Campus-style distribution with differing voltages

In this arrangement, the fire pump and service entrance voltages are not the same.

By Sarah Kuchera, PE, LEED AP, ccrd + WSP, Dallas March 24, 2016

In this example, the design team is working with multiple buildings, with each having an individual power feed from a 15-kV feeder loop around the campus. The 15-kV service is derived from the utility at a central energy plant and then feeds the 15-kV loop. Each building has a substation that transforms the power down to 480 V for distribution within the building. Each building also has a dedicated 480 V fire pump to support its sprinkler system.

The difficulty with this arrangement is that the fire pump and service entrance voltages are not the same. This prevents a direct connection to the fire pump. Thus, the designer will need to transform power to the pump utilization voltage, which can introduce overcurrent protection devices and disconnecting means. How these are configured and sized must meet the requirements of NFPA 70: National Electrical Code (NEC) 695.4(B).

When feeding through a transformer, the electrical engineer needs to design to the requirements of NEC 695.5. The normal power feed from the substation needs to include 125% of the load of the fire pump and its accessories in the calculation for the transformer. A transformer’s primary overcurrent protection device should be capable of carrying the locked rotor amps of the fire pump and accessories, and a secondary overcurrent protection device downstream of the transformer should not be included for the fire pump feed. This will allow the fire pump to continue running and keep the fire protection system operational to protect the building and its occupants. Here are some key items to consider with this arrangement:

  • The overcurrent protection device should be sized to carry the lock-rotor current of the pump and its accessories indefinitely, as this is the primary source of power. This pump should continue running even during adverse conditions.
  • The overcurrent protection device serving the fire pump shall be selectively coordinated with the other devices to prevent a fault elsewhere in the system that would disable the fire pump.
  • All disconnecting means serving the fire pump and its accessories shall be clearly identified to prevent inadvertent disconnection of the power source.
  • Consideration should be given to the routing of the feeders so they are protected from physical harm or damage by fire.

The feeder from the switchgear to the fire pump controller (located in a different room) will be routed overhead and encased with 2 in. of concrete so that it is considered outside the building per NEC 230.6(2).

If the building is supported by a 15-kV distribution loop that is an "A/B"-type arrangement fed from separate utility substations, the designer could provide two feeds to the fire pump from this loop; one from each of the services as defined by NEC 695.3(C)(1). This should be coordinated with the authority having jurisdiction to validate that these two sources would then be considered as a reliable source.

Sarah Kuchera is senior vice president and electrical engineer at ccrd + WSP. Her duties have included project management, production coordination, and project engineering on a variety of different projects including hospital, retail, hospitality, and office buildings.