Incoming Service Considerations

Article 230 of the National Electrical Code: “covers service conductors and equipment for the protection of services and their installation requirements”. Figure 1.1-22 provides the scope of pertinent references that apply to incoming service equipment. These range from conductor types from overhead service utility drops to underground utility feeds and their proper installation. 

Parts V, VI and VII of Article 230 spell out the common requirements for lowvoltage service equipment <1000 Vac. These parts cover locations permitted, various marking requirements including Section 230.66 that requires service equipment be listed and marked as Suitable for Use as Service Equipment, (SUSE). Also included is Section 230.71, which limits the number of incoming main service disconnects to a maximum of six. 

Section 230.95 of this Article requires equipment ground fault protection for service disconnect(s) 1000 A and above when applied on solidly grounded wye services, where the phase to ground voltage exceeds 150 V. 

Article 250 of the NEC contains the requirements for grounding and bonding of electrical systems. Specific details pertaining to grounding for the incoming service equipment begin at Section 250.24. 

These include application of the grounding electrode conductor in Section 250.50 to its sizing in accordance with Table 250.66. Requirements for bonding of service equipment begins in Section 250.90. Sizing of the main bonding jumper and system bonding jumper are also covered in Table 250.102(C)(1). 

A more in-depth discussion of ground fault protection can be found in Section 1.5 of this Design Guide.

Figure 1.1-22. Application Zones of 2014 NEC Articles Related to Incoming Utility Services

The NEC Article 230 does not specifically require that electrical service rooms be fire rated rooms or that sprinklers be provided. However, survivability requirements for fire pump disconnects in local building code requirements, in addition to NEC Article 450 or additional utility specifications may require fire rated rooms, particularly if mediumvoltage service is being supplied. 

Space allocation should be considered when laying out equipment in a service room. Both low- and medium-voltage utility metering typically adds an additional equipment structure, or structures, to an incoming service lineup. These are used to accommodate the current transformers and potential taps or voltage transformers necessary for the external utility revenue meter to calculate usage.

Article 110 of the NEC covers a broad range of requirements for electrical installations. It includes provisions that govern the construction and spatial requirements for egress, clearances and working space in rooms containing electrical distribution and service equipment. 

Table 1.1 4 includes combined tables from NEC Article 110, showing the minimum “depth of the working space in the direction of live parts” required in front and behind medium-voltage equipment and low-voltage equipment.

Table 1.1-4. NEC Minimum Depth of Clear Working Space at Equipment

Additional work space may need to be allocated for OSHA required grounding practices, prior to servicing deenergized medium-voltage equipment. As an example, 6-foot-long insulated hot sticks are typically used to keep personnel at a safe distance, while applying portable ground cables. This procedure is utilized to discharge any residual capacitive voltage present on cables terminating in a medium-voltage transformer primary cable compartment or in the rear cable compartment of medium voltage switchgear.

As renewable energy or cogeneration is added, power systems are becoming more complex and so too is their service interface for utility power. Many Public Service Commissions have adopted Standard Interface Requirements (SIR) for Distributed Energy Resources (DER) based on IEEE 1547. These are intended to protect the utility system from user owned generation back feeding into a fault or dead cable on the utility grid. 

Utilities may have their own specifications and tariffs for the interconnection of this Dispersed or Distributed Generation (DG). These include capacity limitations and/or the addition of charges for the “spinning reserves” they must keep on hand, should the user’s DG assets fail or load increase. 

Consequently, the design engineer must be aware that special relaying protection may need to be included in the design. Also, additional analysis of the utility tariffs and rate structures may be necessary to validate the projected payback of participation in peak demand reduction programs using owner-supplied generation.