Location: United States, Colorado

While diagnosing an unrelated electrical issue during renovation, I discovered that the junction box containing a fuse and switch, which provides power to my furnace, reads hot with a non contact voltage tester.

It seems to read hot on all of the metal surfaces: the junction box body, plate, and fuse cover.

Photo of the junction box: photo of junction box

This seems clearly wrong to me, but the furnace operates normally and I've never had problems with it. Unless there is something unusual about these fuses, it seems to me that a hot wire has come loose or otherwise shorted with the box itself.

Is this a safety issue? Is this box wired correctly? I think yes, but it'd be good to get more input.

  • 1
    Do not trust NCVT, they could be very misleading.
    – Ruskes
    Jun 21, 2022 at 20:55
  • 1
    do you have a multi-meter? they don't suffer the same false-positives. Use it to check the voltage between case and ground. if 0v, you're fine.
    – dandavis
    Jun 21, 2022 at 20:58
  • 1
    if it's wired correct, a hot wire shorting to the case would instantly blow the breaker/fuse. Then there's the old finger test: if it's not shocking you when you touch it, it's probably not hot.
    – dandavis
    Jun 21, 2022 at 20:59
  • What model NCVT? And, as noted by @dandavis use a multimeter to check case (which should be grounded) to a known ground elsewhere. Jun 21, 2022 at 21:05
  • 1
    @crip659 the initial reason for concern was that generally in NCVT-1 mode this tester doesn't give a reading unless it's very close, usually almost touching romex on the hot side.
    – Justin
    Jun 21, 2022 at 21:37

2 Answers 2


The NCVT could detect a hot wire even up to 1 inch far away.

So, to be sure use Multimeter, since NCVT could be misleading.

Here is a simple test/proof.

Just wave it in front of wall socket without going in to actually make contact.

You have what we call a false positive.

No need to do anything, but if you want, ground that box, just in case the hot ever touches it.


It's very common for people to wire metal boxes like plastic ones, ignoring the box altogether and wiring grounds "thru" or not at all. In that case, the metal box would not be grounded, and a hot fault to ground would simply sit there with an energized box.

So this is the time to make sure all grounding is in good order from the supply and in the box.

Note that your furnace does play a role in air conditioning: the furnace's air handler distributes the A/C.

If you can see the back of the box with a mirror, there should be a #10-32 screw poking out the back of the box, which is the terminal one normally uses for the grounding to the box. If not, then it probably wasn't grounded. If the cable simply doesn't have a ground wire in it, the rules for retrofitting grounds have gotten rather liberal, and this would be easy work in an unfinished open-joist basement.

  • 2
    Sounds like the right thing to do is: Shut off power to this at the main panel (and verify that the NCVT now shows "nothing") and then open it up and take a look. Jun 21, 2022 at 22:51

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