Article 645 is optional — and that surprises people.
Article 645 provides alternate installation rules for an information technology equipment room. You may use it if the room meets all of the stated conditions. You are never required to, and plenty of well-built data centers do not.
Article 645 relaxes certain wiring method requirements — most usefully, it permits listed IT equipment cable and power cables under a raised floor without the usual wiring-method constraints. In exchange it requires a set of conditions, most notably a disconnecting means that shuts down all electronic equipment and all HVAC serving the room.
§ What 645 covers
- Equipment, power supply wiring, interconnecting wiring and grounding of information technology equipment and systems within an information technology equipment room.
- It is a supplementary article — it modifies and relaxes the general rules for a defined space, it does not replace the Code.
§ The conditions that must all be met
Disconnecting means complying with 645.10 is provided.
A separate HVAC system is provided that is dedicated to the IT equipment room, or the HVAC serving other areas is provided with fire/smoke dampers at the room boundary.
All IT equipment installed in the room is listed.
Only persons with authorized access occupy the room.
The room is separated from other occupancies by fire-resistant-rated construction with protected openings, as required by the applicable building code.
Only electrical equipment and wiring associated with the operation of the IT room are installed in the room.
These are conjunctive. If any one condition ceases to be true — a non-listed piece of equipment gets installed, the separation is compromised by a later renovation, unauthorized access becomes routine — the room no longer qualifies for the alternate rules, and the installation reverts to needing to comply with the general requirements.
§ The disconnecting means — 645.10
A means must be provided to disconnect power to all electronic equipment in the IT equipment room or designated zone, and a similar means to disconnect the power to all dedicated HVAC serving the room or zone and cause fire/smoke dampers to close. Controls must be grouped, identified, and readily accessible at the principal exit doors.
This is the requirement operators argue about most, because a big red button that drops an entire data hall is, from an availability standpoint, a hazard in its own right. Accidental activation of an EPO has taken down data centers.
The Code addresses this through the alternate route: if you do not use Article 645, you do not need 645.10 — you comply with the general Code requirements for wiring methods instead. Many modern facilities take that path deliberately, run standard wiring methods, and avoid the EPO entirely.
Where an EPO does exist: guard it against accidental activation, ensure the identification is unambiguous, and make sure the operations team knows what it drops and what it does not. An EPO that is understood is a control; one that is a mystery is a liability.
§ When not to use 645
- You do not want an EPO. The most common modern reason. Comply with the general Code, avoid 645.10.
- Your room cannot meet all conditions — mixed occupancy, non-listed equipment, shared HVAC without dampers.
- You have no raised floor. Much of the practical benefit of 645 relates to under-floor wiring. Slab-on-grade halls with overhead containment gain less.
- Multi-tenant colocation where "only persons with authorized access" is difficult to maintain in the sense the Code intends.
A facility designed under 645, then modified over time — a non-listed device installed, a wall penetrated, access widened — with nobody re-evaluating whether the room still qualifies. The alternate rules were relied on for the wiring method; the conditions that justified them no longer hold.
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