Data center is solidly grounded

Syska Hennessy Group designed a data center that required solidly grounded underground duct banks.


Recently, Syska Hennessy Group led the design of a large data center that required a 25 kV utility service, solidly grounded, in underground duct banks. The service was 25 kV, 100% insulation level, copper conductors with ethylene propylene rubber (EPR) insulation.

The facility has an on-site 25 kV/13.2 kV substation to supply a 13.2 kV distribution system in underground duct banks. The system is impedance grounded and can be operated with one phase grounded for an unlimited time. When design engineers couldn’t find a cataloged 15 kV class cable with 173% insulation level, Syska used a 25 kV-class cable with 100% insulation level, copper conductors, and EPR insulation. The data center’s electrical infrastructure 600 V class facility distribution system was designed solidly grounded in overhead conduits and includes the following:

  • 600 V class copper conductors with thermoplastic high heat nylon (THHN) insulation
  • 600 V class facility distribution system, solidly grounded, in underground duct banks
  • 600 V class copper conductors with rubber high heat waterproof (RHHW-2) insulation
  • 600 V class plug-in bus ducts with copper conductors for distribution to IT equipment cabinets
  • 600 V class feeder bus duct with copper conductors for maintenance bypass and load bank circuits that are only occasionally used
  • 600 V class cable bus with copper conductors for large capacity feeders in the critical distribution system.


Christopher M. Johnston is a senior vice president and the chief engineer for Syska Hennessy Group's critical facilities team. Johnston specializes in the planning, design, construction, testing, and commissioning of mission critical 7x24 facilities, and leads team research and development efforts to address current and impending technical issues in critical and hypercritical facilities. With more than 40 years of engineering experience, he has served as quality assurance officer and supervising engineer on many projects.

Anonymous , 06/11/13 08:36 PM:

If the 13.2kv system is ungrounded you need only a 133% cable insulation. When the system is high impedance grounded why use a 173% cable insulation? Should the 133% insulation suffice?
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