Rewire Doorbell Plans – Meeting Electrical Code Standards

doorbelliot

Behind my doorbell panel, a 24VAC transformer is currently wired into mains (USA) using wire nuts. I am interested in tapping into mains to power an IoT device that would integrate with the doorbell (similar to this implementation). I wish to continue allowing the existing transformer to draw from mains while also powering a USB power adapter, all while staying compliant with electrical code. The resulting implementation would continue to fit within the doorbell enclosure.

Some options I was considering:

  • Split the existing connections (e.g. using Wago 221-413 connectors) and expose a non-grounded connector for use with a USB power adapter
  • Replace the direct connection to the transformer with a grounded connector (e.g. this), rewire the transformer to a plug, and drive both that transformer and the USB power supply with a splitter

Though less preferable, I was also considering tapping into the 24VAC output from the transformer to drive 5VDC, or having my device be battery-powered.

Any advice on how I should best proceed would be greatly appreciated.

enter image description here

Best Answer

Wow, now that I see your picture, this thing is a hatchet job.

No, no. That transformer cannot just be stuffed loose in a plastic (!!) Box. There is no separation between mains and low voltage. What a fiasco.

I would extract the doorbell wiring from the box and bring it out to the wall through a hole outside the box. Now, with only mains wiring in the box, I would install 2 standard receptacles (path of least challenge) with 2-gang cover plate, and fit up a plug-in "wall wart" transformer for the doorbell. Use another socket for any bog standard USB supply from the local gas station.

It's all about separation

There should be a metal curtain between your mains and low-voltage wiring. This is where we get into mains wiring methods. This separation is easy with doorbell transformers; they make transformers that mount on a 1/2" knockout or a junction box cover plate. The mains wiring is contained; the 24V comes out to open screw terminals.

This concept of "metal curtain" must be respected. The single best way to achieve that is to install a common receptacle and plug a quality USB power supply (Apple iPad is my pick) in the receptacle.

  • Install any common 1-gang deep "handy-box" steel junction box right next to the doorbell box. That holds the receptacle. Assuming the power supply cable going into the doorbell box is full-current-rated, reroute it into the handy-box instead, remembering minimum slack (6" of wires free inside the box).

  • Connect from the handy-box into the old doorbell box. I'm a conduit person so I would go into a standard knockout via a steel conduit nipple to the handy-box, and use THHN wire between; this also carries ground to the metal doorbell box. But you could use a few inches of appropriate Romex (most places sell it by the foot, remember 6" tail on each end). Or alternately, just install a power cord (with strain relief) on the doorbell box.

  • Wire it up in the usual, Code-legal way, so both the receptacle and doorbell box get power.

    • In all things mains, hook up safety ground first always, and never disconnect it, not even for troubleshooting (it's never the problem).

  • Plug the USB power supply into one socket, and (if needed) the doorbell box's new line cord into the other.

Related Topic