Typical Neutral Configuration
Conductor Size Full Capacity One-Third Capacity One-Sixth Capacity One-Twelfth Capacity
#2 AWG Aluminum 10 × 14 AWG 6 × 14 AWG N/A N/A
1/0 AWG Aluminum 16 × 14 AWG 6 × 14 AWG N/A N/A
4/0 AWG Aluminum 13 × 10 AWG 11 × 14 AWG N/A N/A
350 kcmil Aluminum 20 × 10 AWG 18 × 14 AWG 14 × 16 AWG N/A
500 kcmil Aluminum N/A 16 × 12 AWG 20 × 16 AWG 10 × 16 AWG
750 kcmil Aluminum N/A 20 × 9 AWG 30 × 16 AWG 10 × 14 AWG
being equal, these losses are lower where there is less neutral conductivity. A cable with a one-third neutral has 53 percent of the losses of a cable with a full-capacity neutral if the cables are spaced 7.5 inches center to center. The circuit ampacity of full-neutral cables in three-phase circuits is also reduced because of these shield losses. This problem is signifi- cant in larger cable sizes, particularly if the cables are not closely grouped. For instance, on a 350-kcmil circuit carrying 390 amperes, the losses on a circuit with 7.5-inch cable spacing will drop from 12 kW/1,000 feet to 6.4 kW/1,000 feet if a one-third neutral is used instead of a full-capacity neutral. Elevated losses and reduced ampacities are not gener- ally a problem on three-phase circuits of 1/0 AWG aluminum or smaller if the cables are grouped in a single trench. Additional infor- mation on circuit ampacity rating for various neutral configurations is given in Section 4.
Longitudinally Corrugated Shield
The L.C. shield has been developed as a way to provide greater conductivity in larger cables. The shield generally consists of a copper sheet that is installed with its major axis parallel to that of the cable. The sheet is then folded around the cable and sealed to itself on the opposite side. Circumferential corrugations are fabricated in the resulting tube to add flexibility and ensure that the shield will uniformly bend with the cable. The seal applied between the two sides of the copper is usually an adhesive elastomer. The tube gen- erally does not have a metal-to-metal connection with the cable insulation shield at this point be- cause allowance must be made for the cable in- sulation to thermally expand during operation at elevated temperatures. Not only is the tempera- ture change higher in the insulation than it is in the cable shield, but all dielectrics have a sub- stantially higher coefficient of thermal expansion than that of copper. Because the metallic shield must have good contact with the semiconduct- ing insulation shield to function effectively, a tight fit must be maintained at all times. There- fore, the insulation expansion is accommodated by flexibility in the elastomeric seal. The return of the shield to intimate contact as the cable cools is assisted by the external insulating jacket.
Equation 2.2
where: A = Cross-sectional area, in cmils b = Tape thickness, in mils dm= Mean diameter, in mils W = Width of tape, in mils L = Overlap of tape, in mils
A = 4bdm× W 2(W – L)
L.C. shields are commonly available in five-mil thickness but, for applications requiring additional fault current capability, shield thicknesses of eight or 10 mils can be furnished. However, L.C. shields should be sized to carry expected system neutral currents. Use of L.C. shields as the system neutral will require evaluation of available system fault currents and protective system clearing times. Ac- cessories such as shield (neutral) bonding clamps must also be carefully evaluated for long-term continuous current and fault current capacity.
The L.C. shield does provide a limited degree of resistance to water vapor transmission. It is clearly superior to concentric neutral configura- tions for water vapor transmission. It is some- what better than helically applied copper tape shields because the length of the straight joint is less than the helical joint. Moreover, the elas- tomer at the lap point does provide a better seal, although, under static pressure, the elastomeric seal cannot be depended on to prevent moisture from migrating into the cable insulation.
Flat Copper Tape
This is perhaps the oldest conductive shield con- figuration. The tape generally consists of a five-mil (0.005-inch) thick copper tape helically applied over the semiconducting insulation shield. The tape is usually installed with a 12.5 percent overlap. Tape shields may be fabricated from bare copper or may be tinned copper. Because of the small cross section, the conductivity of flat copper tape shields is relatively low compared with the central cable conductor. Equation 2.2 gives the effective cross-sectional area of an overlapped tape.
CONCENTRIC NEUTRAL CONFIGURATIONS
As experience has been gained with under- ground installations under a variety of condi- tions, the utility industry has developed several specialized variations of the basic concentric neutral configurations. Each of these arrange- ments has an advantage for a particular set of installation conditions.
Bare Concentric Neutral
The first widely accepted concentric neutral cables were of a bare concentric neutral (BCN) config- uration. In this design, the concentric neutral strands were laid over the semiconducting insu- lation shield and no jacket was applied. When the cable was directly buried, this arrangement had the advantage of exposing the concentric neutral conductors to the surrounding soil. The result was a very effective ground, especially where soil re- sistivity was low. The low resistance between neutral and earth meant more of the system neu- tral current could return to the source by way of the earth, thereby reducing current in the con- centric neutral and circuit voltage drop. Further- more, the low resistance between the neutral and earth reduced neutral-to-earth voltages dur- ing both normal operations and fault conditions.
The bare concentric neutral is also considered the best possible arrangement for personnel safety in case of a dig-in. The neutral size en- sures the ability to adequately conduct fault cur- rents until protective devices operate. The high- er conductivity of the concentric neutral will produce lower voltages on the neutral at the fault location. The low resistance between the neutral and earth will significantly reduce the touch potential at the dig-in site. Most important, the concentric neutral physical arrangement en- sures the object penetrating the cable will have established a good neutral connection before contacting the energized center conductor.
