Understanding conductor requirements for services
Part 1 of this two-part series, which ran in last month's August issue, explored how to ensure your service entrance equipment meets Code. Starting with general requirements, the article tackled service equipment details and moved into discussions on multiple disconnects and service overcurrent protection. Now it's time to focus on installation requirements for service conductors and equipment.
Compared to other conductor requirements, those for service conductors can seem unduly restrictive. For example, service conductors for one structure can't pass through the interior of another [230.3]. To avoid problems, start a service installation by deciding which conductors are part of the service. What you decide here will determine how you do the rest of the job.
To identify a service conductor, first determine whether the distribution point you're working with is on the line side (service) or load side (premises). Conductors and equipment on the load side of service equipment are feeder conductors, covered by Articles 215 and 225 — not Art. 230 (Fig. 1). Examples of feeder conductors include conductors supplied from a battery, photovoltaic system, or generator.
To understand where service conductors begin, it's important to know that the “service point” is the point of connection between the premise wiring and utility supply conductors. Article 100 tells us that service conductors run from the service point to the service disconnecting means (service equipment — not meter). Keep in mind that service-entrance conductors may be either overhead (with a service drop) or underground (with a service lateral).
Article 230 requirements for a conductor that is inside differ from those for one that's outside. The problem here is that the distinction between inside and outside is sometimes not clear. For example, is a conductor inside a building if it's under the bottom floor of that building?
Fortunately, Art. 230 provides specific criteria, thus solving the problem. A conductor is “outside a building” when installed [230.6]:
Under at least 2 inches of concrete beneath a structure.
Within a structure in a raceway that is encased in at least 2 inches of concrete or brick.
In a vault that meets the construction requirements of Art. 450, Part III.
In conduit under not less than 18 inches of earth beneath a structure. Note: Conduit is a specific type of raceway. EMT, for example, is not conduit. See Chapter 3 (Articles 342 — 353).
Where moisture could enter through an underground raceway and come in contact with live parts, the raceway must be sealed or plugged [300.5(G)]. An example would be service equipment located at the bottom of a hill with underground service laterals supplied from the top of the hill.
Don't install service conductors in the same raceway (or cable) as feeder or branch-circuit conductors [230.7] (Fig. 2). Otherwise, overcurrent protection for the feeder or branch-circuit conductors could be bypassed if a fault occurred between the service and non-service conductors. However, this rule doesn't prohibit the mixing of service, feeder, and branch-circuit conductors in the same “service equipment enclosure.”
This requirement may be the root of the misconception that you can't install “line” and “load” conductors in the same raceway. It's true. You can't install service conductors in a raceway with feeder or branch-circuit conductors, but you can install line and load conductors for feeders and branch circuits in the same raceway or enclosure.
Overhead service conductors must maintain a clearance of 3 feet from doors, porches, balconies, ladders, stairs, fire escapes, windows that open, or similar locations [230.9]. However, you don't have to maintain the 3-foot distance for conductors that run above a window.
Maintain a vertical clearance of at least 10 feet above surfaces from which people might reach overhead conductors [230.24(B)].
Don't install service conductors under an opening through which materials might pass, or where the conductors will obstruct building entrances (Fig. 3). And never use a tree or other vegetation to support the conductors [230.10(C)].
Overhead service-drop conductors
How does a service drop conductor differ from a service conductor?
Basically, the drop conductor is the conductor that runs from the final overhead span of the service conductor to your service entrance. Don't let drop conductors be readily accessible [230.24].
Maintain clearances to building openings [230.9], swimming pools [680.8], roofs, and final grade.
Service drop conductors that run over roofs must maintain a minimum clearance of 8 feet above the roof surface, for a minimum distance of 3 feet in all directions from the edge of the roof. Four exceptions exist:
Apply the final grade clearances, if the area above the roof has pedestrian or vehicular traffic.
For 120/208V or 120/240V service-drop conductors, overhead conductor clearances from the roof can be reduced 3 feet if the slope of the roof exceeds 4 inches for every 12 inches.
