3.6 Floor Outlets, Poke-Throughs & Access-Floor Cable Support
Key Takeaways
- A poke-through device is a listed, fire-rated assembly installed in a cored hole through a concrete floor slab; it restores the F-rating and T-rating of the floor that the core drill destroyed.
- Concrete-pour floor boxes are set and leveled before the slab is poured, while after-set boxes are installed into an existing slab, and both must be listed for the abuse and moisture conditions of the finished floor.
- Every core drill through a structural slab requires ground-penetrating radar or X-ray scanning first, because cutting a post-tension tendon or main rebar is a structural failure and a life-threatening release of stored energy.
- In a raised access floor used as an air plenum, cabling must be plenum-rated (CMP/OFNP) or in an approved raceway, must be supported off the slab in trays or baskets, and must be routed so it does not block cold-aisle airflow.
- Cable entering a floor box or poke-through must keep the 4x outer-diameter static bend radius, so deep floor boxes, radius-controlled entry fittings, and a 12 to 36 inch service loop below the slab are all required.
Floor Outlets, Poke-Throughs & Access-Floor Cable Support
Open-plan offices, trading floors, conference rooms, auditoriums, and laboratories rarely have a wall within reach of the workstation. In those spaces the telecommunications outlet comes up through the floor. The BICSI Installer 1 Exam Content Outline lists three separate tasks that live below the finished floor line — prepare telecommunications outlet at floor, install poke throughs, and install cable support systems under the floor — all inside the 31%-weighted Establish Pathways and Space area.
Working in a floor slab raises the stakes considerably compared with a drywall partition. A slab is a structural element and a rated horizontal fire barrier. A mistake is not a patch of drywall; it is a structural repair or a life-safety violation.
1. Poke-Through Devices
A poke-through is a factory-engineered, listed assembly that drops into a cored hole in a concrete floor and delivers power and/or telecommunications to the floor above while restoring the fire rating of the slab.
+-----------------------------------------------------------------------------+
| POKE-THROUGH ASSEMBLY (SECTION) |
| |
| [FINISHED FLOOR - LEVEL 2] |
| ====[ Flush Cover Plate / Recessed Service Fitting ]==== |
| | Jacks, receptacles, or a combination service head |
| +----+------------------------------------------------+ |
| | CONCRETE SLAB [ INTUMESCENT COLLAR / WRAP ] | <-- Expands under |
| | (2-hour rated) [ Listed Poke-Through Barrel ] | heat to seal |
| +----+------------------------------------------------+ the core |
| | Below-slab housing / dividers (power | telecom) |
| [CEILING PLENUM - LEVEL 1] |
| ---> Conduit or cable feed from the TR pathway |
+-----------------------------------------------------------------------------+
Key Characteristics
- Fire performance: The device is tested as a complete through-penetration firestop system. The listing states the F-rating it restores (commonly 1 or 2 hours), the T-rating, and often a scrub-water / W-rating for wet-mopped floors. Section 4.1 defines those ratings.
- Core diameter: Poke-throughs are listed for a specific core size — typical devices use a 2 in, 3 in, or 4 in (50, 75, or 100 mm) core. Coring oversize voids the listing.
- Physical separation: Devices that carry both power and telecommunications contain an internal barrier that keeps the two systems in separate compartments, satisfying the separation principle covered in Section 3.4.
- Capacity is small. A poke-through barrel is a narrow tube. Count the cables the service head needs, check the manufacturer's fill table, and remember the 4x outer-diameter static bend radius still applies inside the housing.
- Activation limits: Building codes limit how many through-penetrations may be made per unit area of a rated slab, and the AHJ enforces it. Do not core a new hole for every desk move; feed from an existing device when possible.
2. Floor Boxes: Pour-In-Place vs. After-Set
| Type | When It Is Installed | Notes |
|---|---|---|
| Concrete-pour (cast-in-place) box | Set, leveled, and secured to the deck before the concrete is poured | Must be sealed against wet concrete intrusion, screeded flush, and protected until finish. Once the pour cures, its location is permanent. |
| After-set box | Installed into an existing slab after a core or saw cut | Adjustable-height rings let the installer set the cover flush with carpet, tile, or terrazzo. |
| Recessed / flush service fitting | Either | A hinged or sliding lid sits flush with the walking surface; cords exit through a gasketed door slot. |
| Floor monument (tombstone) | Surface of the finished floor | A raised housing over a poke-through or floor box, used where a flush lid is not required. |
Listing matters. A floor box in an area that gets wet-mopped must be listed as scrub-water resistant. A box in a wet location needs a listing for that location. Metallic floor boxes are conductive components in the floor and are bonded to the telecommunications grounding system per the rules in Sections 4.2 and 4.3.
3. Underfloor Duct and Cellular Floor Systems
In buildings designed with distribution in the slab, the cabling pathway is cast into the concrete.
