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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, data sets 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 to 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 locations specific; ground temperature, the depth of buried systems, ensuring the back fill is suitably compacted, and if soil conditions are unknown, undertake specific soil analysis for Soil Resistivity.

Secondly, the ERA 69-30 series reports separately calculate two installation or reference methods
•    Cables buried directly in the ground and;
•    Cables in ducts buried in the ground

The Requirements for Electrical Installations BS 7671/HD 60364-5-52 makes no distinction between the two.

The ERA 69-30 series reports recognise that cables buried directly in the ground have a higher current carrying capacity than that of Cables in ducts buried in the ground.


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