Plenum, riser and general-purpose spaces

Lesson 3 explained the ratings; this lesson explains the spaces. A cable's rating is only right or wrong in relation to the space it runs through, and a single cable run can pass through three kinds of space on its way from a telecommunications room to a desk: a vertical shaft between floors, a ceiling that returns air to the air handlers and a wall cavity in a corridor. Each space has its own requirement, and the run must meet the strictest one it passes through unless it changes cable or enters a raceway on the way. At the Kellerman Building, Sanjay hands Maya the mechanical drawings as well as the cabling drawings on her first day of the riser retrofit. She needs both, he explains, because only the mechanical drawings show which ceilings are plenums.

Plenums

The building code defines a plenum as a chamber that forms part of an air duct system. The most common plenum in an office building is the space above a suspended ceiling when that space is used to return air to the air handling units instead of a ducted return. Air from every room on the floor is drawn up through grilles in the ceiling, travels across the ceiling space and is pulled back to the fan. Anything burning in that space puts its smoke straight into the air system, which is why the code requires FT6 cable in plenums, whatever the building's construction type.

A plenum is not always obvious from below. A ceiling with return grilles is likely a plenum; a ceiling with ducted returns probably is not; a ceiling above a corridor may be a plenum while the ceiling above the offices beside it is not. Raised floors used to supply air to computer equipment are plenums too, and so are some spaces under stages and in mechanical chases. The mechanical drawings, or the mechanical engineer, settle the question. Cobalt Line marks plenum areas on its cabling drawings in a colour wash so the crews can see them at a glance.

Fabricated ducts, the sheet-metal ducts that carry supply or exhaust air, are a different and stricter case. The codes allow wiring inside them only in very limited circumstances, and cabling installers should never run cable through or inside a duct. If a pathway seems to need it, the route is wrong.

Risers

A riser is a vertical run of cable from floor to floor, usually in a shaft or through a stack of sleeves in aligned telecommunications rooms. The Canadian codes do not have a separate riser rating as the US codes do. Instead, the vertical tray test behind FT4 measures the same tendency to carry fire upward, and in buildings required to be of noncombustible construction, exposed cables in a shaft must be FT4 or better. In combustible buildings, FT1 is the minimum for exposed cable. Every floor penetration of the shaft must also be firestopped, as lessons 8 and 9 explain, so that the shaft does not become a chimney.

General-purpose spaces

Everywhere else is general space: wall cavities, ceilings that are not plenums, cable trays in open areas and the inside of telecommunications rooms. Here the requirement depends on the construction type of the building. In buildings required to be of noncombustible construction, exposed cables with combustible jackets must be FT4 unless they are in a totally enclosed raceway, in masonry or concrete, in a service room separated by a fire separation of the required rating, or within the short length at a service entrance the code allows. In combustible construction, FT1 is the minimum.

Construction type

Whether a building is required to be of combustible or noncombustible construction is decided by the building code, from the building's occupancy, height and area. A school, a hospital and a high-rise condo are usually required to be noncombustible; a small wood-frame office may be combustible. Cobalt Line does not decide this; Sanjay gets it from the architect's code analysis, which is on the cover sheet of the architectural drawings, and writes it on the cabling drawings. The difference matters: in a noncombustible building, an FT1 cable exposed in a ceiling is a defect even when the ceiling is not a plenum.

Raceways change the rules

Enclosing cable in a raceway changes what is exposed to fire. Cable inside a totally enclosed noncombustible raceway, such as EMT or rigid metal conduit, is not exposed, so the building code's flame-spread requirements for exposed cable do not apply to it. That is why an FT4 cable may run through a plenum inside EMT. The building code also allows short exposed lengths of FT4 cable within 9 m of such a raceway in a plenum, so a drop from the end of a conduit to a device need not be FT6. Non-metallic raceways and cable trays do not change the requirement in the same way; a cable in an open tray in a plenum must still be FT6, and the tray itself must be suitable.

The strictest space wins

A cable run is only as legal as its least suitable segment. A run from the third-floor telecommunications room at Kellerman to a desk passes through the riser for one floor, across a return-air ceiling and down a wall. The riser and the plenum both apply, so the cable must be rated for both, which in practice means a plenum rating unless the plenum section is in raceway. Crews sometimes pull a general-rated cable for a run that starts in a non-plenum ceiling, not noticing that the last ten metres cross a corridor plenum. Maya's check is to trace each run on the drawing and list every space it passes through before choosing the cable.

