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I live in a century house whose electrical was last worked on in the 1960s at the latest. Without opening a wall, I’d like to make some deductions about the odds of having in-service knob and tube wiring.

My multimeter has a NCV detection feature that alerts + / - 4 inches or so from lightswitches and receptacles, and also detects voltage in the wall along a logical line (e.g. up the stud bay from the lightswitch but not the next bay over).

Is it fair to say that if I follow the NCV detector beeps from a lightswitch or receptacle, and find two parallel tracks a foot or two apart, I probably have Knob & Tube?

Is the inverse true? If the NCV only alerts along one 6-8 inch wide line, is that a pretty reliable indicator that both conductors are closer together than K+T would be and so I likely have some later style sheathed wiring?

newcoder
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    Have you checked in other places where the wiring may be exposed? How about the attic or the basement or crawl space if there is no basement? – jwh20 Jun 04 '20 at 11:04
  • why do you expect two parallel tracks a foot or two apart? When I've seen knob and tube the neutral and hot often take vastly different routes in the house. – Fresh Codemonger Jun 04 '20 at 11:34
  • @FreshCodemonger My hope for parallel tracks was thanks to the example photos I’ve encountered of knob and tube happening to show sections where the hot and neutral fit in the same photo (illustrating the airspace relied on for insulation). I hadn’t realized the routes can tend to be vastly different. That sounds like although one could follow the neutral on a loaded circuit, it could be difficult to tell apart from daisy-chained receptacles. – newcoder Jun 04 '20 at 13:43
  • @jwh20 Basement is a no-go since the house was separated into apartments in what appears to be the 1960s or earlier (wide wood paneling, not all receptacles have a ground, bakelite switches found), at which time the basement was finished and got a separate meter. Finished attic may be original, may be added from the remodel, previous owner and county records did not clarify. Either way the attic unit needs more insulation. Attic and main floor share the original meter with an ancient wiring convention. Next stop is the crawlspace above the attic the next time I want to get up before the sun. – newcoder Jun 04 '20 at 13:55
  • All of the K&T I have worked on were parallel runs as the hot and neutral start and stop at the same place they are almost always parallel runs. – Ed Beal Jun 04 '20 at 14:17
  • Who cares? Just put an AFCI breaker on it and don't worry about it. K&T isn't dangerous, just archaic. AFCI is going to catch all the stuff people worry about with K&T. Or aluminum wiring. Or cloth wiring. Or Southwire Romex. – Harper - Reinstate Monica Jun 04 '20 at 14:24
  • @Harper-ReinstateMonica The big issue is the attic unit was well temperature controlled til an old shade tree died & a hailstorm totaled the roof (which was really 4 layers of shingles). Now we’re having water damage from ice dams in winter & the attic unit gets 90+ degrees F in summer with the same two 8000 BTU window units that used to be ok (860W each, 2 separate 15A circuit breakers shared with the rest of the apartment) running full blast. We’re trying to assess the risks since the wiring is under heavy load already & local insulation crews have entirely new-construction experience. – newcoder Jun 04 '20 at 14:44
  • Seems like the adage on this site is that K&T is fine until you mess with it, and major insulation sounds like messing with it. Correct me if I’m wrong, but AFCI will catch sagging and arcing against a pipe, but may not catch a K&T wires that, say, get encased in spray foam and can no longer cool as designed, keep carrying over 10A continuous load (existing AC plus modern life), and cause the foam to smoulder since there’s nothing for it to arc against until something melts, right? If I can prove in-service K&T we’re hoping to pivot to a foam-over-finished-wall insulation solution. – newcoder Jun 04 '20 at 14:51
  • I don't know about spray foam, but blown insulation around K&T is well understood at this point. A bunch of states had laws forbidding blown insulation around K&T wiring. HAD past tense. The laws were susperstition-driven. When they switched to a data-driven approach, there are inspector's reports made for every house fire, and the data was plainly saying this wasn't a problem. So they repealed those laws. Makes sense; conduits are allowed 9 wires at wire-limit amp loads. A K&T wire is only one. – Harper - Reinstate Monica Jun 04 '20 at 15:35
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    It's a pain, but you can also crack outlets/switches open and see what you've got. Caution: be very careful pushing/pulling wires. – Aloysius Defenestrate Jun 07 '20 at 01:02

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Yes your testing plan can work but you will need a load on the circuit to be sure. With no load on the circuit the NCV will only detect the hot, if the circuit is daisy chained as long as the last receptacle or outlet in the chain is loaded there should be the return voltage on the neutral and the tester should detect it. No load and only the hot will be detected so it would look like it was updated but may be K&T. I have always liked using a plug in light but today’s LED’s may not have enough to be detected the field may be weak, a 60w lamp or a hair drier On low will work well .

Ed Beal
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