In large facilities, it is common for IT equipment to be arranged in a single room or data center. Depending on the extent of the business’ IT needs, an entire building may be constructed for this purpose and designed to meet specific criteria and qualify as an IT room.
Article 645 of the National Electrical Code provides specific requirements that must be met before the rules in Article 645 can be applied to an IT room. In other words, to be able to use the provisions in Article 645, all requirements in 645.4 must be satisfied. These include the following (in part):
- A disconnecting means according to 645.10 must be provided.
- There is a separate HVAC system dedicated to the room and equipment.
- Listed communications equipment and IT equipment are installed.
- The room is accessible to and occupied only by operators and maintainers of the equipment/system.
- The room is separated from other occupancies in the building by fire-resistant-rated construction.
- Only equipment and wiring associated with the IT room’s operation is installed in the room.
Section 645.4 and NFPA 75, Standard for the Fire Protection of Information Technology Equipment, provide significant detail and other information on the construction requirements for an IT room. If it is not constructed to meet the criteria and items 1–6 are not provided, then all the rules in NEC chapters 1–4 apply. The driving text in 645.4 indicates that Article 645 shall be permitted to provide alternate wiring methods to the provisions of Chapter 3 and Article 708 for power wiring, parts I and III of Article 725 for signaling wiring, and parts I and V of Article 725 for optical fiber cabling if meeting the conditions in items 1–6.
Section 645.15 includes specific IT system equipment grounding and bonding requirements. The primary requirement is that all noncurrent-carrying metal parts of such equipment be connected to the equipment grounding conductor (EGC) of the supply branch circuit or feeder in accordance with Article 250. There is an exemption from the grounding requirement, but only where the IT equipment is double-insulated.
Power distribution units
To minimize possible differences in potential in the grounding systems for power circuits supplying IT equipment, it is common for these centers to be equipped with a single or multiple power distribution units (PDUs). The term “power distribution unit” is not defined in the NEC , but it is described in 645.17. PDUs used for IT equipment are permitted to have multiple panelboards within a single cabinet, provided that the PDU is utilization equipment listed for IT application.
PDUs are typically built with transformers and panelboards in a single enclosure or assembly. This unit is supplied by a feeder and branch circuits are routed to the IT equipment from the PDU, which usually provides a convenient shunt trip feature that affords easy compliance with the disconnecting means rule in 645.10.
Section 645.14 indicates that the power systems derived in listed IT equipment (PDUs) supplying IT systems through specially constructed receptacles and cable assemblies are not considered as separately derived for the purposes of applying the grounding requirements for separately derived systems. This means that installing, wiring and grounding these circuits and systems must all be done in accordance with the specific instructions provided with the IT equipment. Some of these PDUs require a connection to the building electrode system, while others do not.
The listed equipment provides the requirements that installers must follow for grounding of such circuits and systems. The PDU supplies usually isolated/insulated grounding circuits and receptacles as premanufactured “whips.” The grounding and bonding connections for all such circuits are made within the PDU because that is the power source. Thus, EMI in the grounding circuits supplying the IT equipment is kept to a minimum, because these circuits are relatively short and do not extend throughout the building or structure. The informational note following 645.15 indicates that the grounding and bonding requirements provided in the product standards that apply to listed IT equipment ensure that the rules in Article 250 are complied with.
The term “signal reference grid” in 645.15 refers to a common conductive structure such as a computer floor, copper interconnected sheet strips or copper mesh grid installed under the raised floor. IT equipment is often connected to the signal reference grid to equalize or minimize the potential differences between components. All equipment in the room should be connected to the grid, as should the EGC in each supply branch circuit supplying the equipment.