For 120/208V or 120/240V service-drop conductors, conductor clearance over the roof overhang can be reduced to 18 inches if no more than 6 feet of overhead conductors pass over no more than 4 feet of roof (Fig. 4).
4) The 3-foot vertical clearance that extends from the roof doesn't apply when the point of attachment is on the side of the building below the roof.
When not run over a roof, overhead service conductors must maintain a vertical clearance to final grade [230.24(B)]. There are four specifications:
10 feet above finished grade, sidewalks, or platforms/projections from which they might be accessible to pedestrians for 120/208V (or 120/240V circuits where the voltage doesn't exceed 150V-to-ground).
12 feet above residential property and driveways and those commercial areas not subject to truck traffic for 120/208V, 120/240V, or 277/480V circuits.
Same as No. 2, but 15 feet if the voltage exceeds 300V to ground.
18 feet over public streets, alleys, roads, parking areas subject to truck traffic, driveways on other than residential property, and other areas traversed by vehicles.
Attachment and support
The point of attachment for service-drop conductors must be at least 10 feet above the finished grade and must be located so the minimum service conductor clearance required by 230.9 and 230.24 can be maintained [230.26]. This means that the lowest part of the drip loop must never be less than 10 feet above the finish grade. The points of attachment might need to be raised to provide the clearances from building openings required by 230.9 (and from other areas by 230.24).
Multiconductor cables used for service drops must be attached to structures by fittings identified for use with service conductors [230.27]. Open conductors must be attached to fittings identified for use with service conductors or to noncombustible, nonabsorbent insulators securely attached to the structure.
The service mast used as overhead conductor support must have adequate mechanical strength (or braces/guy wires to support it) to withstand the strain caused by the service-drop conductors [230.28]. Some local building codes require a minimum 2-inch rigid metal conduit for the service mast. Many electric utilities contain specific requirements for the service mast. Only electric utility service-drop conductors can be attached to a service mast. 810.12 and 820.44(C) specify that aerial cables for radio, TV, or CATV cannot be attached to the electric service mast, and 810.12 prohibits antennas from being attached to the service mast. Communications cables, such as those for telephone [800.133(C)] and broadband [830.133(B)], are not permitted to be attached to the service mast.
Underground service-lateral conductors installed by the electric utility must comply with the National Electrical Safety Code (NESC), not the NEC [90.2(B)(5)]. Underground conductors not under the exclusive control of the electric utility must comply with the NEC.
Each service drop or lateral can supply only one set of service-entrance conductors [230.40]. However, five exceptions apply, which you'll find listed right below 230.40.
Service-entrance conductors can be spliced or tapped per 110.14, 300.5(E), 300.13, and 300.15 [230.46]. While the NEC allows this, maintenance considerations often make this method inadvisable. Remember, the NEC doesn't tell you how to design an optimal installation [90.1].
If your service is 600V or less, you can use any of the 16 wiring methods listed in 230.43. A 17th wiring method (cable tray) is covered separately in 230.44. Cable trays used to support service-entrance conductors can contain only service-entrance conductors, unless a solid fixed barrier separates the service-entrance conductors from other conductors.
If the authority having jurisdiction (AHJ) determines that the raceway isn't subject to physical damage, you can use Schedule 40 rigid nonmetallic conduit if you prefer. Otherwise, you must use one of the following:
Rigid metal conduit,
Intermediate metal conduit,
Rigid nonmetallic conduit (Schedule 80),
Electrical metallic tubing, or
Other means acceptable to the AHJ.
Remember, looks can be deceiving. What appears to be a feeder might actually be a service conductor. How can you be sure you're applying the right requirements?
The answer lies in the way the NEC is arranged. There's a good reason why the definitions are located upfront in Art. 100. So when you're faced with an installation that might be a service — for example, because the conductors enter from an outside wall — look at the Art. 100 definition of service. Cross-check your conclusions by looking at the Art. 100 definition of feeder and rule that out before proceeding.