# Required rating for each run: the strictest space along the route sets the minimum
ORDER = ["FT1", "FT4", "FT6"]            # weakest first
NEED = {"riser": "FT4", "plenum": "FT6", "wall": "FT4", "tr room": "FT4", "ceiling": "FT4", "conduit in plenum": "FT1"}
def required(spaces):
    worst = 0
    for sp in spaces:
        worst = max(worst, ORDER.index(NEED[sp]))
    return ORDER[worst]
runs = {
    "3-14 desk":       ["riser", "plenum", "wall"],
    "3-22 meeting rm": ["tr room", "ceiling", "wall"],
    "3-31 kitchen":    ["tr room", "ceiling", "conduit in plenum", "wall"],
    "WAP 3-05":        ["tr room", "plenum"],
}
for run, spaces in runs.items():
    print(f"{run:16s} {' > '.join(spaces):40s} needs {required(spaces)}")

3-14 desk        riser > plenum > wall                    needs FT6
3-22 meeting rm  tr room > ceiling > wall                 needs FT4
3-31 kitchen     tr room > ceiling > conduit in plenum > wall needs FT4
WAP 3-05         tr room > plenum                         needs FT6

The example assumes Kellerman, which is required to be of noncombustible construction. The conduit section in the third run needs no flame rating because the cable is enclosed there, but the rest of the run still needs FT4.

Information technology rooms

Rooms for computer equipment often have raised floors with cables underneath. Where the space under the floor is used for air supply, it is a plenum and the rules above apply. The CEC and the building code have specific provisions for some computer rooms, and data centre specifications usually go further, requiring plenum-rated or low-smoke cable throughout. Cobalt Line follows the specification and confirms the requirement with the designer whenever a raised floor appears on a drawing.

Abandoned and leftover cable

Cable left in a plenum or ceiling after it stops being used still burns. Owners and their specifications increasingly require removal of abandoned cable during renovations, and inspectors may call it out where it creates a fire load or blocks access. On the Kellerman retrofit, the specification requires every abandoned cable in the ceilings and riser to be removed, and Maya's crew pulls out more cable than they install on some floors.

Spaces that are easy to miss

Some spaces catch even experienced crews. The space above a ceiling in an exit stair is part of the stair enclosure, and cables not serving the stair should not pass through it at all. Corridors in hospitals and schools may be smoke compartments or rated corridors, and the ceiling above them may be a plenum even when the rooms beside it are ducted. Elevator machine rooms and electrical rooms are service rooms with their own rules. Mechanical penthouses may contain air plenums formed by the building structure. Underfloor spaces in older buildings may connect to the air system through forgotten openings. Each needs the drawings or the engineer to confirm what it is.

Renovations add another trap: a ceiling's use can change while the cable stays. When an owner converts a ducted ceiling to a return-air plenum to save on ductwork, all the existing cable in that ceiling may suddenly be below the required rating. The mechanical engineer should tell the owner, and Cobalt Line's service contracts ask the owner to notify the company of mechanical changes, but in practice the closeout crew finds such changes during routine work. When they do, Maya reports the condition to the owner in writing with photos, because cable that met the code when installed may now need replacing or enclosing.

Kellerman has both problems. Its twelfth floor was converted to a plenum return during a mechanical upgrade in 2019, and the old tenant cabling above it was never replaced. Removing it as abandoned cable solves the problem there, and the new cabling is plenum-rated throughout.

The safest approach on any job with mixed ceilings is the one many owners now choose: specify plenum-rated cable everywhere above ceilings, so that later changes in mechanical design or use cannot leave cable under-rated. The extra cost of plenum cable over riser cable is small compared with the cost of replacing cable in an occupied building.

Showing the spaces on the drawings

The simplest way to avoid rating mistakes is to make the spaces visible to everyone. Sanjay's cabling drawings for every Cobalt Line job carry a ceiling space plan: plenum zones washed in one colour, rated walls drawn heavy, riser shafts outlined and the required cable rating written in each zone. The plan is printed for each crew lead and posted in the site office. When the mechanical design changes, the plan is revised and reissued with the change clouded, so that nobody pulls cable to an old version. Inspectors appreciate it too; Lorraine Dufour, the inspector on Kellerman, asks for a copy at the first rough-in so she can check legends zone by zone.