It is in apartment on 10th floor. I suppose it may be the ventilation tube, but I can't understand why the ventilation tube has such a thick walls. And why it ends there, instead of going to some grill in the ceiling...

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    Any chance you can get a closer shot of it? Also...ewww at the NM in a high-rise apartment building! – ThreePhaseEel Apr 5 '15 at 7:48
  • So far there is no way to get a closer shot without opening the ceiling.:( I've made this shot after opening the outlet which looks like ventilation grill but has nothing connected to it. – Jim Apr 5 '15 at 9:40

Looks like a plenum return to me, especially since you said there's a ceiling grill into this area. Basically the false ceiling is acting as part of the ductwork. Very common in commercial buildings, less so in residential... but not at all unreasonable.

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    Does make the "not remotely plenum rated" NM wire look even worse, though. You won't die from the smoke off the network wire (if it's the usual blue=plenum wire) but the power wiring will do you in...ewww indeed. – Ecnerwal Apr 5 '15 at 20:56
  • What, you:re expecting the typical cable installer of landlords to have a Clue? ;-J – keshlam Apr 5 '15 at 21:35
  • No, I'm just sad to see the results of a throughly bribed building inspector endangering more lives where code has "solved" this particular means of killing people... If it was followed. – Ecnerwal Apr 5 '15 at 21:39
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    Not until there's a fire... at which point flammable and dripping plastic will make the problem worse ... – keshlam Apr 5 '15 at 21:48
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    the toxic smoke from the burning plastic makes it worse faster. The insulation on plenum rated cables is not quite fireproof, but has a low level of toxic smoke production compared to "normal" plastic insulation. The thick walls on the tube are almost certainly (thermal) insulation, as has been mentioned in other answers/comments. – Ecnerwal Apr 5 '15 at 23:16

Looks like somebody forgot to connect an HVAC supply duct -- probably to the vent you opened to take the picture! The "thick walls" on the duct are probably insulation, which is why I suspect it is a supply duct.

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