AFCI for knob and tube circuit?


Old 09-11-17, 08:23 AM
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AFCI for knob and tube circuit?

We just bought an old house, and on inspection, found one knob and tube circuit that goes to second floor. Unfortunately the walls are all original plaster and woodwork. There are closets on the first and second floors, so we could send wires up to the attic and then have them come down into rooms, but that still will necessitate some plaster damage.

In the meantime, I found the circuit in the panel (GE 200 amp) and replaced the breaker with an AFCI breaker. I have read that knob and tube is less susceptible to overheating and can handle more current than Romex. I also read that it is most susceptible to damage where it enters metal boxes due to brittle insulation.

We only use a couple lamps in these rooms. Are we protected with the AFCI, or am I giving myself a false sense of security?

Thank you
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Old 09-11-17, 09:30 AM
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Assuming you used the correct amperage AFCI breaker for the wire size, you should be protected. The only additional protection the AFCI breaker gives you is protection against arcing faults.

I have read that knob and tube is less susceptible to overheating and can handle more current than Romex.

I do not agree.
Old 09-11-17, 09:43 AM
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Thanks, Joe. I used a 15 amp breaker (that's what I replaced too). The braided wire was spliced to plastic coated wire in the panel box.
Old 09-11-17, 09:45 AM
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The AFCI is good, also adding GFCI protection would be better.

The statement on K&T carrying greater current has a grain of truth but overall very misleading for the reason you identified. The true part is that bare copper hanging free in air can indeed carry much more current than insulated cable assemblies. However, a wiring system is only as good as its weakest point, and as you pointed out the parts of the wires that are insulated are very fragile and any additional heating in these areas greatly increases the risk of failure or fire. This makes the K&T circuit overall worse than modern Romex. K&T is also supposed to have no thermal insulation on it, however in many old houses the K&T has been buried in fiberglass or cellulose during a remodel and has been rendered unsafe.
Old 09-11-17, 10:56 AM
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I'd been recently working at a few homes removing K&T wiring. Some insurance companies are putting up a red flag with K&T wiring and requesting (demanding) its replacement by a licensed electrician. Since you recently moved in.... check into it.
Old 09-11-17, 09:08 PM
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Most of the knob and tube I've seen, with some rare exceptions is only #14 which is only good for 15 amps.

You're safest putting a 'dual function" breaker in which provides AFCI and GFCI protection. As an added bonus you can legally install 3 prong receptacles as long as you affix a " AFCI/GFCI protected outlet no equipment ground" sticker.
Old 09-17-17, 10:44 AM
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What would the AFCI/GFCI breaker do differently vs AFCI only? Protect against shock?
Old 09-18-17, 07:05 AM
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AFCI protects against sparks/arcs/fire.

GFCI protects against people/animals being electrocuted.
Old 09-19-17, 10:05 AM
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To throw in one more consideration for K&T wiring... It's generally safer when used on lighting circuits. You know there are x number of 40/60w bulbs (or LEDs at considerably less draw), so you'll never really be pushing the limits of the circuit in terms of overheating.

Where it becomes more of an issue is with receptacles. Plug in a vacuum, or window AC, or space heater, and all of a sudden you're drawing a lot more current through wires that can have a tendency to overheat (depending on insulation, etc).

I would definitely recommend planning an upgrade at some point. A good electrician can likely do one circuit with little damage.

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