I have an old-school intercom installed in the house that I am in the process of disconnecting. When I remove the master unit from the wall, I found 120v mains coming in directly to a transformer behind the unit, with absolutely no separation between the HV and the LV side (and all the wires to the various head units).

Given that I'm pulling the system out, I don't mind, as ultimately I'm going to put the 120v wire into a junction box and add a doorbell transformer into the cavity left by the master unit.

I'm having a hard time locating the breaker though. It isn't the light or outlet breakers that are near by in the kitchen, and it's un-labeled. There are a lot of breakers, so ideally I'd like to avoid going one-by-one. There is a 60-htz hum coming from it and my non-contact voltage detector confirms it is hot.

Is there an easier way to determine which breaker it's on than just trial-and-error for all of them? If there was an outlet, I'd use a tracer to find the breaker. But given this just goes into a pair of wire nuts and then the transformer, I don't want to touch it while live...

  • 1
    You can binary search by turning off half of the candidate breakers at a time but that still will take some time Jul 8 '20 at 15:31
  • Is the transformer still connected? Jul 8 '20 at 16:25
  • @A.I.Breveleri Yes, it is still connected...
    – ircmaxell
    Jul 8 '20 at 16:25

Trial and error flipping breaker handles isn't so bad especially if you have an assistant or can attach something that makes noise (a fan, a radio, etc) so that you can hear at a distance when you find the correct breaker. Honestly it's the quickest thing you could do.

Some of us might have been known to wire 120 v devices while the circuit is live. While of course I couldn't recommend that a person should do such a reckless thing.. if a person did choose to wear gloves and work carefully, it wouldn't be impossible to attach a receptacle to those bare wire ends so that the plug-in tracer could be used (or a plug-in noise maker, for that matter). Were a person to attempt this, it might be most safely accomplished if the receptacle is prepared with some "pig tails" (wire leads 6 inches long or so), wrapped in electrical tape to insulate the bare terminals, and then the pig tails could be attached to the live 120 v leads with wire nuts.

If the old-school intercom is still at least somewhat functional you might be able to set it up in a way that it makes noise. Some had a built-in AM/FM radio, or an audio input jack, or maybe you can just turn up its volume and listen for the background crackly static sound to go quiet while you're going through the breakers.

  • There's enough space and the wire is long enough where I could likely accomplish that live. I'm just not a fan of working live anymore (been hit once or twice)...
    – ircmaxell
    Jul 8 '20 at 16:28

I would purchase a circuit tracer if you don’t want to flip breakers. A quick internet search brought up sperry cs61200p for 39.15$ This kind of tracer is handy to have you connect to a live circuit and scan over the breakers the strongest signal is the right breaker. I have some cheap ones similar to this For residential and high dollar ones +800$ that don’t need power to be on but I do this for a living. The less expensive ones work when you have it turn the breaker off and the signal disappears that is your double check for safety.

  • I have one that works on an energized circuit, but I don't have that adapter to connect to bare wires (and would prefer not working with live wires to do it).
    – ircmaxell
    Jul 9 '20 at 16:03
  • You stated you did not want to flip breakers this is really the only other option.
    – Ed Beal
    Jul 9 '20 at 17:36

Minimal fuss to other circuits:

  • Shut down the stuff that would be offended by a power outage.
  • Shut off the main breaker,
  • Disconnect transformer and wire in a receptacle
  • Turn on the main breaker, plug in & trace.
  • Turn on the stuff that would be offended by a power outage.

You need a very noisy device that can run off the LV side of the transformer, or a suitable LV lamp and a long wire. Since it's LV you can connect something with practically no risk.

The idea of course is that you need to hear it or see it while at the service panel, so you can tell when you've cut the power to the transformer.

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