Functional blocks (subsystems) within data centers, such as UPS, room air conditioning, and chillers can be implemented as single monolithic units or as a number of devices (modules) working together to share the load. For example a 1MW UPS requirement can be satisfied by any of the following combinations of devices: • a single 1 MW UPS • four 250 kW UPSs • ten 100 kW UPSs • one thousand 1 kW UPSs The individual UPS devices may or may not have “device modularity”, but the UPS subsystem is considered to be modular if it is comprised of multiple UPS devices. Subsystem modularity is ubiquitous in larger data centers where subsystems like PDUs and CRAH units are almost always comprised of multiple units. Three major drivers of subsystem modularity are fault tolerance, concurrent maintenance, and logistics. - **Fault tolerance** is provided when the subsystem can survive the failure of one of the modules without interruption of the load. - **Concurrent maintenance** is the related situation where a module can be taken off line for testing or maintenance without interruption of the load. - **Logistics** of moving devices within a facility makes it highly beneficial when an individual module is small enough to be moved via a passenger elevator, shipped via an enclosed truck, and moved through doorways and over interior flooring surfaces without difficulty. These factors drive data center designs away from huge monolithic subsystems, and toward subsystems comprised of multiple modules, especially if the subsystems are deployed indoors. As in the case of device modularity, subsystem modularity is often an important element in a modular data center design, but subsystem modularity does not, by itself, mean a data center has a modular architecture. To have a modular architecture, the design must specify how the different subsystems are deployed together.