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PPKSE

Electrical Installation Designs. CABLE SIZE CALCULATIONS. The size of a cable to be used for an installation depends upon:. the current rating of the cable under defined installation conditions and the maximum permitted drop in voltage as defined by Regulation 525-01(pg 100). PPKSE.

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PPKSE

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  1. Electrical Installation Designs CABLE SIZE CALCULATIONS The size of a cable to be used for an installation depends upon: • the current rating of the cable under defined installation conditions and • the maximum permitted drop in voltage as defined by Regulation 525-01(pg 100). PPKSE

  2. Electrical Installation Designs CABLE SIZE CALCULATIONS Some symbols and definitions will be used in cable size calculations as: Iz is the current carrying capacity of the cable in the situation where it is installed. It is the tabulated current for a single circuit at an ambient temperature of 30°C. Ib is the design current, the actual current to be carried by the cable. In is the rating of the protecting fuse or circuit breaker. PPKSE

  3. Electrical Installation Designs CABLE SIZE CALCULATIONS I2 is the operating current for the fuse or circuit breaker (the current at which the fuse blows or the circuit breaker opens). Ca is the correction factor for ambient temperature. Cg is the correction factor for grouping. Ci is the correction factor for thermal insulation. Cr is the 0.725 correction factor to be applied when the semi enclosed fuses protect the circuit. PPKSE

  4. Electrical Installation Designs Current Cable Rating The factors which influence the current rating are: • the design current, Ib , the actual current to be carried by the cable. The cable current carrying capacity, Iz must be equal to or greater than the circuit design current, Ib; • the type of cable either PVC, PVC/PVC, MICC, copper conductor or aluminium conductor (refer Table 5.2) PPKSE

  5. Electrical Installation Designs Current Cable Rating c) the installed conditions either clipped to surface or installed with other cables in a trunking. The correction factor may be need applying for installation condition is the grouping correction factor, Cg given in Tables 4B1, 4B2 and 4B3 (pg 217&218), of the IEE regulations and the correction factor to be used when cables are enclosed in thermal insulation, Ci. The Cg is not applied applied if the cable is to be clipped direct to a surface or not in contact with other cables. PPKSE

  6. Electrical Installation Designs Current Cable Rating • Cont…c) • Regulation 523-04 gives us three possible correction values: • where one side of the cable is in contact with thermal insulation, we must read the current rating from the column in the table which relates to Reference Method 4. • where the cable is totally surrounded over a length greater than 0.5m, we must apply a factor of 0.5. • where the cable is totally surrounded over a short length, the appropriate factor is given in Table 52A of the IEE Regulations should be applied. PPKSE

  7. Electrical Installation Designs Current Cable Rating • the ambient temperature, the cable resistance increases as temperature increases and insulation may melt if the temperature is too high. The correction factor may be need applying for this condition is the ambient or surrounding temperature correction factor, Ca, which is given Tables 4C1 and 4C2 of Appendix 4 of the IEE Regulations. PPKSE

  8. Electrical Installation Designs Current Cable Rating • the type of protection – for how long will the cable have to carry a fault current? Choose the type and rating of protective device (fuse or circuit breaker) to be used, In. • To ensure correct protection from overload, it is important that the protective device operating current or the operating current for the fuse or circuit breaker (the current at which the fuse blows or the circuit breaker opens), I2 is not bigger than 1.45 times the current carrying capacity of the cable, Iz. PPKSE

  9. Electrical Installation Designs Current Cable Rating It is important to appreciate that the operating current of a protective device is always larger than its rated value. In the case of a back-up fuse, which is not intended to provide overload protection, neither of these requirements applies. PPKSE

  10. Electrical Installation Designs Current Cable Rating When the semi enclosed fuses is used to protect the circuit, the 0.725 correction factor may be need applying for this condition. PPKSE

  11. Electrical Installation Designs Current Cable Rating • Here then are the eight basic steps in a simplified form: • Determine the design current Ib. • Select the rating of the protection, In. • Select the relevant rating factors (CFs). • Divide In by the relevant (CFs) to give tabulated cable current carrying capacity, It. • Choose a cable size to suit It. • Check the voltage drop. • Check for shock constraints. • Check for thermal constraints PPKSE

  12. Electrical Installation Designs VOLTAGE DROP CALCULATION PPKSE

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