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I have these Aeotec Multisensor 6 devices that are pretty neat, and they have a recess-mounting bracket. They can be battery- or USB-powered. I’d like to USB-power them and recess them in my ceilings, but that would mean installing receptacles in the attic crawl space, and then plugging in a USB adapter and running the cable to the sensor.

My googling suggests this may not be allowed, as it would be “permanently installed.”

For measuring temperature, it might be better for the sensors to be placed on the wall, rather than on the ceiling where it’s hottest. In this case, to neatly power them, I’d have to put a receptacle in the wall, and this would not be allowed, at least not without an access panel. Would it actually be allowed if there was an access panel? I guess in that case it would be more of a cabinet, and thus acceptable, right?

Are there any other alternatives that would be code-compliant?

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    Installing an outlet in your attic, so long as it's accessible for maintenance purposes should be completely legal. Plugging a switch operated lamp into the outlet for lighting in the attic should also be completely legal. I can't fathom why plugging in a USB wall-wart would be any different. What "googling" shows you that this is illegal? You might want to edit your post to include the proposed code violations so the electricians can confirm or dispel the the concerns.
    – FreeMan
    Jan 5 at 12:42

3 Answers 3

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Frame Challenge - It isn't about 120V AC power at all, it is about USB power.

Everybody is into WiFi and other wireless solutions these days. "No wires!" they say, until they realize they need power. The professional solution, used frequently for VOIP phones and security cameras, is Power over Ethernet, or PoE. That leads to two solutions:

  • Devices that support PoE

A device designed to support PoE may cost more. Not because the technology is inherently more expensive. In fact, including an AC->USB power adapter and a wireless chipset (WiFi, Zigbee, etc.) will likely cost the manufacturer more than a PoE chipset and an ethernet jack. But consumer marketing is all "USB adapter and wireless" so that's what gets produced. Search some more, you may find the right combination at a reasonable price, or you might find only industrial grade sensors that cost a lot more.

  • USB power via PoE

It is possible to turn the power from a PoE port into USB power. A web page explaining this is here and here is a sample device from Amazon:

PoE to USB

Note that this is not a USB extender. It doesn't let you take your USB mouse/keyboard/printer/webcam/etc. and put it 300' away via PoE. There are devices that can do that sort of thing, but they are much more complex and not necessary here as you only need USB as a power source.

Of course, now you need to run Ethernet cable (e.g., Cat 5e) to each device. But that can be done much more easily than 120V AC power, and you don't (in most places) need an electrician, permits, conduit, UL-listed boxes, etc.

The power needs to come from somewhere. You can use a device (PoE injector) to add power, but the better solution, especially if you have multiple devices is to add an Ethernet switch that includes PoE ports.

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    POE is underrated as a solution - I've run ethernet to a separate building to get a light out there without having to involve an electrician, and ended up also running a raspberry pi there to do backups.
    – Criggie
    Jan 5 at 22:15
  • If the aim is simply to provide USB power. POE would be overkill. Running 12/24V terminated with a buck converter would be significantly cheaper. Plus cabling would be cheaper and more durable than with UTP.
    – Aron
    Jan 6 at 4:09
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    @Aron But UTP + PoE may be more "off the shelf" and if OP switches to devices that (even if they aren't designed for PoE for power) can communicate over ethernet then that would be a big advantage. Jan 6 at 4:14
  • POE isn't a bad solution. Pro/Con. But UTP is a major investment for most DIYers (crimping your own RJ45 sucks); much more than running a pair of 16 gauge.
    – Aron
    Jan 6 at 4:23
  • I never crimp my own RJ45. Run cable to wall jacks. If stuck in an attic, use surface mount jacks. Much more reliable, and much easier, to punch down jacks than to crimp plugs. Also provides an easy upgrade path - e.g., if you want to replace an outdoor sensor with a camera. Jan 6 at 4:33
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Maybe instead of placing the receptacles in the attic, you could place them in the ceiling. You would install so called clock outlets or recessed outlets. A recessed outlet is often used for wall-hung TVs, clock outlets are similar, but feature a small mounting hook that you might be able to use to hang your sensors from if you're lucky. You would now also need to find a USB power adapter that is small enough to fit into the recessed cavity of the receptacle, and you could then simply mount your sensor over the top of the receptacle.

You could also have a mounting place made for the sensor that would cover the outlet if needed and would make it easy to access. There are 3D printing services who can design and produce this type of stuff pretty reasonably.

As user statueuphemism pointed out in a comment, if you use a box like in the first picture, which will accept any type of regular receptacle, you can even just get a receptacle that has USB ports in it. Then you wouldn't need an additional power adapter at all.

Some examples of available styles of these outlets: enter image description here enter image description here

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    I personally like this solution and you can even buy outlets that have built-in USB to remove the need for a wall-wart at all. Jan 5 at 13:56
  • Good point. However, then you might lose the recessed feature of the outlet, which might make it look less neat. But I guess the box in the first picture would work with that setup. Jan 5 at 14:06
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    Using a recessed outlet that offers only a USB connection (no standard outlet) also has the added benefit of baffling the next homeowner. Extra points if it's a vaulted ceiling that's hard to get up to.
    – bta
    Jan 6 at 3:10
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give that they are battery powered they don't need much current, it should be fine to just use really long USB cables. This is a SELV circuit and SELV has many fewer requirements under code.

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    Standard USB cables are normally maximum 5 meters ~ 16 feet due to technical limitations of the USB protocol. That is not an issue for "power only" but does limit what may actually be available. On the other hand, ethernet cable (again, not necessarily if used only for power) can go up to 100 meters ~ 328 feet. Jan 5 at 19:02
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    @manassehkatz-Moving2Codidact power only is a huge issue with Standard USB cables. Standard USB cable gauge would have huge losses over long runs at any appreciable current.
    – Aron
    Jan 6 at 4:11

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