What Is Slab-Mounted Concealed Chase Routing? (Code Specs)
Slab-mounted concealed chase routing is the placement of building services in a shallow, secured channel above or beside a concrete floor slab, then concealing it with a finish. The chase must not weaken the slab, block required access, mix incompatible services, or defeat fire-rated construction. Its dimensions are project-specific; codes set service and access rules, not one universal chase size.
A well-planned chase can keep wiring, pipes, or ducts out of sight without making a room harder to heat, cool, clean, or maintain. The challenge is that “hidden” cannot mean inaccessible or unsafe. I treat a concealed chase as a coordinated building-services route, not a reason to cut into concrete or cover over parts that need inspection.
The rules depend on the locally adopted building and electrical codes, approved construction documents, and product listings. Check those requirements with the building department and the relevant licensed trades before work begins.
What slab-mounted concealed chase routing means
A slab-mounted chase is a low-profile enclosure fixed above or next to a concrete slab. It carries selected services while leaving the slab intact unless an approved design allows a penetration. “Concealed” describes the finished appearance; it does not remove requirements for access, support, service separation, or fire protection.
A chase may be built along a wall, at a floor edge, or as a raised floor detail. Its width and depth must fit the actual services, their supports, bends, insulation, and access needs. There is no general code dimension that applies to every chase.
This distinction matters: routing services in a chase above a slab is different from cutting a groove into the slab. A surface chase can still affect structure if its anchors enter concrete, so even that work needs an approved attachment plan.
Planning the route: slab survey, permits, and service coordination
A slab survey establishes what the concrete contains and what the proposed work may affect. Before choosing a route, identify the slab type, thickness, reinforcement, embedded utilities, rated assemblies, and nearby equipment. Confirm permit and inspection rules locally; a scan is useful information, not permission to cut or drill.
Start with drawings and records, then compare them with the actual site. Plans may be incomplete or out of date. Locate likely tendons, reinforcing steel, conduits, and pipes using suitable scanning methods, and have a qualified person interpret the results when needed.
Post-tensioned and reinforced slabs need special care. A shallow sawcut, drilled anchor, or core can strike a tendon or reduce structural capacity. Do not cut, core, or drill until embedded elements have been located and the required structural approval has been obtained. Scanning reduces uncertainty but does not authorize a change to the structure.
Before installation, coordinate the route on a drawing. Show chase width and depth, bends, supports, access panels, equipment clearances, door swings, and any slab attachments or penetrations. Also identify which trade is responsible for each service and for sealing any rated opening.
Access and clearances: electrical equipment, boxes, and ducts
Access rules apply to the equipment and connections inside or near a chase, not just to the chase opening. Electrical working space, junction-box access, duct sizing, and service separation each have their own rules. A chase should be sized only after those needs are known, then checked against approved plans and product instructions.
For covered electrical equipment, NEC 2023 §110.26(A)(1) sets working-space depth. For equipment rated 0–150 volts to ground, 36 inches is the minimum depth under the applicable table condition. Section 110.26(A)(2) requires a width of at least 30 inches or the equipment width, whichever is greater. Section 110.26(A)(3) requires 6 feet 6 inches of height or the equipment height, whichever is greater.
These are working-space rules, not a general chase-size prescription. The required depth can vary with the conditions in the NEC table, and the rule applies to equipment covered by that section. Keep required working space clear; do not use it as a storage recess or cover it with a chase.
NEC 2023 §314.29 requires boxes to remain accessible without removing building finish or structural parts. A junction box may have a suitable access panel, but it must not be buried behind permanent cladding. Plan the panel before finishing so it remains usable and identifiable.
Duct size and airflow also have no universal chase clearance. Duct dimensions and routing affect pressure loss and system performance. Residential duct design commonly uses ACCA Manual D; have the system designed for its airflow needs and verify clearances against applicable code and manufacturer instructions.
| Item | Verified rule or design basis | What it means for the chase |
|---|---|---|
| Electrical working-space depth | NEC 2023 §110.26(A)(1): 36 in. for covered equipment rated 0–150 V to ground under the applicable condition | Keep the required equipment space outside the chase footprint |
| Electrical working-space width | §110.26(A)(2): 30 in. or equipment width, whichever is greater | Do not narrow the space with trim, panels, or stored items |
| Electrical working-space height | §110.26(A)(3): 6 ft 6 in. or equipment height, whichever is greater | Check overhead finishes and obstructions |
| Raceway fill | NEC Chapter 9, Table 1: 53% for one conductor, 31% for two, 40% for more than two, subject to notes and configuration | Size raceways by the applicable fill calculation, not by appearance |
| Junction-box access | NEC 2023 §314.29 | Provide access without removing finish or structural parts |
| Duct dimensions | System design and applicable rules; no universal chase clearance | Confirm airflow, pressure loss, insulation, and service access |
Raceway fill percentages are not permission to pack any raceway to those figures without checking the table notes, conductor types, and raceway setup. Service separation also depends on code, listings, and manufacturer instructions. Coordinate electrical, plumbing, gas, and duct routes rather than assuming all can share one undivided space.
Execution: supports, raceways, penetrations, and firestopping
A compliant installation uses listed materials and approved attachment details suited to the service and location. Install supports at the required intervals, protect services from damage, and avoid unapproved slab penetrations. Where a route passes through a rated assembly, use an approved firestop system compatible with that assembly and the penetrating service.
A practical sequence is:
- Confirm the approved route, permits, and inspection points before materials are installed.
- Mark the chase and verify the route against the slab survey and coordinated drawings.
- Install listed raceways, boxes, supports, and service components as required by their instructions.