- Underfloor duct: rectangular metal ducts cast into the slab on a grid. A larger header (feeder) duct runs from the TR and crosses smaller distribution ducts. Access is through preset inserts (installed at pour) or afterset inserts (cored into the duct later).
- Cellular floor: the corrugated cells of a metal floor deck are closed off and used as raceways, again fed by a header duct.
- Trench duct: a wide, shallow, removable-cover trench in the slab, common in equipment rooms.
For the installer the practical rules are the same as for conduit: respect the fill limits from Section 3.2, never mix telecommunications and power in the same compartment, use bushings and radius fittings at every insert, and pull with lubricant and a controlled tension.
4. Raised Access Floors
A raised access floor sits on pedestals above the structural slab, creating a service void of typically 6 to 36 in (150 to 900 mm). It is the dominant approach in data centers, computer rooms, and trading floors.
+-----------------------------------------------------------------------------+
| ACCESS FLOOR CABLE SUPPORT (SECTION) |
| |
| [FLOOR PANELS + CUTOUTS WITH GROMMETS] |
| =========[]=========================[]========================== |
| | | |
| [PEDESTAL] [PEDESTAL] |
| | +-----------------------------+ | |
| | | WIRE BASKET / TRAY (COMMS) | | <-- Cables OFF the slab |
| | +-----------------------------+ | and above the airflow |
| | <=== COLD SUPPLY AIR ===> | |
| | +-----------------------------+ | |
| | | POWER WHIPS - separate tier | | <-- Physical separation |
| ===+=====+=============================+====+=== per Section 3.4 |
| [STRUCTURAL SLAB] |
+-----------------------------------------------------------------------------+
Rules for Under-Floor Cable Support
- Support the cable — do not lay it on the slab. Cables resting on the structural deck are walked on during panel lifts, sit in any water that reaches the slab, and are impossible to re-dress. Use wire basket tray, ladder rack, or listed under-floor cable support brackets clipped to the pedestals.
- If the void is an air plenum, the plenum rules apply. As Section 1.3 established, a pressurized under-floor supply void is an environmental air space. All cable routed in it without an approved raceway must be CMP or OFNP listed, and the support hardware must be suitable for the space.
- Do not dam the airflow. Run cable trays parallel to the airflow direction wherever possible, keep bundles out from under perforated tiles, and never allow a cable pile to grow into a wall across a cold aisle. A cable dam is a thermal event waiting to happen.
- Separate power from telecommunications. Maintain the Section 3.4 separation distances, or place power whips on a different tier from the telecommunications basket.
- Protect every panel cutout. Cable passing through a floor panel cutout must be protected by a grommet or brush seal. The bare edge of a cut floor panel will shear a jacket, and an unsealed cutout leaks conditioned air.
- Bond the metal. Access floor pedestals, stringers, and metallic trays are bonded to the telecommunications grounding system with a minimum 6 AWG conductor as covered in Section 4.3.
- Keep the void accessible and clean. Removed panels must be replaced, and no abandoned cable may be left below the floor — the removal mandate of Section 9.1 applies to the under-floor plenum exactly as it applies to the ceiling.
5. Core Drilling: The Non-Negotiable Safety Sequence
[!CAUTION] Never core a structural slab without scanning it first. Modern concrete floors contain reinforcing steel, electrical conduit, radiant tubing, and in many buildings post-tensioned steel tendons under enormous stored tension. Severing a tendon can whip the strand through the slab, cause explosive spalling, and compromise the structure. Cutting main rebar can trigger a structural repair costing more than the entire cabling contract.
- Obtain written permission from the general contractor, structural engineer of record, and building owner. Coring is almost always a permitted activity with its own approval chain.
- Scan the location with ground-penetrating radar (GPR) or X-ray and mark the rebar and tendon layout on the slab. Shift the core, do not shift the scan.
- Control the area below. Post a spotter on the floor below, barricade the drop zone, and account for the core slug — a 4-inch concrete plug falling one story is a fatality hazard.
- Manage water and slurry. Wet coring produces slurry that will find its way into the space below. Dam and vacuum it.
- Firestop immediately. The moment the core is complete, the slab's rating is broken. Install the listed poke-through, sleeve, or firestop system before leaving the area, and tag the penetration with its UL system number as required in Section 4.1.
- Wear the PPE. Eye protection, hearing protection, respiratory protection for dry coring silica dust (a regulated OSHA hazard under the respirable crystalline silica standard), and gloves.
What is the defining functional characteristic of a listed poke-through device installed in a cored concrete floor slab?
An installer is about to core a 3-inch hole through the second-floor slab of an occupied office building to add a poke-through. What must happen before the drill turns?
In a data center where the raised access floor void is pressurized with chilled supply air, which set of practices is required for under-floor telecommunications cabling?