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?

  • 2
    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 4, 2020 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. Jun 4, 2020 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 4, 2020 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 4, 2020 at 13:55
  • 1
    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. Jun 7, 2020 at 1:02

1 Answer 1


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 .

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