15.5 Data Processing Rooms and Communications Systems

Why This Appears Twice on the Blueprint

The PSI content outline lists Data Processing and Communications Equipment under Special Occupancies, Equipment, and Conditions on the NEC Knowledge portion (6 items) and again under the same area on the Calculations portion (3 items). It is a small share of each, but it is the sub-topic candidates are least likely to have tabbed, because Article 645 sits alone in Chapter 6 and communications rules now span the Chapter 7 limited-energy articles and Chapter 8.

Article 645 — Information Technology Equipment

The article is optional

This is the fact the exam is really testing. Article 645 does not require anything of an IT room. It offers alternative rules — most importantly, permission to run branch-circuit wiring and interconnecting cable in the space under a raised floor — in exchange for meeting a set of preconditions. A data room wired entirely under Chapters 1 through 4 is perfectly legal and never touches Article 645.

645.4 — the six conditions

The allowances of Article 645 apply only where all of the following are met:

1. A disconnecting means complying with 645.10 is provided.
2. A dedicated HVAC system serves the IT equipment and is separated from other areas of the occupancy, or an HVAC system serving other areas is provided with fire/smoke dampers at the room boundary.
3. All IT and communications equipment installed in the room is listed.
4. The room is occupied by, and accessible only to, persons needed for the maintenance and functional operation of the IT equipment.
5. The room is separated from other occupancies by fire-resistant-rated walls, floors, and ceilings with protected openings.
6. Only electrical equipment and wiring associated with the operation of the IT room is installed in the room.

Miss any one and none of the article's relief is available.

645.10 — the disconnecting means

A means must be provided to disconnect power to all electronic equipment in the IT equipment room or designated zone, and a similar means must disconnect power to all dedicated HVAC systems serving the room and cause all required fire/smoke dampers to close. The controls must be grouped, identified, and readily accessible at the principal exit doors. A single means is permitted to disconnect both the electronic equipment and the HVAC.

In the field this is the mushroom-head emergency power off (EPO) button beside each exit, driving a shunt trip. Two points get tested:

• The disconnect is not required to disconnect lighting or other non-electronic equipment in the room.
• 645.10(B) permits critical operations data systems to substitute an approved procedure with monitoring and an orderly shutdown rather than a hard EPO.

645.5 — supply circuits and cables

• 645.5(A) — branch-circuit conductors supplying one or more units of IT equipment must have an ampacity not less than 125 percent of the total connected load. This is the calculation item hiding inside this sub-topic.
• 645.5(B) — IT equipment may be connected by power-supply cords not exceeding 15 ft.
• 645.5(E) — where the 645.4 conditions are met, listed wiring methods are permitted under a raised floor, including RMC, IMC, EMT, ENT, PVC, metal and nonmetallic wireway, surface raceway, FMC, LFMC, LFNC, and Types MI, MC, AC, and TC power and control tray cable.
• Abandoned cable under a raised floor must be removed unless it is tagged for future use.

645.18 requires surge protection for critical operations data systems, and 645.25 requires an available fault current field marking based on engineering supervision.

Plenum above the room

645.3(B) points to the plenum rules where the space above an IT room is used for environmental air: general wiring per 300.22(C), and only plenum-rated cable types — Class 2/3 plenum cable, plenum fire alarm cable, Types OFNP/OFCP optical fiber, Type CMP communications cable, and Type CATVP coaxial.

Limited-Energy and Communications Lookups in 2026

Chapter 8 no longer has its former independence. Do not answer a current exam question using the old shortcut that Chapters 1–7 apply only when Chapter 8 explicitly cites them. Read the applicable general requirements together with the limited-energy provisions and their specific permissions or exceptions.

Article 750 now concerns limited-energy grounding and bonding. Energy management systems, formerly in Article 750, are in Article 130. The identical article number in an old edition is therefore particularly misleading.

For a structured-cabling question, identify the circuit class, cable listing, route and environment before choosing an installation rule. A general-purpose cable marking is not permission to install the cable in an environmental-air space. A plenum route requires the appropriate listed cable or an expressly permitted wiring method. Likewise, check the applicable support, separation and bonding provisions; low operating voltage does not eliminate those requirements.

An exam may describe an entrance cable exposed to lightning, an indoor cable near power conductors, or a shield requiring bonding. These are different questions. Find the rule for the exposure or installation, then follow its cross-references. The new structure replaces several old 800-series installation lookups; do not carry an old section number forward solely because the cable type is unchanged.

Reference: 2026 NEC adoption review, including the Chapter 8 and limited-energy reorganization.