Cast-in boxes are positioned and secured before the concrete pour, becoming part of the element from the start. Post-installed boxes are added after the element has cured, by drilling or cutting an opening. Cast-in is generally the lower-effort, higher-quality-control route at production scale, because the position is set once and the concrete forms around it cleanly. Post-installation is the fallback when a position wasn’t planned in advance, or when a change comes after the element already exists — and it comes with real trade-offs in labour, finish quality and structural consideration that a cast-in approach avoids.
What “cast-in” means
The box is fixed in its final position on the mould or rebar before the pour (see Positioning Electrical Boxes in Precast Concrete Elements for the fixing methods this involves — magnetic, mould-knockout, or BULO bracket, depending on model and mould type), and the concrete is placed and vibrated around it. Once the element is struck, the box is exactly where it was positioned, with no additional work needed to create the opening. Our Concrete & Precast Boxes range is specified as suitable for casting temperatures up to +90 °C — a real, checkable limit worth confirming against your own mix and cure process rather than assuming any box tolerates any casting condition.
What “post-installed” means
The element is cast without the box, and the opening is created afterward — drilling a core, saw-cutting, or breaking out an opening — then the box is fitted into the finished opening, typically with mechanical fixing or adhesive/mortar. This is standard practice when a position is decided after casting, when a design changes late, or when retrofitting a box into an element that wasn’t originally designed for it.
Cost comparison
| Factor | Cast-in | Post-installed |
|---|---|---|
| Labour at the plant | One positioning step during mould setup | None at casting stage |
| Labour after curing | None | Drilling/cutting plus fitting — additional labour and time |
| Rework risk | Low if position/tolerance was correct on a proven trial element | Higher — cutting into cured concrete risks damaging reinforcement or the element itself |
| Tooling/consumables | Fixing hardware used during casting | Coring/cutting tools, fixing compound for the retrofit |
| Schedule impact | Built into normal production cycle | Adds a separate step, usually after the element is otherwise finished |
Quality comparison
Cast-in positioning, once proven on a trial element, repeats reliably across a production run — every element gets the same result because the process, not a person’s judgement on site, sets the position. Post-installation depends on accurate marking-out and careful cutting on every single opening; the risk of an off-position hole, or of cutting close enough to reinforcement to be a structural concern, rises with every element cut. It generally also leaves a less clean finish around the opening than a box that was cast in from the start.
When post-installation is still the right call
- Retrofitting an element that was cast without knowing a box would be needed
- A late design change after elements are already cast
- Very low-volume or one-off work where the setup cost of a cast-in process isn’t justified
A checklist for choosing between the two
- Is the box position known before the pour? If yes, cast-in is almost always the better route at any meaningful volume.
- Is this a one-off addition to an already-cast element? Post-installation is likely the only realistic option.
- If post-installing, confirm the opening location against the reinforcement layout before cutting — not just against the visible surface.
- If cast-in, run a trial element first before committing to full production.
- Whichever method, confirm the box model’s IP rating and material are appropriate — a post-installed opening in an exterior element, for example, may need additional sealing consideration that a cast-in position was designed around.
What we do differently
We don’t push cast-in as the answer for every situation — it’s the better route when the position is known early, and post-installation is a legitimate answer when it isn’t. What we do is make sure the model itself is specified correctly for whichever method your project actually uses, and confirm that with you before you commit.
Read: Positioning Electrical Boxes in Precast Concrete Elements → · Start a market-fit review →