Load ratings of cable

Abstract

Operating and test data concerning the maximum safe loading of impregnated-paper-insulated lead-covered cable are presented. The results of the study may be summarized as follows: 1. The occasional operation of cable at higher temperatures than are permitted by present temperature rules effects considerable economy. 2. During emergencies, temperatures of 5 to 35 degrees centigrade (depending on kind of cable) above those permitted by the rules are safe for the insulation. 3. For extra-high-voltage solid-type cable, void formation in insulation and expansion of lead sheaths may limit allowable temperatures and temperature ranges. 4. Cracking of lead sheaths due to reciprocating cable movement into manholes may limit the temperature range for usual daily loading. Limitation is more severe for longer conduit lengths up to 500 feet, but changes little with increase from 500-to 1,000-foot lengths. 5. Cracking of sheaths in manholes due to cable movement may be reduced by improving manhole conditions. 6. For many cables a balanced design requires a lead-alloy sheath that gives increased resistance to effects of cable movement and of internal pressures. 7. Continuous field temperature surveys are essential to efficient use of large conduit and cable systems. 8. Only a small fraction of the cable ever operates at the higher temperatures. 9. Data on center empty-duct temperatures and on average heat losses over 24-hour periods give satisfactory results in heat calculations. 10. Other practices which increase load ratings are the use of different ratings for various periods of the year, the replacement of poor soil in special cases, and the use of extra-large conductors in warmer conduits.