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Cables Buried and/or Ducted Current Carrying Capacities - The rating differences between BS7671 and the ERA in ElectricalOM

The Requirements for Electrical Installations BS 7671 in Appendix 4 publishes ratings taken from the harmonised document, HD 60364-5-52. Within ElectricalOM, we also publish ratings from the ERA 69-30 series reports.

IEC60287 outlines the calculation methods for deriving the current ratings for cables. Both the Requirements for Electrical Installations BS 7671 and the ERA 69-30 series reports are based on this standard.

It is often noted by our clients that the calculated ERA 69-30 current ratings are substantially higher than the information provided in The Requirements for Electrical Installations BS 7671.

There are two distinct reasons for this.

The first difference between the two is the conditions or correction factors used in the calculations. The following table provides information on the key differences:



Correction Factor BS 7671/HD 60364-5-52 ERA 69-30
Ca Ground temperature (°C) 20°C 15°C
Cd Installation Depth (m) 0.7m 0.5m
Cs Soil thermal Resistivity (K.m/W) 2.5 K.m/W 1.2 K.m/W

Ca - Ground temperature (°C) – The lower ground temperature will allow the current rating of the cable to be increased.
Cd - Installation Depth (m) – The shallower depth also improves the current rating.
Cs - Soil thermal Resistivity (K.m/W) –  a value of  1.2 K.m/W is where the soil is considerably moister. This is considered a more typical value for many parts of the UK.

The difference in these three key values leads to a significant derating of the current rating of buried cables.
 
Modecsoft recommends, however, that in all designs, the individual circumstances of the installation are accounted for. Correction factors applied should reflect the location-specific ground temperature, the depth of buried systems, ensuring the backfill is suitably compacted, and, if soil conditions are unknown, undertake specific soil analysis for Soil Resistivity.



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