On a construction site, electrical conduit installation is one of the most time-sensitive tasks in the entire build programme. Get it in too late — after the slab is cast or the walls are plastered — and you are looking at core-drilling, chasing, and expensive rework. Get it right, and the electrical system runs cleanly through the structure with no structural damage and no surprises downstream.
This guide is for everyone working on site: civil engineers, structural engineers, architects, quantity surveyors, site supervisors, and electricians. Understanding the conduit installation sequence is not just the electrician’s job. It is a coordination responsibility that affects every trade on the project.
Quick Answer: Electrical conduit is installed in two stages. First fix happens before concrete is poured and before walls are plastered — conduit is embedded in slabs and chased into block walls. Second fix happens after the structure is complete — cables are pulled through and fittings are installed. The critical stage for site coordination is always first fix: once concrete is cast, the conduit must already be in place.
What Electrical Conduit Is and Why It Matters
A conduit is a pipe or channel that protects and routes electrical wiring through a structure. Think of it the same way you think about reinforcement in concrete — you cannot see it once the structure is finished, but it determines whether everything works correctly.
Without conduit, bare cables are exposed to mechanical damage, moisture, and heat. In a building, conduit is also what allows wiring to be replaced or upgraded in the future without breaking open walls. It protects occupants from electrical faults and gives the building a service life that matches its structural life.
In Kenyan construction, three types of conduit are commonly used:
- PVC conduit — the most common type on residential and light commercial buildings. Lightweight, cost-effective, and adequate for most indoor applications.
- Flexible conduit — used at connection points and junction boxes where a rigid conduit cannot make the final turn to equipment or fittings.
- Galvanised steel conduit — used in industrial buildings, plant rooms, and locations with high fire risk or physical damage exposure requirements.
The Two Stages: First Fix and Second Fix
Electrical work on a building project happens in two distinct phases. Every professional on site needs to understand this sequence, because the first phase directly gates structural work.
First Fix: Before the Structure Is Closed
First fix is everything that gets buried inside the building before finishing. On a typical residential or commercial project, this includes:
- Laying conduit runs through floor slabs and roof slabs before concrete is poured
- Chasing conduit into masonry block walls before plastering
- Setting box positions for switches, socket outlets, and distribution boards
- Pulling draw wires through all conduit runs for use during second fix
The rule is simple: first fix must be complete before the slab is cast and before walls are plastered. If this step is missed, the only recovery options are core-drilling through hardened concrete or chasing through finished plasterwork — both expensive, damaging, and avoidable.
Second Fix: After the Structure Is Complete
Second fix happens once the structure is watertight, walls are plastered, and the building is ready for fitting-out. This is when cables are pulled through the pre-installed conduit using the draw wires, socket outlets and light switches are fitted, the distribution board is installed and circuits are connected, and the system is tested and commissioned by the licensed electrical engineer.
Second fix is largely self-contained. The critical stage for cross-trade coordination is always first fix.
Conduit Installation Inside Reinforced Concrete Slabs
This is where the most significant coordination happens between the structural and electrical teams. When conduit is embedded in a slab, it creates a void in the concrete cross-section. Managed correctly, there is no structural issue. Managed poorly, and you compromise the slab.
During slab conduit installation, structural engineers and site supervisors must confirm the following:
- Conduit diameter must not exceed one-third of the slab thickness at any section. For a 150mm slab, this means a maximum conduit diameter of 50mm.
- Multiple conduits must not be bundled together. Spacing must allow concrete to consolidate between them.
- Conduit must not be placed in the compression zone of the slab — typically the top portion. Position conduit within the middle third of the slab depth where possible.
- Concrete cover requirements still apply over conduit, measured the same way as cover to reinforcement.
The installation sequence in a typical reinforced concrete slab is: formwork set and propped, bottom reinforcement placed and checked, electrical conduit runs laid in position on top of the bottom reinforcement, draw wires pulled through and labelled, top reinforcement placed over the conduit, site engineer confirms conduit positions before pour sign-off, then concrete placed and consolidated around the conduit runs.
