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Service Conductors and Service Equipment

Sep 1, 2001 12:00 PM, By Mike Holt, Mike Holt Enterprises, Inc.

Knowing where the service ends can put an end to some typical problems.

The National Electrical Code refers to the phrase “service equipment” in almost 50 Code sections. Understanding where the service begins and ends is critical in the proper application of many Code rules — particularly those for grounding, bonding, and disconnecting means. To understand how to apply these rules, review the following definitions in Art. 100.

Service point. This is the connection point between the serving utility facilities and the premises wiring.

Service conductors. These run from the service point to the service disconnecting means (the service equipment, not the meter). Service-entrance conductors can enter an installation from overhead (service drop) or underground (service lateral).

Service equipment. This necessary equipment, which usually consists of circuit breakers or switches and fuses and their accessories, is connected to the load end of service conductors that serve a structure and constitutes the main control and cutoff of the supply. Service equipment does not include the metering equipment, such as the meter and/or meter enclosures (Sec. 230-66).

Reviewing these definitions should help you understand that service conductors originate at the serving utility (service point) and terminate on the line side of the service disconnecting means (service equipment). The following conductors and equipment on the load side of service equipment are considered feeder conductors:

• Secondary conductors from customer-owned transformers.

• Feeder conductors from generators, UPS systems, or photovoltaic systems.

• Feeder conductors serving remote structures.

You must install feeder conductors in accordance with the requirements in Arts. 215 (feeders) and 225 (outside wiring). In addition, Sec. 250-30 contains the grounding requirements for separately derived systems, and Sec. 250-32 outlines the grounding requirements for separate structures. Do not apply the grounding requirements for services (Sec. 250-24) to feeder conductors and equipment.

Neutral-to-case connections in violation of Secs. 250-6 and 250-142(b) create a condition where neutral current has multiple return paths to the grounded (neutral) conductor of a power supply. This allows objectionable neutral current to flow on the metal parts of the electrical system and building, which can lead to fire, shock hazard, and power quality issues with digital equipment.

Too many electrical workers (and inspectors) think service conductors include the secondary conductors of transformers and supply conductors to a remote building. This misunderstanding has led to several improper and unsafe neutral-to-case connections at disconnects and panelboards. To ensure a safe electrical installation, know where the service begins and ends. You must ground service equipment, separately derived systems, and separate buildings and structures in accordance with Secs. 250-24, 250-30, and 250-32, respectively.

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