Plenums

[crownjason86]

If an AHU closet in a dwelling is being used to distribute air with louvered doors or is the return, can you use liquid tight or is metal flex required? I'm familiar with NEC 300.22 that is talking about Plentum ceilings but would just like some thoughts on this.

 

[augie47]

Welcome to the Forum.

A good question.
Based on this wording in 3-00.22, "This section shall not apply to habitable rooms or areas of buildings, the prime purpose of which is not air handling", I would think that is not a plenum space.
The referenced "NFPA 90A-2015" might shed some light on it.

 

[LarryFine]

Welcome to the forum.

Interesting question. My mom's townhome was built with the entire storage closet as the return pathway, with a louvered grill over a hole through the wall and the filter directly on the air handler.

However, nothing else was done in the closet as a plenum space. The standard breaker panel, regular NM in flex fed the electric furnace, water heater, etc., and there was a plain pull-chain light socket.

She used a lot of talcum powder, so everything in the closet was coated with it. I rebuilt the return with a 16" flex duct and relocated the filter to a return grill that held the filter at the outer wall.

 

[Fred B]

Not sure the exact space configuration the OP has in mind

NFPA90A - 2024
Definitions:
3.3.25.1 Air-Handling Unit Room Plenum.
An individual room containing an air-handling unit(s) used to gather air from various sources and combine the air within the room before returning it to the air-handling unit.
3.3.25.2 Apparatus Casing Plenum.
A sheet metal construction attached directly to a fan enclosure, fan coil unit, air-handling unit, or furnace bonnet for the purpose of connecting distribution ducts.

300.22 NEC-2023
"this section shall apply to the installation and uses of electrical wiring and equipment in ducts used for dust, loose stock, or vapor removal; ducts specifically fabricated for environmental air; and other spaces used for environmental air (plenums)."

This seems to indicates section not limited to ceiling plenum.