The concept of modular data centers (MDCs) originated about six or seven years ago as a portable method of delivering information technology data center capacity without the high cost and long construction time. Manufacturing these data centers at a factory can reduce fabrication cost and time as well as facilitate shipping using a train, truck or a combination of both. Power and cooling can be installed as part of the MDC or provided by the customer, which can be connected quickly and relatively inexpensively at the site. These units can be leased or purchased, with expansion and contraction of the MDCs costing musch less than permanent data center construction methods.


Underwriters Laboratories Subject 2755, Outline of Investigation for MDCs, permits listing as a unit, further streamlining the installation and the subsequent approval process. Subject 2755 covers pre-engineered data center modules consisting of an outer cabinet or container that houses information technology equipment (ITE) and various support items, such as electrical service and distribution equipment; heating, ventilating and air conditioning (HVAC) systems; and other similar applications.


The new Article 646 in the 2014 National Electrical Code (NEC) covers MDCs. Article 646.2 defines them as prefabricated unit(s), rated 600 volts or less, consisting of an outer enclosure housing multiple racks or cabinets of ITE (servers in racks) as well as varied support equipment, such as electrical service and distribution equipment, HVAC systems, filters, transformers, frequency drive units and other similar items. MDCs can be manufactured as containerized data centers, where large shipping containers are modified and then fitted with cooling systems and computer server racks, or as prefabricated modules, where the manufacturer uses heavy duty sheet metal to fabricate the server enclosures and infrastructures. 


Based on the definition, something that houses only support equipment that is not part of a specific MDC, such as HVAC or power distribution systems, is not considered a data center.


Based on 646.3(L), wiring methods and materials for power distribution within the MDC must comply with the requirements in NEC Chapter 3 for that type of wiring method, unless modified by Article 646. Wiring in areas in the MDC under a raised floor that is constructed and used for ventilation, as described in 645.5(E), can use the wiring methods permitted for power cables, communications cables, connecting cables, interconnecting cables, cord-and-plug connections, and receptacles associated with the ITE. In other words, many of the wiring methods under the raised floor may not be required to be low-smoke and fire-resistant wiring methods that would normally be required where these areas are used as plenums (other spaces used for environmental air). Section 646.3(N) requires an approved means to disconnect all electronic equipment in the data center and to shut any HVAC fire or smoke dampers, similar to the method required in 645.10.


In accordance with 646.5, a permanent nameplate must be attached to each MDC equipment enclosure, and this nameplate must be visible after data center installation. The nameplate permits the inspector, installer and maintenance personnel to ascertain the supply voltage, the number of phases, the frequency and the full-load current (FLC) of the unit. The unit’s FLC must not be less than the sum of the FLC for all motors and all other equipment that may be in operation at the same time during normal conditions of use. There may be unusual types of loads, duty cycles or frequencies (such as frequency control devices for large motor applications) within the data center, and these loads are figured as part of the full load. Where these unusual loads affect the size of the conductors, requiring larger-than-normal conductors or smaller, reduced-size conductors, the nameplate must indicate the required capacity. On occasion, more than one incoming supply circuit may be provided with the nameplate that displays the relevant information for each incoming circuit.


For listed equipment, the FLC shown on the nameplate may be the maximum measured 15-minute average FLC. The manufacturer’s name must be located on the nameplate. When powered by a separate service, the short-circuit current-rating of the service must be marked. Branch circuits supplying ITE are assumed to be loaded to not less than 80 percent of the branch-circuit rating with a 100 percent duty cycle. The size of the supply conductors to the MDC, based on 646.6(A), must be sized at 125 percent of the FLC rating.


There is much to be learned about these new MDCs. Read and learn all of Article 646 when dealing with them.