Sizing of power cables for circuit breaker controlled feeders (part 3)

3. Criteria Starting and running voltage drops in cable

This criterion is applied so that the cross sectional area of the cable is sufficient to keep the voltage drop (due to impedance of cable conductor) within the specified limit so that the equipment which is being supplied power through that cable gets at least the minimum required voltage at its power supply input terminal during starting and running condition both.

Cables shall be sized so that the maximum voltage drop between the supply source and the load when carrying the design current does not exceed that which will ensure safe and efficient operation of the associated equipment. It is a requirement that the voltage at the equipment is greater than the lowest operating voltage specified for the equipment in the relevant equipment standard.
So before starting with calculation for voltage drop let us first analyze that what is the permissible voltage drop as per relevant standards and guidelines and what is the possible logic behind selecting these values as the permissible values.

Indian standard 1255- CODE OF PRACTICE FOR INSTALLATION AND MAINTENANCE OF POWER CABLES UP TO AND INCLUDING 33 kV RATING in its clause 4.2.3.4 mentions the permissible value for different cross sectional sizes of Aluminium conductor in volts/kM/Ampere for cables from voltage grade of 1.1kV till 33kV. Since we calculate voltage drop in terms of percentage of source voltage, this clause is not very widely used in basic as well as detailed engineering fraternity.
Its complex unit requires to be multiplied by cable length and ampacity. However one can definitely check for any cable size and length, what value is obtained in terms of percentage?

IEEE standard 525 – Guide for the Design and Installation of Cable Systems in Substations in its annexure C, clause number C3 mentions that Voltage drop is commonly expressed as a percentage of the source voltage. An acceptable voltage drop is determined based on an overall knowledge of the system. Typical limits are 3% from source to load center, 3% from load center to load, and 5% total from source to load. These values are indicated diagrammatically below.

6.6kV substation layout

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