Electrical distribution systems (switchboards, panelboards, power distribution units, feeders, etc.) serving data centers must be designed to be resilient as well as reliable, especially when they serve healthcare occupancies. Although typically addressed by locating distribution equipment so that is resilient, the electrical feeders are often overlooked. The feeders are the electrical conductors that connect the electrical equipment together, including switchboards, disconnect switches, transformers, and panelboards.
Data centers are the backbone of healthcare systems that store all the information for medical records, diagnostics, research, support systems etc. and allow for healthcare professionals to treat patients effectively. Therefore, designing proper protection of the feeders within the data center is critical for a resilient and reliable electrical system to maintain data center productivity. As such, data centers serving healthcare occupancies should be designed to meet the requirements of a Critical Operations Power System (COPS) as defined in the National Electrical Code (NEC) Article 708.
When considering the protection of the feeders, you need to consider protection from direct physical harm, protection from fire damage, and protection from water damage. Each application requires a different means of protection and is vital to ensuring the electrical system is resilient. Part II of NEC Article 708 (708.10) discusses all three of these protection measures to provide guidance to the facility and the engineer during design.
When considering protection against physical damage of the feeders for COPS, the NEC only permits the feeders to be either installed within conduit or be provide as MI cable. This requirement therefore prohibits the use of direct burial of cable or utilizing MC or AC system cable. However, it should be noted that flexible metal conduit can be utilized where necessary such as the connection from a panelboard to a transformer.
Rigid Metal Conduit (RMC) and Intermediate Metal Conduit (IMC) offer optimal protection. Rigid Metal Conduit is thick threaded tube made of either coated steel, stainless steel, or aluminum. Intermediate Metal Conduit is steel tubing. Although metal conduit is required when installed exposed or buried, if encased in concrete Article 708 does permit the use of schedule 40 or 80 rigid polyvinyl chloride conduit (PVC), reinforced thermosetting resin conduit (RTRC), electrical metallic tubing (EMT), and flexible nonmetallic or jacketed metallic raceways. All these criteria can be found in NEC Article 708.10 (C)(1) through (C)(3).
Protection from fire damage is equally as important, especially since fires can start anytime and are always unexpected. Having suitable protection in such cases is reassuring. The major difference when designing in accordance with Article 708 compared to Article 700 (Emergency Systems) is that even with a building that is provided with sprinklers, Article 708 still requires additional fire protection of the feeders. The use of Mineral Insulated (MI) cable provides the cables with 2-hour fire rated protection and provides the additional benefit of meeting the requirements outlined for physical protection of the cables.
MI cable is used for critical applications and harsh environments. The disadvantage of MI cable is that it can be costly. Therefore, alternates to MI cable would include provide a 2-hour assembly around the cables, concrete encasing the conduits, or using another listed 2-hour rated cable product. The protection time offered by these product ratings gives the owner time to a keep data center running while initiating the proper means for extinguishing the fire, which adds to resiliency.
When considering protection from water damage, Article 708 discusses installing the feeders above the 100-year floodplain. But if not possible, the feeders still need to be protected from damage from water that will inevitably enter conduit in underground applications. Feeders installed in wet locations below the level of the 100-year floodplain are required to be wet-location specific, such as a THWN or XHHW insulated cable.
Protection of feeders is not only essential to data center reliability and resiliency, but also fiscally beneficial to owners through avoided cost of repairs. Maximum protection minimizes the risk and cost of unforeseen shutdowns and downtime to repair damaged feeders. Protection of feeders should always be a priority in any data center design. Proper feeder protection avoids unexpected expenditures, lost data and damaged equipment and assures data center uptime.
Written by:
F&T Staff