18/2 Plenum Speaker Wire: The Air-Space Rated Line
Quick answer / TL;DR: 18/2 plenum speaker wire is the two-conductor 18 AWG line in the plenum-rated jacket – the CMP stock the air-handling space requires, above the CMR the wall's cavity takes. The 18 AWG pair serves the short concealed run, and the plenum rating is the figure the ceiling's return air, the duct's chase and the space the building's air crosses read. It is the line for the path the air moves through, and the CMR in wall stock is the line for the wall's cavity the air does not.
The 18/2 plenum speaker wire is the two-conductor line at the thin gauge in the jacket the air-handling space requires: the 18 AWG pair, the two conductors the speaker's circuit takes, in the plenum-rated stock the ceiling's return air and the duct's chase are built to hold. The 18/2 is the count and the gauge, and the plenum is the jacket's rating – the figure above the CMR the air the building moves reads rather than the wall's cavity alone.
This is the air-space line's view: the 18/2 figures and the run they serve, the plenum rating the jacket carries, the difference from the CMR in wall stock, and the installs where the air-handling path makes the plenum the required line.
The 18/2, and the Run It Serves
The 18/2 is the line's count and gauge: the two conductors – the plus and the minus the speaker's circuit takes – at the 18 AWG, the thin conductor the short concealed run holds. The 18 AWG's resistance is the figure the run's length reads against, and the short throw the thin gauge serves is the one the ceiling's speaker and the plenum's path take – the amplifier's position to the ceiling's speaker at the distance the cavity holds, the run the 18's loss carries within the rule's fraction. The 18/2 is the pair the ceiling's short run needs, and the plenum is the jacket the path through the air space requires – the count, the gauge and the rating the install's three figures.
| The line's figure | The value | What it serves |
|---|---|---|
| The conductors | Two, the plus and the minus | The speaker's circuit at the ceiling's position |
| The gauge | 18 AWG, the thin conductor | The short concealed run the cavity holds |
| The jacket | The plenum rating, the CMP stock | The air-handling space the path crosses |
| The path | The ceiling's return air, the duct's chase | The space the building's air moves through |
The table is the line's four figures, and the reading is the path the plenum serves. The conductors and the gauge are the circuit's figures – the pair and the thin conductor the short run holds. The jacket is the rating's figure, and the path is the space the air moves through. The 18/2 plenum is the line the three figures select together: the pair at the thin gauge for the ceiling's short run, and the plenum jacket for the air-handling space the path crosses.
The Plenum Rating, and What It Adds
The plenum rating is the jacket's figure above the CMR, and what it adds is the smoke and the flame behavior the air-handling space reads. The wall's cavity is the CMR's path – the enclosed space the drywall covers – and the air-handling space is the plenum's: the ceiling's return air, the duct's chase, the space the building's air crosses on its way through the system. The fire in the air-handling space is the fire the air carries, and the plenum jacket's material is the figure the smoke and the flame's spread are rated against. The CMR's material is the wall's cavity's wire, and the plenum's is the line above it for the space the air moves through – the rating the path requires rather than the one the gauge or the price selects.
The CMP marking, and the check at the roll
The plenum rating's marking is the CMP on the jacket, and the check at the roll is the print the install reads before the run goes into the ceiling. The CMP is the plenum's listing – the communication wire's rating for the air-handling space, the material the smoke and the flame's figures hold – and the print runs along the jacket at the interval the listing sets. The CMR's print is the wall's cavity's figure, and the absent print is the general-purpose stock the air space does not allow. The check is the jacket's print at the spool, and the plenum run's line is the one the CMP confirms before the ceiling's panels close.
The Difference from the CMR In Wall Stock
The difference between the plenum line and the CMR in wall stock is the path the rating answers, and the figure is the space the run crosses. The CMR is the wall's cavity's stock – the enclosed space the drywall covers, the listing the path's fire behavior reads. The plenum is the air-handling space's – the ceiling's return air, the duct's chase, the space the building's air crosses – and the rating above the CMR is the one the air's path requires. The same pair at the same gauge, the two jackets rated for the two spaces, and the run's path is the figure that selects between them. The install that crosses the air-handling space is the one the plenum stock serves, and the wall's cavity is the CMR's – the rating by the path, and the path is the space the wire's run crosses.
The mixed path, and the rating the run takes
The mixed path is the run that crosses both spaces, and the figure is the rating the stricter stretch requires. The run that starts in the wall's cavity and continues through the ceiling's return air is the one the plenum stock serves for the whole of it – the line rated for the air space carrying the wall's stretch without the second wire's joint – or the two stocks joined at the boundary the path changes at. The plenum rating is the higher of the two, and the run that crosses the air-handling space anywhere along its length is the one the plenum stock covers, the joint the boundary's option and the single stock the simpler dress. The rating the run takes is the figure the path's strictest stretch states.
18/2 plenum speaker wire is stocked as the two-conductor thin-gauge line in the plenum-rated jacket, the CMP print along the wire, for the ceiling's return air and the duct's chase; the catalog lists the gauge, the rating and the length per roll, and the air-handling space is the path the rating serves.
Real-World Applications
The office's ceiling speaker: the 18/2 plenum stock through the return air space, the CMP print the inspector reads, and the thin gauge the ceiling's short run holds. The home's duct chase: the run the air-handling path crosses, the plenum jacket the space requires, and the ceiling's panel the check preceded. The venue's ceiling array: the 18/2 stock along the joist space the air moves through, the rating above the CMR the path's figure, and the run the air space's jacket serves end to end. In each the 18/2 plenum speaker wire is the line the air-handling path selects – the pair at the thin gauge, the jacket at the plenum rating, and the run the building's air moves through the space the wire crosses.
Related reading: The rules the rating runs under, in full: speaker wire in wall code — speaker-wire-in-wall-code
Related reading: The gauge the run's length selects: what gauge speaker wire for in wall — what-gauge-speaker-wire-in-wall
Frequently Asked Questions
What is 18/2 plenum speaker wire?
The two-conductor 18 AWG speaker line in the plenum-rated jacket – the CMP stock the air-handling space requires, above the CMR the wall's cavity takes. The 18/2 is the count and the gauge, the thin pair the short concealed run holds, and the plenum is the jacket's rating for the ceiling's return air, the duct's chase and the space the building's air crosses. The line is the three figures the air-handling path selects together.
When do I need plenum-rated speaker wire?
When the run crosses the air-handling space – the ceiling's return plenum, the duct's chase, the space the building's air moves through. The plenum jacket's material is rated for the smoke and the flame the air would carry, and the wall's cavity is the CMR's path rather than the plenum's. The run's route is the figure that states which rating the path requires, and the CMP print at the jacket is the check the install reads at the spool.
Can I use CMR in wall wire in a plenum space?
The CMR is the wall's cavity's listing, and the air-handling space is the path the plenum rating serves – the rating above the CMR for the space the air crosses. The plenum space's fire behavior is the figure the higher listing answers, and the CMR stock is the line the wall's path carries rather than the air space's. The run that crosses the air-handling space is the one the plenum stock serves, and the mixed path's run takes the plenum rating for the strictest stretch.
Is 18 gauge thick enough for ceiling speakers?
For the short concealed run, yes – the 18 AWG's resistance holds within the rule's fraction at the ceiling's throw, the amplifier's position to the speaker at the distance the cavity holds. The longer the run, the more the thin conductor's loss spends, and the gauge the run's length selects is the figure the install reads against the speaker's impedance. The 18/2 is the short run's line, and the distance the ceiling's path measures is the figure that confirms it.
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