Prefabrication and modular construction

Speed to market is one of the strongest forces in data center construction. Each month a facility is delayed is a month of lost revenue on capacity worth hundreds of millions of dollars, and demand for AI computing has pushed owners to deliver large campuses in 12 to 18 months instead of the traditional two to three years. One of the main ways to build faster is to move work off site: manufacturing building components, electrical and mechanical systems, and even complete data halls in factories, then shipping and assembling them on site. This lesson covers the forms of prefabrication, its advantages and limits, transport and lifting constraints, approvals, interfaces and tolerances, and how modular strategies support phasing.

Why prefabricate

Prefabrication and modular construction (often described as design for manufacture and assembly, DfMA) offer:

• Speed: factory work proceeds in parallel with site work (grading, foundations, utilities), so the overall schedule shortens; modules arrive ready to set and connect.
• Quality: factories offer controlled conditions, jigs, repeatable processes and factory acceptance testing (FAT) before shipment, so fewer defects reach the site.
• Labor: skilled trades are scarce in many data center markets; factories use labor more efficiently and in locations where it is available.
• Safety: less work at height and in congested sites.
• Repeatability: standard modules reused across campuses capture learning and simplify maintenance.

The trade-offs are less flexibility to change designs late, larger up-front design effort, transport costs and limits, the need for precise interfaces, and dependence on factory capacity, which itself became a constraint as demand surged.

Forms of prefabrication

Prefabrication ranges from components to whole buildings:

Large campuses typically combine a conventionally built or precast shell with highly prefabricated electrical and mechanical systems, which carry most of the value and complexity. Electrical "power blocks" (medium-voltage switchgear, transformers, UPS and batteries in skids or enclosures) are repeated for each increment of capacity.

Schedule benefits

The main schedule benefit comes from overlapping work that would otherwise be sequential. In conventional construction, electrical rooms are built after the structure is closed in; with prefabricated power modules, they are built in a factory during site preparation and foundations, and installed as soon as their pads are ready.

Transport limits

Everything built off site must travel by road, and sometimes rail or sea, to the site. Module dimensions are therefore set by transport rules:

In the US, federal and state rules set a legal width of 8.5 ft (2.6 m), height commonly 13.5 to 14 ft (4.1 to 4.3 m) depending on the state, and a gross weight of 80,000 lb (36,300 kg) on Interstates. Loads beyond these need state oversize and overweight permits for each state crossed, route surveys, escort (pilot) vehicles and travel restrictions; superloads may need engineering review of bridges and police escort.

Common module sizes are therefore about 3.6 to 4.3 m wide (permitted as oversize but routine), 3.6 to 4.3 m tall on low-bed trailers, and 12 to 18 m long. Designers check the route, including bridges, overhead wires, turns, rail crossings and the site entrance, early in design.

Lifting and setting

Modules, skids and precast elements are placed by crane. Lift planning considers the load's weight and center of gravity, rigging (spreader bars, slings), the crane's rated capacity at the required radius (capacity falls rapidly as radius increases), ground bearing under the crane outriggers or tracks, wind limits, and exclusion zones. Critical lifts (often defined as over 75% of the crane's capacity, over occupied areas or involving multiple cranes) require engineered lift plans.

Approvals and certification

Factory-built buildings and equipment must still meet building, electrical and fire codes, but inspections are partly moved to the factory:

In the US, many states run modular (industrialized) building programs in which plans are reviewed and units inspected in the factory by the state or third-party agencies, and approved units carry a state insignia accepted by local building officials, who then inspect foundations and site connections. ICC/MBI standards 1200 and 1205 cover the design, plan review, inspection and installation of off-site construction. Electrical assemblies must be listed by a nationally recognized testing laboratory (such as UL) or receive field evaluation, and the NEC applies to their installation.

Early engagement with the authority having jurisdiction is essential, because some officials are unfamiliar with large factory-built power and data modules, and approvals can be delayed if certification routes are unclear.

Interfaces and tolerances

A prefabricated module is only as good as its fit with the site. Interfaces include anchor bolts and pads, electrical connections between modules and to the utility, piping connections, controls and monitoring networks, fire protection, and weather sealing between modules. Factories build to tight tolerances, but foundations and site work vary more, so designers specify tolerances for pad elevations and anchor locations, provide adjustable connections (slotted holes, shims, flexible pipe connectors, cable slack), and use digital tools (building information models, laser scanning of pads before setting) to verify fit.

Modularity and phasing

Modular design also means designing the facility as repeated capacity blocks that can be added as demand grows: for example, a campus planned for 300 MW built in 25 MW phases, each with its own power and cooling blocks. Benefits include matching capital spending to demand, adopting newer equipment in later phases, and reducing risk. Structural and civil design supports this by providing foundations, duct banks, roads and utility corridors for later phases (or at least space and connection points), designing expansion joints and building ends for extension, and arranging phases so construction does not disrupt operating data halls.

The limits of modularity

Not everything should be prefabricated. Very large components may exceed transport limits; designs still changing rapidly (such as cooling for new generations of AI hardware) may be better built on site; and modules need enough repetition to justify factory setup. Modular buildings also have more joints and connections to seal and maintain. The best results come from a clear strategy: standardize what repeats, prefabricate what benefits from factory conditions, and leave flexibility where requirements are uncertain.

Lesson 12 places prefabrication within the overall construction sequence, quality assurance and handover of the project.