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I would like to build some electronics to automatically and individually control two receptacles on a 20A circuit.

Is the following a good plan?

  • Fish 12/4 NM through the wall from the receptacles' box to the below
  • Put a box or panel someplace accessible (the rectangle on the left)
  • In that box:
    • Use UL listed COTS parts for everything
    • Follow NEC for wiring methods, spacing, materials, and grounding
    • I'll do what I can to make it neat and tidy
  • Pass low voltage wires out to a separate box (the rectangle on the right) housing low voltage electronics

Wiring diagram

The low voltage conductors on the right will be sized according to the (much lower amperage) requirements of the low voltage circuit.

I can do pretty much whatever I want in the low voltage box, right?

I'll ground as needed on the line side of things (the box if it's metallic, the converter and relays if they require it). But am I required (by NEC) to ground anything on the low voltage side? I'm not talking about using DC "ground" in the electronics, or whether V- will be earth potential, but I'm talking about the 12AWG grounding conductor that comes in from the left... does NEC require that for anything on the low voltage side?

I live in Michigan, USA. This is the other half to this question.

2 Answers 2

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The main issue is inter-mixing AC and low voltage power in the same box. There needs to be a divider between them.

Fortunately, a number of products come to the rescue and automatically have AC on the inside of the box, and low voltage on the outside. Those which come from the HVAC world are cheap.

  • 24VAC transformer that fits in a knockout
  • 24VAC transformer that fits in a junction box lid
  • 24VAC transformer and a large relay/contactor together in a junction box lid
  • Relay that fits in a knockout (e.g. RiB)
  • Relay and micro-transformer that fits in a knockout (e.g. AUBE)

Of course all this stuff is 24VAC, because that's a quasi-standard to to its universal use in HVAC controls.

There is no need to "ground" low voltage stuff. In fact, grounding to V- could be a bad idea, since for instance LED strips are positive common, so that gets weird fast if you ground V-.

You could take 24VAC via standard "thermostat cable" over to your low voltage hobby box, and largely "do as you will" inside that. I assume synthesize 5VDC from the 24VAC, run Arduino, stuff like that.

The only reason I suggest using 24VAC as an intermediate voltage is the ready availability (cheap) of UL-listed components designed for that voltage, whose packaging beautifully complies with NEC requirements. Such things in 5VDC and 12VDC are much harder to obtain.

Further, 24VAC removes voltage drop issues. It has far less to begin with; and even if you suffer some, the relays don't care, and neither will your local switching 24V-5V supply.

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    the Aube "RC840T" has a transformer that's barely large enough to run the built-in relay, powering the arduino from that and signalling the relay from the arduino could be a challelge.
    – Jasen
    Jan 12, 2022 at 1:54
  • @Jasen yes, it would require careful energy management, sort of like Nest does when C wire is not connected, e.g. use the Aube's capacity to charge a 18650 when not picking up the relay, and suspend charging while picked up. But Aube is perfectly well aware that people are using its products for things like smart 'stats, so I except they'll fill the need with units that have upsized transformers. Jan 12, 2022 at 2:08
  • Would 725.136(D) apply? Or are barriers my only option? Jan 14, 2022 at 2:36
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If you put low voltage (actually called extra low voltage) and mains voltage in the same box then the low voltage needs to be treated as mains voltage.

So don't do that, instead use parts that provide a barrier between the low voltage and mains voltage, if you do that then you have separated extra low voltage (SELV). SELV is stuff like doorbell witing and 24V thermostats.

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