ce operation. Thus this cable has 19-gauge wire pairs with load coils inserted at 1830-m (6000 ft) intervals with coils of 66-mH inductance. The most commonly used spacings are B, D, and H. Table 5.4 will be useful in calculating the attenuation (loss) of loaded loops for a given length. For example, in 19H88, the last entry in the table, the attenuation per kilometer is 0.26 dB (0.42 dB per statute mile). Thus for our 8-dB loop loss limit, we have 8/02.6, limiting the loop to 30.77 km (19.23 mi). When determining signaling limits in loop design, add about 15 S per load coil as a series resistor. In other words, the resistance values of the series load coils must be included in the total loop resistance. 5.4.5 "Cookbook" Design Methods for Subscriber Loops 5.4.5.1 Resistance Design Concept. Resistance design (RD) dates back to the 1960s and has since been revised. It was basic North American practice. Our inclusion of resis- tance design helps understand the "cookbook" design concept. At the time of its inception, nearly all local serving area switches could handle loops up to 1300 So resistance. In vir- tually every case, if the RD rules were followed, the attenuation limit of 8 dB would be complied with. The maximum resistance limit defines a perimeter around a local switch which is called the resistance design boundary. For subscribers outside of this boundary served by the switch, long route design (LRD) rules were imposed. LRD is briefly covered in Section 5.4.5.2. The following additional terms dealing with RD are based on Ref. 5. 1. Resistance design limit is the maximum value of loop resistance to which the RD method is applicable. The value was set at 1300 So primarily to control transmission loss. In the revised resistance design (RRD) plan, this value is increased to 1500 S2. 2. Switch supervisory limit is the conductor loop resistance beyond which the operation of the switch supervisory equipment (loop signaling equipment) is uncertain. 3. Switch design limit. With RD procedures, this limit was set at