4

Our doorbell transformer is powered via an original 1950s-era fabric jacketed 2-conductor NM (internal conductors are plastic-jacketed). Photo below shows that NM leaving electrical box; doorbell transformer is sitting out in the open on a wooden shelf on the other side of the visible wood enclosure.

As part of replacing the doorbell transformer to power a Ring doorbell, I'd like to mount the new transformer to a proper metal junction box, which will be secured to a stud in the unfinished garage wall. There is about 3-4 feet of this old but in good condition NM running in the wall to the transformer. I can think of two main choices for connecting the old NM:

  1. Just run it to the new metal junction box and use some sort of box-entry clamp to secure it at a knockout hole. Would I use a modern clamp designed for NM-B, or some special clamp?

  2. Protect it for that short run by running it into something like a liquidtite or flex conduit section (maybe 3/4" trade size?) for the run to the junction box. How would I secure it at the transition to conduit? And would I remove the outer fabric jacket and just run the internal plastic insulated conductors in the conduit to the junction box?

enter image description here

diagram of transformer, side view with 120V connections up: diagram of transformer, side view with 120V connections up

4
  • Aren't doorbell transformers usually "in the open" like that? Maybe for heat dissipation or such?
    – gnicko
    Commented Nov 15, 2022 at 23:59
  • @gnicko Yes. There's 120V wires going in one side of the transformer, into a circular opening that threads into a standard j box knockout opening with locknut. The metal transformer body and lower voltage output terminals are exposed. I want to run the old NM into the other side of the j box and connect their conductors with the transformer's 120V input wires inside the j box. Metal box in photo is circuit breaker box supplying the NM.
    – Armand
    Commented Nov 16, 2022 at 0:22
  • 1
    How accessible is the existing run? I don't like keeping the old NM unless it's a real headache to run new stuff.
    – KMJ
    Commented Nov 16, 2022 at 1:06
  • 1
    @KMJ It's accessible, but I don't yet want to open the electrical panel (remove dead front). As you can see from photo, there are other old NM cables squished in and I don't want to disturb them at this point.
    – Armand
    Commented Nov 16, 2022 at 4:04

1 Answer 1

6

Just run it to the new metal junction box and use some sort of box-entry clamp to secure it at a knockout hole. Would I use a modern clamp designed for NM-B, or some special clamp?

Yes, use a metal box and a standard clamp. Just make sure it is the right size for your box knockout.

clamp

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.