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I found this in the back-corner of our second-floor closet:

power supply

unplugged

Presumably this is not to code. Has anyone seen anything like this before, or have any ideas what it's for? Unplugging it doesn't seem to have broken anything, and there aren't any small electronics on any of the nearby walls.

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    code isn't too concerned with low voltage wiring, are you sure it's a problem? – dandavis Sep 28 '20 at 21:47
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    Well, the missing faceplate certainly isn't Code! As to the low-voltage side of things, can you get us clear shots (or a readout) of the printing on the low-voltage wiring? That's the determinant as to whether the LV wiring is of the proper type to be used in a wall to begin with... – ThreePhaseEel Sep 28 '20 at 23:34
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It looks like the power supply for an old alarm system.

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  • concur with @freshop, You might have the remnants of the old system hanging in a nearby closet up high or down low, possibly under a panel cover. – mark f Sep 28 '20 at 19:50
  • There is an old alarm system downstairs, so this is probably correct. – BlueRaja - Danny Pflughoeft Sep 29 '20 at 11:21
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Nobody mentioned door-bell.
Many electrical doorbells use a AC transformer with output anywhere in the 5 to 20V range.
9 and 12V AC are most common.

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    @Criggie Several years ago my father janked several yards of thin 2-lead wire from behind the drywall when he re-modeled his hallway. He thought it was old alarm-system wiring, no longer used. It wasn't until several days later when someone was at the door he realized he destroyed the door-bell. Of course the walls had been re-done by then. Now he has a wireless door-bell :-) – Tonny Sep 29 '20 at 12:47
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    Doorbell transformers are generally 16vac – batsplatsterson Sep 29 '20 at 15:20
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    @batsplatsterson That is very location specific I think. Here they almost exclusively 12 Vac and the few that are not are 9 Vac. Some very old ones (30+ years) are 8 or 10 Vac. I have never seen a 16 Vac. – Tonny Sep 29 '20 at 15:24
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    Well, the outlet in the picture is clearly a US-style, and OP's profile seems to imply that as well. (Could be Canada, but it'd be the same thing there.) – Darrel Hoffman Sep 29 '20 at 16:53
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    In the USA, 16VAC is the most common but can also be lower (8VAC) so you dont get "buzzed" or 24VAC (uncommon), this is what I have heath-zenith.com/products/… – Richie Frame Sep 30 '20 at 0:11
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I agree it's likely this is an alarm system power supply, I know Honeywell had some 9VAC panels and likely others.

Believe it or not it may be code compliant, the rules for class 2 wiring are pretty loose. It's not really workmanship anyone would brag about but it's a pretty common hack.

If the cable is suitably rated CL2 or better it's compliant. However it looks like a zipcord which is not likely rated for installation inside walls on class 2 circuits. However, at 9VAC, it's not much of a hazard.

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  • Note that even if it's nominally 9V, there's absolutely nothing there that instills confidence that a short on that red cable won't get 1000A going through it. The irony would be if it were supplying power to a /fire/ alarm, particularly if its standby batteries were long dead. – Mark Morgan Lloyd Sep 29 '20 at 13:39
  • @MarkMorganLloyd the only way a transformer of that size could supply 1000A would be if the input were shorted to the output. And then you'd have even bigger problems. – Mark Ransom Sep 29 '20 at 14:35
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    I think @MarkMorganLloyd may have been thinking about a short from a 120V line to the low voltage line, like if a nail or screw were to pass through both of them. – StayOnTarget Sep 29 '20 at 14:38
  • Actually, I saw MADE IN CHINA and at that point ceased making any assumptions whatsoever regarding what's in the box. – Mark Morgan Lloyd Sep 29 '20 at 15:03
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    @alephzero, Course for horses. Brick houses around here (California) have a bad habit of falling down if the ground shakes a little bit. – The Photon Sep 29 '20 at 21:28

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