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 Chapter 8 sits at the very back of the book past the tables.

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.

Chapter 8 — Communications Systems

Chapter 8 stands alone

90.3 is the gateway rule: Chapter 8 covers communications systems and is not subject to the requirements of Chapters 1 through 7 except where the requirements are specifically referenced in Chapter 8. If a Chapter 3 rule is not called out in Chapter 8, it does not apply to a telephone or coaxial cable. This is the most-tested single sentence in the whole topic.

The articles

The 2020 NEC created the general Article 800 and moved the old Article 800 content to 805; the 2023 NEC kept that structure. If a candidate's memory says "800 is telephone," it is a 2017-edition memory.

On the power-limited side, the 2023 NEC also split the old Article 725 apart: Article 722 now holds the cable requirements for power-limited circuits, Article 724 covers Class 1 power-limited circuits, Article 725 covers Class 2 and Class 3 power-limited circuits, and new Article 726 covers Class 4 fault-managed power systems.

Bonding, grounding, and protectors

• 805.90 requires a primary protector where the communications circuit is exposed to accidental contact with power conductors or to lightning.
• 800.100 governs the bonding conductor or grounding electrode conductor for the protector and cable shield: it must be listed, insulated, copper or other corrosion-resistant material, not smaller than 14 AWG, and in one- and two-family dwellings its length must be as short as practicable and not more than 20 ft.
• The bonding conductor lands on the intersystem bonding termination required by 250.94 — the little bar with three or more terminals next to the service. This is the one place where a Chapter 2 rule is explicitly pulled into Chapter 8.
• 820.93 requires the outer conductive shield of coaxial cable to be bonded or grounded as close as practicable to the point of entrance.
• 800.53 asks for at least 6 ft of separation, where practicable, between communications wires on buildings and lightning protection conductors.
• 800.24 requires cables to be installed in a neat and workmanlike manner and supported by the building structure — not draped over or supported by the suspended-ceiling grid or its support wires.

Cable listings and substitution

Communications cable types run from most restrictive to least:

• CMP — plenum. Permitted in ducts, plenums, and other environmental air spaces.
• CMR — riser. Permitted in vertical runs penetrating floors.
• CM / CMG — general purpose.
• CMX — limited use, permitted in dwellings and in raceway.

Substitution runs only upward in restriction: CMP may be used anywhere CMR, CM, or CMX is permitted; CMR may be used where CM is permitted. Never the reverse. The same logic applies to CATVP/CATVR/CATV/CATVX coaxial and to OFNP/OFNR/OFN optical fiber.

Separation from power conductors

800.133 keeps communications conductors out of the same raceway, cable, box, or enclosure as electric light and power conductors, Class 1 circuits, and non-power-limited fire alarm circuits, unless they are separated by a permanent barrier or listed divider. Where they are not in a raceway, a separation of at least 2 in. must be maintained from those power conductors — unless the power conductors are enclosed in a raceway or are metal-sheathed, metal-clad, nonmetallic-sheathed, Type AC, or Type UF cable.