- Keep joints and boxes accessible, and maintain required separation between services.
- Stop if field conditions differ from the plan; obtain review before changing an attachment or penetration detail.
- Test and inspect the services before closing the chase.
NEC 2023 §300.21 and adopted building-code provisions require protection of openings through fire-resistance-rated assemblies. The correct firestop depends on the assembly and service; use an approved, compatible system rather than a generic sealant. Record the system and location before the chase is closed.
Never assume an anchor is harmless because it is short. Confirm its location and permitted embedment from the approved structural detail. For post-tensioned or otherwise reinforced slabs, obtain structural-engineer approval for any proposed drilling, coring, cutting, or anchoring that could affect the slab.
Worked installation scenarios and trade-offs
These examples are planning scenarios, not claims about specific completed jobs. They show how I would apply the code and coordination checks to common layouts. The actual route still depends on site conditions, adopted rules, approved drawings, and the products selected.
Scenario 1: A wall-edge chase for electrical raceway. A homeowner wants to conceal a short raceway along a concrete floor. The route is planned above the slab, with no concrete cut. The installer confirms that the selected raceway and supports suit the location, calculates conductor fill under NEC Chapter 9, and keeps nearby electrical equipment working space clear. Any junction box remains accessible through a planned panel.
Scenario 2: A chase near a post-tensioned slab. A proposed floor-edge detail needs anchors. The slab records and a scan are reviewed, but the scan alone does not approve the anchors. The work pauses until the structural professional approves the attachment location and method. If the detail cannot be approved, the route is revised to avoid the slab attachment.
Scenario 3: A route crossing a rated wall. The chase carries a service through a fire-resistance-rated assembly. The team identifies the assembly and penetrating service, selects an approved firestop system for that exact condition, and documents its placement before concealment. A generic foam or sealant is not a substitute for the required system.
| Design choice | Best fit | Main check |
|---|---|---|
| Surface chase above slab | Routes that can remain out of the concrete | Approved anchors, finish height, access, and service protection |
| Chase beside slab at wall edge | Perimeter routing with room for access panels | Door and equipment clearances; attachment details |
| Slab penetration | Only where the design specifically permits it | Locate embedded elements and obtain required structural approval |
| Shared enclosure for several services | Only when rules and listed products allow the arrangement | Separation, support, access, heat, and manufacturer instructions |
The main trade-off is concealment versus access. A thinner chase may look cleaner but may not fit bends, supports, insulation, or access panels. Set dimensions from the complete installation, then confirm that the finished detail preserves required clearances.
Verification: inspection, testing, and concealed-work records
Verification confirms that each service works and that required access and fire protection remain intact before the chase is closed. Follow the inspection sequence set by the local authority and approved plans. Photograph concealed work, note box and firestop locations, and retain product details for future repair or inspection.
Before closing the enclosure, check that:
- Required inspections have occurred or are scheduled at the correct stage.
- Electrical and other services have been tested as required by their codes and installers.
- Junction boxes and access panels can be reached without removing permanent finish or structural parts.
- Equipment working space remains clear and meets the applicable dimensions.
- Rated penetrations have the correct approved firestop system.
- The final route matches the approved drawings, or approved changes are documented.
If a later repair is needed, those records can help locate the route without guesswork. Do not rely on memory or a single finish mark as the only record of concealed services.
Conclusion
A slab-mounted concealed chase is a low-profile route above or beside a slab, not a license to cut into concrete or hide components that need access. Plan from the slab survey, service requirements, and local code. Keep electrical working space and junction boxes accessible, use approved firestopping at rated assemblies, and obtain structural approval before slab work.
FAQ
Is there a standard depth for a slab-mounted concealed chase?
No. Code does not set one universal chase depth. Size it for the services, supports, bends, insulation, access, and product instructions, then confirm it against the approved design.
Can I cut a shallow channel into a concrete slab?
Not without first locating embedded elements and obtaining required structural approval. A shallow cut can still damage reinforcement, tendons, or embedded services.
Does NEC 110.26 specify the size of the chase?
No. Section 110.26 sets working-space requirements for covered electrical equipment. Those dimensions do not prescribe a general chase size.
What electrical working-space dimensions should I check?
For covered equipment rated 0–150 volts to ground, NEC 2023 §110.26(A)(1) gives a 36-inch minimum depth under the applicable condition. Sections (A)(2) and (A)(3) specify width and height. Check the adopted edition and applicable table condition.
Can a junction box be concealed behind a fixed chase cover?
No. Under NEC 2023 §314.29, a box must be accessible without removing building finish or structural parts. Provide a suitable access opening or panel.
How much raceway fill is allowed?
NEC Chapter 9, Table 1 gives maximum fill of 53% for one conductor, 31% for two, and 40% for more than two, subject to table notes, conductor types, and raceway configuration.
Can electrical, plumbing, gas, and ducts use the same chase?
There is no blanket rule that permits every combination. Confirm separation, listing, support, access, and clearance rules for each service with the adopted code and manufacturer instructions.
What firestop should I use where a chase crosses a rated wall?
Use an approved, compatible firestop system selected for the specific rated assembly and penetrating service. The required system depends on those conditions; generic sealant is not a substitute.
Does scanning a post-tensioned slab make drilling safe?
No. Scanning can help locate embedded elements, but it does not authorize drilling or cutting. Obtain the required structural approval and follow the approved attachment or penetration detail.
When can I close the chase?
Close it only after required inspections, testing, access checks, and firestop verification are complete. Document concealed routes and details before they become hidden.
(This article was written by one of our staff writers, Garrett Vance. Visit our Meet the Team page.)
