for three-phase installations is too large, both the initial cost and the circulating current losses will be higher. However, on single-phase instal- lations, a larger concentric neutral is needed to carry the neutral return current that may be near the magnitudes of the current in the energized conductor. On single-phase installations, a re- duced neutral capacity could produce higher neutral-to-earth voltages and higher losses be- cause of the lower conductivity of the neutral conductor. Conceivably, the reduced-capacity neutral could even be thermally overloaded as the cable approaches normal rated capacity.
For these reasons, RUS requires a full-capacity neutral in single-phase installations and allows a one-third (or greater) capacity concentric neutral on three-phase cable installations. This approach ensures that there will be concentric neutral con- ductivity at least equal to the phase conductor conductivity in both single-phase and three- phase installations. The cooperative engineer should consider the typical use of the cable that is being bought when deciding whether to use full-capacity or reduced-capacity neutrals.
Length
Each purchaser will have different requirements for the length of cable on reels to use on routine installations. Requirements will vary with terrain, the type of equipment used to install cables, and the typical distance between termination points. The cables should be bought in the longest lengths practical for the field crews to use so as to leave less scrap at the reel ends. Constraining factors will be the width and diameter of reels that the cable transport and installation equip- ment can accommodate.
The cooperative engineer must also consider the weight of the full reel when deciding on the standard reel size. As with all other aspects, it is helpful to select the same maximum reel sizes that other cooperatives choose, especially if there is a group purchase arrangement. Doing so makes stocking easier for manufacturers and distributors and consequently reduces the cost for the cooperative.
Cold Weather Bending
Utilities operating underground systems in cold climates have experienced a variety of flexibility
problems with cables caused by the low temper- atures. To lessen these problems, the specifier can insert a section requiring a cold bend quali- fication test. This test will indicate the probability the cable will fail during bending or movement at low temperatures. It is not a measure of cable flexibility. In most cases where the cable operat- ing temperature is always above -17°C (0°F), cable bending problems are not significant.
Feeder Cable Shielding
Section 4of this manual shows that high-capac- ity three-phase cable installations incur much higher losses when high-conductivity concentric neutrals are used. Induced currents that circulate between the neutrals of the three phases cause these losses. Lower conductivity neutral/shield arrangements reduce these losses. Such arrange- ments not only can reduce the economic loss as- sociated with circulating currents, but also can increase cable ampacity by cutting the amount of heat generated in the neutral/shield. Substa- tion exits or other large feeders generally have better load balance with lower neutral currents. Therefore, reduced concentric neutrals will have adequate thermal capacity, especially if they are supplemented by a separate neutral conductor. Where a high-capacity feeder is being installed, the engineer should give particular attention to the size of the neutral and/or shield specified on the cable.
The engineer must also check the magnitude and duration of fault currents on the system when selecting a particular neutral/shield arrangement. Fault current duration is usually not a problem on 200-amp-class single-phase circuits because full-capacity neutrals are used and circuit reclos- ing is not a factor. However, the other extreme is substation feeder exit cables where there is a de- sire to reduce neutral capacity to minimize circu- lating current losses and increase ampacity. In these locations, the fault currents are higher, overcurrent protective devices operate more slowly, and reclosing is often used. All these ele- ments contribute to higher neutral/shield tem- peratures under cable fault conditions. The neu- tral/shield component of underground substation feeder exit cables and express feeders must also carry fault currents for all down-line faults. An additional neutral conductor located in the same
cable purchases to review factory production and testing procedures. To be effective, an individual familiar with cable production and testing meth- ods must be present. Because the expense of this observation is essentially the same for large or small orders, large orders greatly reduce the incre- mental unit cost for observation. Moreover, with group purchasing, there is a greater chance that a staff engineer from one member of the group will have (or be able to develop) the expertise necessary to effectively perform this function.
Group purchasing and larger orders will al- ways lead to a lower unit price. Because all the cable bought under a group plan will be accord- ing to a single specification and of the same construction, the manufacturer can achieve economies through the following:
• Volume purchases of required material;
• Longer, more efficient runs in wire drawing operation;
• Longer, more efficient runs in cable extrusion operation; and
• Wider distribution of fixed costs associated with a single order.
