National Electrical Code 2005: Designing Computer Rooms

By Chris Tapas, P.E., President, Tapas Engineering Services PC, Chicago June 8, 2005

Editor’s Note: This is the fifth in a regular monthly column that covers significant new issues raised by the 2005 Edition of the National Electrical Code.

Computer room designs and construction vary, but common features of most are self-contained HVAC systems, fire and smoke monitoring systems, automatic sprinkler or gaseous agent fire extinguishing systems, fire-rated walls, raised floors, underfloor wiring systems, and emergency power-off systems. Because of the critical nature of the 24/7 operation of most computer rooms, reliability is the critical design factor. An owner must be assured that no single point of failure will take the system totally down.

Whether a dedicated facility or part of a multipurpose building, the physical location of the computer room is extremely important. The raised floor space, air conditioning support, uninterruptable power supply (UPS), generators, and related support equipment must be coordinated with the other areas of the building and properly positioned within the building perimeter in order to optimize their interaction and the overall support of operations. The ultimate design of the computer room must reflect the customer’s expectations.

Enter NEC

The intent of Article 645, Information Technology Equipment, is to allow for modification and, in some cases, relaxation of the general electrical wiring methods required by chapters 1 through 4 of the NEC Code for special equipment installed in computer rooms, but only if the room complies with all the requirements in Article 645. In fact, Section 645-4 establishes special design conditions and uses restrictions that a computer room must comply with in order for the use of Article 645 to be permitted. These conditions include:

1. Disconnecting means complying with 645.10 are provided.

2. A separate heating/ventilating/air-conditioning (HVAC) system is provided that is dedicated for information technology equipment use and is separated from other areas of occupancy. Any HVAC system that serves other occupancies shall be permitted to also serve the information technology equipment room if fire/smoke dampers are provided at the point of penetration of the room boundary. Such dampers shall operate on activation of smoke detectors and also by operation of the disconnecting means required by 645.10.

3. Listed information technology equipment is installed.

4. The room is occupied only by those personnel needed for the maintenance and functional operation of the installed information technology equipment.

5. The room is separated from other occupancies by fire-resistant-rated walls, floors, and ceilings with protected openings.

While the NEC Code covers practical safeguarding of persons and property from hazards associated with electricity, the NFPA occupancy standard is the standard for the Protection of Electronic Computer/Data Processing Equipment, NFPA 75. An awareness of NFPA 75 is important for persons applying Article 645 because the Article 645 is extracted into NFPA 75 and NFPA 75 is mentioned prominently in the fine print notes (FPN) of Article 645. NFPA 75 establishes minimum requirements for the protection of computer rooms and areas from damage by fire or its associated effects, and considers such risk factors as threat of fire to occupants and property, life safety aspects of computer room function, and economic loss of equipment, function and records.

The revision to Article 645.5 Supply Circuits and Interconnect Cable is minimal and is mainly an installation issue. For example: NEC 645.5 does not explicitly prohibit power supply cords under raised floors (the wording still is debatable), and a power supply cord can be connected to a receptacle above the floor. And there are many types of cord-connected information technology equipment today that can be used both inside and outside computer rooms, which for practical reasons mandates use of a UL Listed flexible cord, which in turn makes this an installation issue. If instead Article 645 was revised to explicitly prohibit power supply cords under raised floors of computer rooms, then UL Listed flexible cords should not be installed under raised floors of computer rooms.

The change to 645.17 Power Distribution Units (PDU) reflects present practice. The article now reads, “Power distribution units that are used for information technology equipment shall be permitted to have multiple panelboards within a single cabinet, provided that each panelboard has no more than 42 overcurrent devices and the power distribution unit is utilization equipment listed for information technology application.”

Many Power Distribution Units are listed under UL 60950-1 and this has always required panelboards used in PDUs to be listed to UL 67 (or UL 508A), which limit each panel to a maximum of 42 OCPDs. However, the use of more than one panelboard in a single PDU cabinet is exclusively for a computer room application (since NEC 408.35 prohibits installation of more than one panelboard inside a cabinet outside of computer rooms).

Keep in mind that the major goal of Article 645 is to reduce the spread of fire and smoke. The raised floors common in IT rooms pose special additional challenges to achieving this goal, so Article 645 devotes a fair percentage of its text to raised floor requirements. Fire-resistant walls, separate HVAC systems, and other requirements further help achieve this goal.