# Economic Optimisation of Cables

The economic optimisation of cables is the selection of cable sizing based on the overall cost of a cable installation, including consideration of the power losses throughout the life of the cable. Typically an economic optimisation exercise suggests a cable which is larger than the minimum size required to meet sustained current capacity requirements.

Where economic optimisation of the cable results in a larger size, the supply and installation cost will be greater, but the cost of cable power losses during operation will be less. The overall life cycle cost of the cable is reduced. With lower operating losses, the overall energy efficiency of the electrical cable system will increase.

## IEC 60287-3-2

This section of the current rating standard gives a method for calculation of cable size based on the economic optimisation. The total cost of cable is given by:

$CT=CI+CJ$

*CT* - total cost of the installed length of cable, cu

*CJ* - present value of the Joule losses over the life of the cable, cu

The costs are expressed in arbitrary currency units, cu. The Joule losses consider the energy cost and the costs for the additional supply capacity to provide the losses.

Note: the standard goes into detail on the calculation CI and CJ. As we move to implementing the calculation in myCableEngineering, we will expand this section to illustrate the complete method.

## BS 7671 18th. Edition

The new edition (18th) of the BS 7671 Wiring Regulations are due to be published in July 2018. A key change is a new part, Section 8 on Energy Efficiency.

It is likely that part 8 will recommend increasing the cable size based on accessing savings within a time scale to any additional cost of increasing the cable size. Currently, the draft standard is also suggesting, the calculation method found in IEC 60287-3-2.

## myCableEngineering

The economic optimisation of cables has been on our to do list for a quite a while now. Additionally, we already have already implemented many of the algorithms necessary to carry out an IEC 60287-3-2 calculation. With the future inclusion of this type of calculation in BS 7671, there is now, even more, of an impetus to complete our implementation of the analysis. We do intend to have this up and running before the publication of the new 18th Edition.

## Comments

Add commentExample: 33KV incoming feeder has to be designed for 33KV, 31.5KA, 1sec rated Panel to feed 20MW, feeder length is about 2000 m to be laid in RCC Trenches.

2Runs 33KV, 300 Sqmm, AL Cond., XLPE Cable is recommended to cater 20MW, 350A & fault current of 31.5KA, 1 Sec.

1Run of 300 Sqmm, AL Cond., XLPE Cable can carry 310A in duct @ 30 deg C and about 28KA for 1 sec as per manf. catalog.

Considering De-rating Factors for temperature & grouping etc. actual capacity 237Amps/ Run

So 2 runs cater 474 Amps i.e. more than sufficient to cater required 350A(i.e. 20MW).

As 2 Runs are considered it is supposed to cater 31.5KA, 1sec considering current sharing in parallel runs.

Kindly give your advice on the above.