Conductor Protection

Electrical Plan Review

Conductor Protection
The increase in KVA capacity of power distribution systems has resulted in available short-circuit currents of extremely high magnitude. Fault induced,high conductor temperatures may seriously damage conductor insulation. As a guide in preventing such serious damage, maximum allowable short-circuit temperatures, which begin to damage theinsulation, have been established for various types of insulation. For example, 75°C thermoplastic insulation begins to be damaged at 150°C. The Insulated Cable Engineers Association (ICEA) withstand chart, tothe right, shows the currents, which, after flowing for the times indicated, will produce these maximum temperatures for each conductor size. The system available short-circuit current, conductorcross-sectional area, and the overcurrent protective device characteristics should be such that these maximum allowable short-circuit currents and times are not exceeded. Using the formula shown on theICEA protection chart will allow the engineer to calculate short-circuit current ratings of cable not shown on these pages. This can be used to find short-circuit current ratings where the clearing timeis below 1 cycle. The table below the ICEA chart shows a summary of the information from the ICEA Chart/Formula. The circuit shown in the figure below originates at a distribution panel with anavailable short-circuit current of 40,000 amperes RMS symmetrical. The 10 AWG THW copper conductor is protected by a Bussmann® LOW-PEAK® fuse sized per NEC® 240.4(D) (30A maximum for a 10 AWG conductor).Short-Circuit Protection of Wire and Cable
40,000 Amps RMS Sym. Available Distribution Panel LOW-PEAK® Dual-Element Fuse LPS-RK30SP Short-Circuit To Load 10 AWG THW Copper

Table Of ContentsShort-Circuit Current Withstand Chart for Copper Cables with Thermoplastic Insulation Allowable Short-Circuit Currents for Insulated Copper Conductors*
100,000 80,000 60,000 40,000

SHORT CIRCUIT...