FAQ: How are cable sizes selected

Cable size selection is based on three main factors:

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The current carrying rating is determined by the conductor size and the thermal heating of the cable. The cable spacing, application and insulation materials are relevant to the dissipation of this heat.

Voltage regulation is not usually a problem with well-designed electrical power systems but the voltage drop incurred with excessively long cable runs needs to be accounted for.

Guidance on cable size selection for various temperature ratings and wiring methods, along with guidance on voltage drop calculations, are contained in the National Wiring regulations (for the UK, that's BS 18th Edition).

CABLE SIZING CALCULATION CRITERIA

The minimum cross section of LV voltage cables shall be calculated based on below methods:

1. Short circuit current capacity
2. Current carrying capacity
3. Voltage drop of cable

1. Short circuit current capacity

This criteria defines the minimum cross section required for the cable.

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A = Cable cross-section area in Sq. mm = Short circuit current K = Constant, 143 for XLPE Cu. Cable t = Duration of short-circuit in seconds

The fault current at 415V is 40kA and the duration t is considered to be 0.1 based on relay delay time and breaker opening time.

The next nearest available cable size will be 95 Sq.mm

2. Current carrying capacity

Derating Factors considered for multicore cables laid in duct Installation method : Direct buried Ground temperature : 30°C Soil Thermal Resistivity : 2.5 K.m/W Depth of Laying : 75 cm Cable Grouping: No. of cables in group : 1 Spacing between duct centres : 0 mm Correction Factor Derating factor for variation in ground temperature at 30°C k1 : 0.93 (As per IEC -5-52,

Table B.52.15 )

Derating factor for depth of laying of cable k2 : 1 (As per cable catalogue) Derating factor for soil and thermal resistivity k3 : 1 (As per IEC -5-52,

Table B.52.16 )

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Derating factor for cable grouping factor k4 : 1 (As per IEC -5-52, Table B.52.17 ) Overall correction factor for cables installed in Underground duct (k1 x k2 x k3 x k4) : 0.93