In light of all the advantages of BCN cables, it is unfortunate that there are major durability prob- lems with this design under many installation conditions. These problems are all related to corrosion of the exposed cable neutral. In many cases, the neutral had a significantly reduced cross section after only a few years of service. In other cases, the neutral was completely corroded and the only neutral current path was through
ground rods. This condition was totally unsatisfac- tory from the standpoints of system safety and re- liability. Therefore, the use of BCN cable has been discontinued except in very special conditions.
Jacketed Concentric Neutral
Because of the very serious problems experi- enced with BCN cables, the electric utility indus- try began using the jacketed concentric neutral (JCN) configuration. This configuration has most of the major advantages of the BCN design ex- cept for continuous contact of the neutral with earth. The jacketed configuration reduces access of moisture and corrosive agents to the neutral. Insulating jackets also interrupt the flow of gal- vanic corrosion currents between the neutral and other metallic objects.
JCN design has achieved wide acceptance as a solution to the concentric neutral corrosion problem. However, the cooperative engineer must give special attention to system grounding if jacketed cables are used. Cable identification also acquires additional importance, as jacketed cables are approximately the same dimension and general appearance as many communica- tion cables and water lines. See Section 5in the
Design Manual for detailed information on sys-
tem grounding.
Flat-Strap Concentric Neutrals
Flat-strap concentric neutrals, not to be confused with flat-tape metallic shields, consist of helically applied flat copper straps. These straps are about 0.020 to 0.025 inches (20 to 25 mils) thick and about 0.150 to 0.175 inches wide. The straps are applied so they abut each other and provide 90 percent metallic coverage over the outside of the cable. Conductivity of flat-strap neutrals is generally equal to that of the energized conduc- tor. Flat-strap concentric neutrals have found greatest acceptance in areas where rodents dam- age direct-buried cables. The complete metallic coverage on a cable was originally believed to lessen damage from gophers. However, using this type of cable to lessen rodent damage has had mixed results. Recent research shows that rodent damage is more effectively limited by in- creasing the diameter of the object. Therefore, flat-strap neutrals should not be depended on to prevent rodent damage.
Flat-strap neutral cables should be jacketed. The thick- ness of the flat strap is less than the diameter of the neu- tral wires. Therefore, the com- plete cable diameter will be less. This is an advantage where space is limited.
Concentric Neutral Materials Other Than Copper
The predominant material in concentric neutrals has always been copper. For many years, the generally accepted wire for bare concentric neu- trals was copper with a tin or tin-lead alloy coat- ing. As experience has been gained with a wide variety of materials, engineers have determined that the coating of the copper concentric neutral conductors was not necessary and, in some cas- es, actually led to higher corrosion rates. It is generally believed that, in the early days of con- centric neutral cable manufacture, tinned copper concentric neutrals gained wide acceptance be- cause most flat-tape metallic shields were tinned on jacketed cables. In some cases, that was a holdover from cables on which butyl rubber in- sulation was used and tinning was needed to avoid corrosion. Also, tinned copper was used on earlier cables because of the prevalence of soldered connections, and the coated copper fa- cilitated soldering of these thin shields. Because concentric neutral cables never employ soldered connections and butyl rubber is no longer used for insulation, the need for coating neutral wires has disappeared. Bare copper wires are now uniformly accepted as the preferred material for concentric neutrals, whether bare or jacketed.
During the mid-1970s, a few utilities briefly experimented with aluminum concentric neutral cables. These were applied in a bare configura- tion. Although some laboratory studies showed that the aluminum neutrals would resist many types of soil-induced corrosion, field experience proved quite the opposite. The very complex in- teractions present on an interconnected neutral passing through a variety of soils led to early failure of these cables. It became obvious that aluminum should never be used as an exposed concentric neutral in direct-buried or conduit cable installations.
Experience with low-voltage insulated cables has shown that aluminum conductors can be extremely susceptible to corrosion, even if they are in- sulated from the surrounding environment. Because cable jackets are not absolutely moisture proof, even an encapsulated alumi num neutral conductor may be subject to long-term deterioration from moisture migration. It is un- wise to consider aluminum neutral conductors for primary cables, even in a jacketed configura- tion, when the only advantage to be gained is slight savings in initial material cost.
Another approach that was used for a limited time to try to solve the bare concentric neutral corrosion problem was the use of a composite copper/steel conductor. The particular configura- tion used a copper center core for conductivity, with a heavy steel coating completely surround- ing the copper. For durability during periods of atmospheric exposure, the steel was galvanized. This cross-sectional arrangement offered the def- inite advantage of having steel exposed to the earth in the direct-buried cables instead of cop- per. The exposed steel greatly simplified the ap- plication of cathodic protection systems to the neutral. However, the conductor used in this neutral construction did carry a premium price. Utilities also experienced difficulty in applying this cable to existing systems that already had extensive exposure of bare copper concentric neutrals. Systems containing this cable configu- ration required sacrificial anodes or impressed voltage rectifiers applied to provide protection to the neutral. For additional information on the principles of cathodic protection, see Section 7.
CABLE JACKET
In most cables, the cable jacket is the outermost layer of material that serves as a barrier to mois- ture and mechanical damage. Therefore, it is im- portant to optimize the design and materials of the jacket to obtain maximum performance in these important areas.
For many years, all power cable designs includ- ed a jacket. However, with the advent of the ex- tensive underground residential programs, electric utilities began installing bare concentric neutral