Step six is the checkpoint that prevents most problems. The site engineer must visually inspect conduit positions before authorising the concrete pour. This inspection takes minutes. Fixing it after takes days.
Conduit in Masonry Block Walls
In block walls, conduit is typically chased in after the wall is built but before plastering. The electrician uses a chasing machine to cut a channel in the block, lays the conduit, and holds it in place before the plaster coat covers it.
Key points for coordination here: do not chase through structural elements. Columns and structural walls are not available for chasing. Any conduit in structural members must be planned at design stage and cast in during construction — never cut in afterwards. Additionally, horizontal chasing in block walls should be minimised, as extensive horizontal chases can reduce the wall’s shear capacity. And chase depth must not exceed one-third of the wall thickness.
How to Read Electrical Drawings on Site
Even if you are not an electrician, understanding the basic electrical drawings helps you coordinate first fix effectively. The three key drawings are:
Lighting layout plan: shows light fitting positions, switching zones, and conduit routes. This tells you where conduit runs along ceilings and down walls to switch positions.
Power layout plan: shows socket outlet positions, appliance connections, and the distribution board location. The DB position is particularly important — it must be incorporated into the structural layout with adequate wall space and accessibility.
Distribution board schedule: lists all circuits, breaker sizes, and cable specifications. The quantity surveyor uses this for pricing. The civil engineer uses it to understand service riser locations.
When reviewing these drawings, look specifically for conduit routes that conflict with beams, columns, floor openings, or drainage lines. Flag these at design review stage. Resolving a clash on paper takes minutes. Resolving it after casting takes days and money.
Common Mistakes That Happen on Site
Conduit not installed before the slab cast. This happens when the electrical team is not properly included in the construction programme. The result is core-drilling through finished concrete. Prevention: add conduit first fix as a mandatory checkpoint in the pour readiness checklist.
Oversized conduit bundled in slabs. Grouping conduits together speeds up installation but creates voids that weaken the slab and cause honeycombing around the conduit. Prevention: enforce spacing requirements during the reinforcement inspection.
Conduit placed through columns. This is a structural violation. Any penetration through a column must be designed by the structural engineer and cast in during construction — never cut in after. Prevention: brief the electrical team on structural element boundaries before work begins.
No draw wire pulled through during first fix. Without draw wires, pulling cables through after construction is extremely difficult, especially in long runs with bends. Prevention: confirm draw wire installation as part of the pre-pour inspection checklist.
DB position not coordinated with structural layout. Distribution boards are heavy and must be fixed to load-bearing elements. Placing them on non-structural partitions creates installation and long-term maintenance problems. Prevention: confirm DB positions against the structural drawings before first fix begins.
What Each Profession Needs to Know
Civil engineers and site engineers: Know the conduit installation sequence. Include first fix completion as a pour readiness checkpoint. Inspect conduit positions before any slab cast.
Structural engineers: Specify maximum conduit dimensions and spacing in your drawings. Review MEP drawings for conflicts at design stage — not on site.
Architects: Show DB locations and conduit risers clearly on your service drawings. Coordinate with structural and MEP to resolve spatial conflicts before construction starts.
Electrical engineers and electricians: Submit first fix drawings early in the programme. Coordinate with the civil team on slab cast dates. Never skip the draw wire installation.
Quantity surveyors: Include first fix milestones in the construction programme. Delay on conduit first fix directly delays slab casting and every trade that follows.
The Bottom Line
Electrical conduit installation is a critical early-stage activity on any construction project. It is not something you can catch up on after the concrete is poured. The cost of getting it right the first time is small. The cost of getting it wrong — in rework, delays, and structural compromise — is significant.
Every professional on a construction site benefits from understanding the two-stage installation sequence, the structural rules for conduit in slabs, and the coordination checkpoints that prevent problems before they happen.
That understanding is what turns a reactive site into a well-run one.
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