|NEC Article 230 - Services
|By Mike Holt for EC&M Magazine
How can you correctly and efficiently follow service installation requirements?
It's often a good approach to start a service installation by deciding which conductors belong in the service. To identify a service conductor, step back and see if you are 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 (covered by Articles 215 and 225) originate on the load side of service equipment. When doing service installations, it helps to know some basic definitions.
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].
The routing requirements of service conductors differ from those of other conductors. For one thing, service conductors for one structure cannot pass through the interior of another structure [230.3]. And you apply different rules, depending on whether a service conductor is inside or outside a structure. When are they "outside" as opposed to "inside?" The answer may seem obvious, but it's not [230.6]. Service conductors are "outside" when they are installed:
Under 2 in. or more of concrete beneath a structure.
Within a structure in a raceway that is encased in not less than 2 in. thick of concrete or brick.
in a vault that meets the construction requirements of Article 450, Part III.
In conduit under not 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-in 230.7, we see that you cannot install service conductors in the same raceway or cable with feeder (or branch-circuit) conductors. This rule does not prohibit the mixing of service, feeder, and branch-circuit conductors in the same service equipment enclosure.
This separation maintains the integrity of your fault protection. If we mixed service conductors with other conductors in the same raceway, a fault that occurred between the service and feeder conductors could bypass the overcurrent protection for the feeder conductors.
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). You cannot use trees as supports (230.10).
The rules for underground service lateral conductors (230.30-230.33) are similar to those for overhead service-drop conductors (230.22-230.29). 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 NEC applies.
Size your service-drop conductors per Article 220 calculations. Ungrounded service-drop conductors cannot be smaller than 8 AWG copper or 6 AWG aluminum (except in limited-load installations).
Locate your service-drop conductors so they are not readily accessible. Ensure they comply with the clearance requirements of 230.24.
service-drop conductors to a point not less than 10 ft above the finished grade. Ensure this point is conducive to maintaining the minimum service conductor clearance required by 230.24(B)-you might need to raise your point of attachment (Figure 230-12).
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 Article 220. Size protection devices no smaller than 125 percent 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 5 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.
Raceways for overhead service drops (230.54) require a raintight service head. Locate the service head above the point of attachment (or within 2ft 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 eatherhead, per 230.54(C).
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 service-entrance conductors from the served premises wiring. A pushbutton that activates the electromagnetic coil of a shunt-trip circuit breaker is insufficient (see 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 for how far the disconnect can be from the entrance (if inside) or how far the disconnecting means can be from the structure (if outside).
Permanently mark each service disconnect to identify it as part of the service disconnecting means [110.22]. 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 are 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 up to six switches or circuit breakers mounted in a single enclosure, in a group of separate enclosures, or in/on a switchboard (Figure 230-22).
You cannot 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 cannot connect emergency standby power ahead of service equipment (Figure 230-25).
Service Equipment Protection
You do not 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 (see 240.21(D) and Figure 230-26). The rating of the protection device must not exceed the ampacity of the conductors (see 230.90(A) for 5 exceptions). You must provide ground-fault protection of equipment for each service disconnect rated 480Y/277V, three-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 be expensive-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 Article 230 methodically, you will efficiently meet the requirements for a safe service installation.
Copyright © 2002 Mike Holt Enterprises,Inc.