Section 90.3 explains in specific detail how the National Electrical Code should be applied to installations and systems. Let’s explore an example of how rules within Chapter 6 modify the general requirements contained in chapters 1–4. 

In large facilities such as data centers, it is common for IT equipment to be installed in a single room or data center. Sometimes, an entire building could be constructed, depending on the extent of the business’ needs. This space is often constructed to meet specific criteria to qualify as an IT room, essentially creating a fire-rated isolation (envelope) from the rest of the building.

Code explanations 

Section 645.4 of the NEC 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—which, in some cases, relax the general wiring requirements—all those in 645.4(1) through (6) must be complied with as follows: 

1. A disconnecting means according to 645.10 must be provided.
2. A separate HVAC system (dedicated to the room and equipment) must be provided.
3. All installed communications and IT equipment must be listed.
4. The room must be accessible to and occupied only by operators and maintainers of the equipment/system.
5. The room must be separated from other occupancies in the building by fire-­resistant-rated construction. 
6. Only equipment and wiring associated with the operation of the IT room can be located in the room. 

See 645.4 for complete information about each of these six criteria. Section 645.4 and NFPA 75, Standard for the Protection of Information Technology Equipment, provide more detailed information on construction requirements for an IT room. 

If the room is not constructed to meet the criteria, then the general wiring rules in chapters 1–4 of the NEC must apply to the installation. 

The driving text in 645.4 indicates that Article 645 shall be permitted to provide alternate wiring methods to the provisions of Chapter 3, parts I and II 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) through (6). Section 645.15 includes specific grounding and bonding requirements for equipment in an IT system. The primary requirement is that all noncurrent-carrying metal parts of equipment be connected to the equipment grounding conductor of the supply branch circuit or feeder in accordance with Article 250. There is an exemption from this 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’s common for these centers to be equipped with single or multiple power distribution units (PDUs). This term is not defined in the NEC, but is described in 645.17. 

PDUs are typically built with transformers and panelboards in a single enclosure or assembly supplied by a feeder, with branch circuits routed to the IT equipment.

The PDUs used for IT equipment are permitted to have multiple panelboards within a single cabinet (enclosure), 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 supplied by a feeder, with branch circuits routed to the IT equipment. PDUs usually provide a 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 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 all must be done in accordance with installation instructions provided with the IT listed equipment. 

Some PDUs that qualify as separately derived systems for the purposes of grounding require a connection to the building grounding electrode system; others may not. The listed equipment provides the requirements installers must follow for grounding of such circuits and systems. 

The isolated/insulated grounding circuits and receptacles are usually supplied as premanufactured “whips” by the PDU supplier. The grounding and bonding connections for all such circuits are made within the PDU because it is the power source. Thus, electromagnetic interference 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. 

If IT rooms are constructed to meet all requirements in 645.4, relief is provided from supporting and securing power supply cords, communications cables, interconnecting cables, associated boxes and plugs and receptacles listed as IT equipment and installed under raised floors.