrocution, and a GFCI may detect and stop that from becoming fatal. –  BMitch Oct 29 '13 at 12:34
    
While nearly all AFCI breakers provide Ground-Fault Protection of Equipment (GFPE 30-50 mA), it's nearly impossible to find one that provides GFCI protection (4-6 mA). So given the choice between an AFCI breaker and a GFCI breaker, I'd choose the AFCI breaker. –  Tester101 Oct 30 '13 at 10:56
    
At first I was thinking that an Arc Fault is unlikely with KnT wiring since the hot and neutral conductors are typically placed on either side of the cavity between stud & in some cases are even separated by a stud themselves. The gap between conductors varies with the stud placement (In my house it's 16" On-Center). I would think that this air gap would reduce the chance of an arc-fault in household wiring. But then I realized both conductors need to be stuffed into the same box at some point, perhaps through the same hole. At that point it seems that an Arc Fault is much more likely. –  Stefan Lasiewski Oct 30 '13 at 22:47
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@StefanLasiewski You can also have an arc along a wire, especially near splices. One frequent problem with KnT wiring is the capacity is much lower than modern demands, leading to overloading. Sometimes that overload will result in a failed junction and arcing. –  BMitch Nov 1 '13 at 10:57

An AFCI is a great addition to K&T wiring. While the conductors in K&T are separated by large distances, and even studs, they do come together at junction boxes which are often metallic. An AFCI adds a layer of peace of mind to the situation.

Be sure to measure your K&T wire to determine gauge. It can take slightly more current than the modern equivalent at the same wire size, but to be safe stick with the modern values. You'll probably need a 2-pole AFCI, as K&T was often wired with a shared neutral. Two pole AFCI for knob and tube wiring Fora more complete writeup of K&T retrofit see http://diy.stackexchange.com/a/20279/5960

Note: when I replace a fixture in a K&T circuit I address the metal box weak point with some new loom, slipped over the wires as they come into the box. The old loom, which could easily be 50 to 100 years old, is sometimes but not always brittle. I'll also pull out the wire and wrap the exposed area with friction tape. This is the only point in the entire K&T system that even needs insulation.

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You'll probably need a double AFCI, as K&T was often wired with a shared neutral. Whoa, what? Can you explain how a double AFCI will help here? Yes, I definitely think I've seen a shared neutral up there. –  Stefan Lasiewski Dec 6 '13 at 16:34
    
FYI, I'm also in Berkeley. I've heard many of the same arguments that you have. The East Bay seems to be Knob & Tube central. –  Stefan Lasiewski Dec 6 '13 at 16:34
    
@StefanLasiewski I clarified the answer: I mean a 2-Pole breaker like the Siemens Q215AFCP (Carried by Lanner Electric). Note that the AFCI might detect existing faults in your system, and trip right away. –  Bryce Dec 8 '13 at 8:40

You will probably find that installing an AFCI breaker will be very annoying. Many electricians don't like them because they go off very unreliably. For example, if you run a vacuum cleaner, or a sump pump, those could trigger the AFCI circuit even if there is no arc fault. I believe it is code to install AFCI breakers in new electrical work, but many electricians were replace them back with regular breakers after inspection, because of how annoying they are.

If you have already replaced the older outlets with GFCI, that's about all you really need to do.

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It's not likely that a licensed Electrician is changing out items after inspection. If they are, they should lose their license. Secondly, this is old, outdated information that constantly propagates throughout the internet. Modern AFCI breakers are less prone to nuisance tripping. If you're going to form an opinion based on others opinions, please check the date of the sources. –  Tester101 Nov 1 '13 at 21:01

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