Group purchasing of large cable quantities has a minimum effect on delivery practices. Manufac- turers will usually ship parts of the larger order to destinations specified by group members at no extra cost. In some cases, groups have negotiated warehousing arrangements with manufacturers for release of cable on a designated schedule throughout a year. This arrangement reduces the cash flow burden on the cooperative. It also gives the manufacturer additional flexibility by allowing the major production runs to be sched- uled at more convenient times.
Another advantage to group purchasing on a standardized specification is the feasibility of having a single distribution point where the group maintains a cable stock. The ability to re- ceive large orders coupled with reduced ware- house space requirements at the individual group members’ sites may make this approach reasonable in some cases. This option is particu- larly attractive when group purchase and stock- ing of other utility materials is also practiced. trench or conduit with the insulated cables can
supplement this capability. The engineer should pay particular attention to this set of conditions when selecting a reduced neutral size.
CABLE PURCHASING PRACTICES Vendor Prequalification
Because cable is one of the keys to a reliable and cost-effective underground distribution sys- tem and some types of cable defects are not ob- vious at the time of manufacture and will be recognized only years later, all cable needs to be manufactured by reliable producers. It is in the cooperative’s best interest to review the qualifi- cations of vendors and select those that have a proven capability to produce a high-quality in- sulated conductor.
Prequalification of vendors ensures that all parties quoting on a cable order have a proven ability to produce a high-quality cable meeting a particular specification. Prequalification avoids situations in which a vendor with questionable qualifications submits an unrealistically low price. Under these circumstances, the utility is typically required to honor the bid, which may lead to additional long-term cost through premature cable failure. It is only logical that if most of the utility industry is carefully prequalifying vendors, those found unqualified by others will have lower prices and better lead times because of lower demand for their products. This possibility makes it even more important to participate in an effective vendor prequalification program.
Group Purchase
One way to simultaneously improve cable prices and quality is to engage in group purchasing of cable. This practice has several advantages to both the vendor and the cooperative.
Larger quantities (more than 50,000 feet) often lead to better overall quality control. During the initial part of a cable manufacturing run, larger orders mean that the front and tail ends of a par- ticular run can be scrapped. This additional cost for nonqualifying material is then spread over a larger order, thereby reducing the unit price.
Active quality control is an important part of any utility purchasing program. This quality con- trol should include factory visits during major
After a cooperative has analyzed its cable needs, written a comprehensive specification, and fol- lowed good purchasing procedures, one critical step remains before installation can begin. This step is the acceptance and inspection of the cable delivered by the manufacturer. Cable ac- ceptance involves several simple and inexpen- sive steps that can yield big dividends. The cooperative engineer must follow these steps to make sure that a quality product is delivered to installation crews.
STEP 1. VISUALLY INSPECT FOR SHIPMENT DAMAGE
Visually inspect cable reels for any damage that may have occurred in transit. Signs of possible damage include impressions or nicks on the out- side layer of cable or the reel lagging. If possi- ble, this inspection should take place while reels are still on the delivery vehicle.
STEP 2. CHECK TAGS
Visually check each reel to determine that it has proper tags and labels as described in the speci- fications. Make sure that information on the reel tags agrees with purchase-order information. For example, be sure that wire size, insulation thick- ness, neutral configuration, and jacket descrip- tion all conform to the specifications and purchase order. Cable length should fall within the bounds described by the purchase order. If cable was ordered cut to specific lengths, the engineer should check the tag and sequential jacket markings (if available) to be sure that enough length is available for the required run.
STEP 3. CHECK DIMENSIONAL TOLERANCE
Make a simple measurement of basic cable di- mensions on one reel of each cable size in a shipment to confirm that labeling is correct. Measure these dimensions:
• Conductor size and stranding,
• Insulation thickness,
• Concentric neutral wire size and number of strands, and
• Jacket thickness.
Section 11, Cable Testing, gives further infor- mation on allowable dimensional tolerances.
STEP 4. CONDUCT CABLE ACCEPTANCE TESTING
Once on each order or once for each 50,000 feet of cable, the cooperative should conduct a com- plete set of dimensional and electrical perfor- mance tests on the cable to make sure it complies with the purchase specifications and referenced industry standards. These tests in- clude the following:
• Conductor shield resistivity test;
• Insulation shield resistivity test;
• Dimensional analysis of all components;
• Microscopic examination for voids, contami- nants, and shield interface protrusions; and
• Insulation shield stripping test.
An outside laboratory will need to help with these tests. Section 11 gives additional informa- tion on these tests.