Plenum

Yes.

But many places now want plenum rated cable that's anywhere behind something. Varies by area.

If BOTH incoming and return air flows are ducted then what you are saying is correct. In practice though only incoming flow is ducted and so basically any cavity above the drop ceiling can facilitate air return hence moving air hence new oxygen in case something burns up there.

So, if your fire marshal tells you it's plenum it's really hard to argue because usually he is correct, both technically (you don't control the return air) and by law.

When the fire marshal tells you something it's true even if it's not. Arguing with them is asking for the doors to be padlocked. I work with architects and they'll debate the gray in the building codes all day long but when the fire marshal arrives, he gets obeyed. He can ruin you life faster than almost anyone on the planet not using a weapon.

Including an ex-wife (so I have been told).

Plenum, generally, has to be run in areas where wiring and air ducting are together, whether ceiling or floor. The explanation is that in case of fire, fumes may be carried by the hvac. Therefore, if a flame retardant, or slow burn material is used, there is less chance of noxious fumes being carried to people at outside reaches of the hvac in other rooms or other floors.

Interesting, the EU has banned polyvinylfloride cables and allows traditional polyvinylchloride cables.

There is no "perfect" cable. Different regulator bodies with different priorities on what is bad vs very bad will come to different conclusions about what is good or required.

... that depends on whether your children are involved.

-- Robert

DTC wrote in part:

Which is worse: a cable that produces worse fumes or a cable which produces them at lower temp?

US NFPA says the latter, EU might well say the former. Heaven forbid they agree upon anything. But in reality, they both have different building mixes and codes that gives rise to different preferences.

-- Robert

Lets hope they dont' start mandating metal conduit for all runs.

The US PoV is PVC cables are *fuel* and thus do not belong where there's a draft.

The EU thinks nerve gas is a worse problem than fuel...