It’s often good to start a service installation by deciding which conductors belong in the service. To identify a service conductor, step back and see if you’re dealing with a service or a load side distribution point. Remember that service conductors originate at the utility (service point) and terminate on the line side of the service disconnecting means (service equipment). But feeder conductors, which arecovered by Art. 215 and 225, originate on the load side of service equipment.
When conducting service installations, it helps to know some basic definitions (Sidebar below and Fig. 1).
Routing. The routing requirements of service conductors differ from those of other conductors. For one thing, service conductors for one structure can’t pass through the interior of another structure (230.3).
Different rules also apply, depending on whether a service conductor is inside or outside a structure. How do you know the difference? The answer may seem obvious, but it’s not [230.6]. Service conductors are “outside” when they are installed according to the following criteria:
- Under 2 in. or more of concrete beneath a structure.
- Within a structure in a raceway that’s encased in no less than 2 in. of concrete or brick
- In a vault that meets the construction requirements of Art. 450, Part III.
- In conduit buried in no less than 18 in. of earth beneath a structure
Once you’ve established whether service conductors are inside or outside a structure, do you need to install separate raceways for them? Yes. According to 230.7, you can’t install service conductors in the same raceway or cable with feeder (or branch-circuit) conductors (Fig. 2). This separation maintains the integrity of fault protection. A fault that occurs between service and feeder conductors that are mixed in the same raceway could bypass the overcurrent protection for the feeder conductors.
This rule doesn’t prohibit the mixing of service, feeder, and branch-circuit conductors in the same service equipment enclosure.
Clearance is also an issue—you must observe a 3-ft clearance from building openings and a 10-ft clearance above various surfaces (230.9). In addition, you can’t use trees as supports for service conductors (230.10).
Service-drop conductors. The rules for underground service lateral conductors (230.30 through230.33) are similar to those for overhead service-drop conductors (230.22 through-230.29). Service-drop conductors installed and/or controlled by the electric utility must comply with the National Electric Safety Code (NESC), not the NEC [90.2(B)(5)]. Otherwise, the requirements of the NEC apply.
Size service-drop conductors per Art. 220 calculations. Ungrounded service-drop conductors can’t be smaller than 8 AWG copper or 6 AWG aluminum, except in limited-load installations.
Locate service-drop conductors so they aren’t readily accessible. Ensure that they comply with the clearance requirements of 230.24.
Attach overhead service-drop conductors to a point no less than 10 ft above the finished grade. This point must make it possible to maintain the minimum service conductor clearance required by 230.24(B); you may need to raise your point of attachment.
Service–entrance conductors. A service drop or lateral can supply only one set of service-entrance conductors, with three exceptions (230.40). Size your service-entrance conductors per Art. 220. Size protection devices no smaller than 125% of the continuous load.
You can install service conductors in one of 16 wiring methods enumerated in 230.43. You must provide protection from physical damage per one of the five methods enumerated in 230.50. Support service-entrance cable within 1 ft of service heads and raceway connections, and at intervals not exceeding 30 in.
Weatherheads. Raceways for overhead service drops (230.54) require a raintight service head. Locate the service head above the point of attachment or within 2 ft of the point of attachment if this isn’t practical. Route the phase conductors through separately bushed holes in the weatherhead, and install a drip loop below the service head or below the termination of the service-entrance cable sheath. Arrange the service drops and service-entrance conductors to prevent water from entering the service equipment. Install the point of attachment below the weatherhead, per 230.54(C).
Service equipment. You must supply the service disconnect with a main bonding jumper (250.28), to allow for a neutral-to-ground connection, per 250.24(B). The service disconnect must be capable of disconnecting all ungrounded service-entrance conductors from the served premises wiring. A pushbutton that activates the electromagnetic coil of a shunt-trip circuit breaker is insufficient (230.71(A) and 230.76).
Install the service disconnect at a readily accessible location outside the structure, or inside near the point of entry of the service conductors. Check with the AHJ to find out how far the disconnect can be located from the entrance (if inside) or how far the disconnecting means can be located from the structure (if outside).
Permanently mark each service disconnect to identify it as part of the service disconnecting means (110.22) (Fig.3). When a structure has two or more services, install a plaque at each service location to show the location of the other service [230.2(E)].
You’re limited to six disconnects for each service permitted by 230.2, or each set of service-entrance conductors permitted by 230.40, Ex. 1, 3, 4 or 5. The service disconnecting means can consist of as many as six switches or circuit breakers mounted in a single enclosure, in a group of separate enclosures, or in/on a switchboard.
You can’t connect equipment to the supply side of the service disconnect enclosure, except for the seven categories identified in 230.82. These include meters and tap conductors for certain purposes. You can’t connect emergency standby power ahead of service equipment.
Service equipment protection. You don’t have to provide service conductors with short-circuit or ground-fault protection, but you must provide overload protection via the feeder protection device.
Each ungrounded service conductor requires overload protection at the point where the service conductors terminate, per 240.21(D) (Fig. 4). The rating of the protection device must not exceed the ampacity of the conductors. Refer to 230.90(A) for five exceptions. You must provide ground-fault protection of equipment for each service disconnect rated 480Y/277V, 3-phase, 1,000A or more.
Getting it right. To properly apply the installation requirements for service conductors and equipment, you must first define where your service entrance starts and ends. Unfortunately, this is often a “missing step” in the design process. The results of that error can result in rework or excess cost if you’re lucky, and catastrophic failure if you’re not. But if you start at that point and then follow Art. 230 methodically, you’ll efficiently meet the requirements for a safe service installation.
Sidebar: Service Entrance Definitions
Service point - The point of connection between the utility and the premises wiring.
Service conductors - The conductors from the service point to the service disconnecting means (service equipment, not meter). Service conductors include service-entrance conductors for overhead (service drop) and underground (service lateral).
Service equipment - The necessary equipment, usually consisting of circuit breakers or switches and fuses and their accessories, connected to the load end of service conductors to a structure (or an otherwise designated area), and intended to constitute the main control and cutoff of the supply. Service equipment does not include the metering equipment (230.66).