Source: https://www.ecmweb.com/code-basics/exploring-requirements-information-technology-equipment
Timestamp: 2019-04-23 11:07:25+00:00

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If you've ever seen a shutoff switch located just outside an information technology (IT) room, you may have wondered who in their right mind would install a backup power system for a critical operations space like an IT room and put a shutoff switch near the door to that same room. Does the fact that the NEC requires such an arrangement mean we've found an error in the Code? Actually, it's not an error.
The reason for this requirement would be very clear to you if you were part of a fire response team arriving at the scene with your water hose. Shutting off the main power prevents electrocution in the rest of the building, but inside the IT room you would face energized loads supplied by the UPS. Without an exterior switch, you would need to enter the room to de-energize the loads. Fire personnel don't have time to search for sources of power — and any guesswork could prove lethal. As a result, the signage required by 702.8(A) explaining the location of the UPS is simply not adequate. Emergency response personnel need to enter the IT room without wasting time.
What about loss of IT function? During a fire, that's the least of your worries. The rest of the time you can reduce the chance of unauthorized shutdowns with breakaway locks and proper signage. However, compliance with Art. 645 involves more than installing an exterior shutoff switch. In addition to protecting fire personnel, the requirements of this Article seek to reduce the spread of fire and smoke by requiring such things as fire-resistant walls and separate HVAC systems for IT rooms. A large percentage of Figure 1 also addresses a common IT room feature: the raised floor.
When Art. 645 applies. What exactly is an IT room? The NEC does not define one, but NFPA 75, Standard for the Protection of Electronic Computer/Data-Processing Equipment, gives us some insight. An IT room is an enclosed area specifically designed to comply with the construction and fire protection provisions of NFPA 75, which defines an IT room and describes the construction requirements of the room.
Art. 645 1 gives us the electrical requirements for an IT room. Does this mean that if you have IT equipment you must create an NFPA 75- and NEC 645-compliant IT room? No. An IT room is not required. Rather, it is something that may be desired by the designer or owner. Because it allows less stringent wiring methods than would normally be required, an IT room is often desired, but never required.
Has a disconnecting means compliant with 645.10.
Has a dedicated heating/ventilating/air-conditioning (HVAC) system and is separated from other areas of the occupancy.
Contains only listed IT equipment.
Is occupied only by persons needed for the maintenance and operation of IT equipment.
Is separated from other occupancies by fire-resistant rated walls, floors, and ceilings with protected openings.
What if you don't want to comply with the rules above? Simple. Don't call it an IT room, and follow the rest of the NEC. Just be mindful that you will not be able to take advantage of the allowances of Art. 645.
Supply circuits and interconnecting cables. Each branch-circuit conductor for data-processing equipment must have an ampacity not less than 125% of the total connected load [645.5].
A flexible cord must be no longer than 15 feet with an attachment plug.
A cord set assembly, where run on the surface of the floor, must be protected against physical damage.
To interconnect data processing units, use cables listed for IT equipment. Where exposed to physical damage, the interconnecting cables must be protected by a means approved by the authority having jurisdiction (AHJ).
The raised floor must be of suitable construction, and the area under the floor must be accessible.
Branch-circuit wiring methods must be securely fastened in place per 300.11.
You must also install the branch circuits for receptacles or hard wired equipment in one of the following kinds of raceway: rigid metal conduit, rigid nonmetallic conduit, intermediate metal conduit, electrical metallic tubing, electrical nonmetallic tubing, metal wireway, nonmetallic wireway, surface metal raceway with metal cover, nonmetallic surface raceway, flexible metal conduit, liquidtight flexible metal conduit, liquidtight flexible nonmetallic conduit, Type MI cable, Type MC cable, or Type AC cable (Fig. 1 above).
You can use nonmetallic raceways within the raised floor area because this space is neither subject to physical damage nor required to comply with the environmental airspace requirements of 300.22(C). Although it seems strange to install nonmetallic raceways in an air handling space, it is not prohibited because the air does not mix with the rest of the building. When you think about it, if there is a fire inside of the IT room the noxious gases emitted by nonmetallic wiring methods will only affect the IT room itself, which is not occupied by people on a regular basis [645.4(4)].
Ventilation in the underfloor area must be for the IT room only and arranged so air circulation will cease upon the detection of fire or products of combustion.
Openings in raised floors must protect cables against abrasions.
Cables enclosed in a raceway.
Interconnecting cables listed and installed with equipment manufactured prior to July 1, 1994.
Signal or communications cables of the following types: Type TC [Art. 336]; Types CL2, CL3, and PLTC [Art. 725]; Types NPLF and FPL [Art. 760]; Types OFC and OFN [Art. 770]; Type CM [Art. 800]; Type CATV [Art. 820].
Signaling and communications cables installed within the raised floor area in an IT room aren't required to be plenum rated (Fig. 2 above).
Conductors with green or green with one or more yellow stripes, insulated 4 AWG and larger, and marked “for use in cable trays” or “CT use” are permitted for equipment grounding (bonding) of the raised floor parts.
Remove these, unless they are in metal raceways.
You know to secure branch circuit supply conductors in place [300.11 and 645.5(D)(2)], but what about IT cables? Art. 645 does not require you to secure power, communications, connecting, or interconnecting cables that are part of listed IT equipment (Fig. 3 below). You may decide to secure them in place for other reasons, and the NEC certainly won't stand in the way of doing that.
Fire alarm circuits, Art. 760.
Fiber optic cable, Art. 770.
Communications circuits, Articles 800 through 820.
If wiring systems penetrate the fire-resistant room boundary, seal them per 300.21.
Disconnecting means. Earlier, we mentioned an exterior shutoff. What exactly are the requirements?
A disconnecting means must disconnect power to IT equipment in the IT room and to dedicated HVAC systems that serve the room. When you have a UPS in an IT room, you must install a disconnecting means per 645.10. This must disconnect power to all UPS systems within the IT room — and it must disconnect all supply and output circuits, and the battery, from its load [645.11].
The control(s) for the disconnecting means must be grouped, identified, and located so as to be readily accessible at the principal exit doors. You can use a single means to control the electronic equipment and HVAC systems. If you use a button as a means to disconnect power, pushing it “in” must disconnect the power.
The typical means of establishing the control function of the disconnect is through the use of a normally open (NO) momentary pushbutton at each principal exit door. Pressing the emergency button closes the contacts and opens the shunt-trip circuit breaker(s), thereby turning off all power.
Grounding (bonding). Metal parts of an IT room must be grounded (bonded) to an effective ground-fault current path, per Part VI of Art. 250, with an equipment grounding (bonding) conductor of a type specified in 250.118 [250.4(A)(3)].
If an IT room has isolated ground receptacles, ensure they are grounded (bonded) to an effective ground-fault current path per 250.146(D) and 406.2(D). If they are “grounded” to a separate ground rod and not to an effective ground fault current path, they are an electrocution hazard and probably a source of electrical noise. They also violate all accepted electrical standards including the NEC [250.4(A)(5)].
Two goals. Some requirements of Art. 645 seem to conflict with the purpose of having an IT room to begin with. But remember, the NEC isn't an engineering manual — it's a people and property protection standard [90.1].
IT rooms have special requirements for two reasons: to protect fire personnel from backup power sources and to reduce the spread of fire/smoke.
As you perform design and installation work for IT rooms, always keep these two goals in mind. Doing so will help you correctly interpret and apply the requirements of Art. 645.

References: Art. 645
 Art. 645

Art. 645
 Art. 645
 Art. 645
 Art. 760
 Art. 770
 Art. 250
 Art. 645
 Art. 645