Power Loss in Cables
Voltage Drop in Cables
The ETCI Rules recommend that the voltage drop in a circuit should not exceed 4% of the nominal voltage in installations and parts of larger installations, rated not greater than 80 Amps.
This is to ensure that loads are supplied with the correct operating voltage after cable resistance etc. is taken into account.
For a 230 Volt single phase supply, the volt drop should not exceed 4% of 230 Volts which is equal to 9.2 Volts.
Also for a 400 Volt three phase supply, the volt drop should not exceed 4% of 400 Volts which is equal to 16 Volts.
Calculation of volt drop in cables is necessary in order to decide on the correct size of cable for a given load.
To calculate the correct size of cable the following factors must be considered:-
· The Load current
· The Length of the run in metres
· The Installation Method
· The Type of cable
· The Resistivity of the conductor
· The Cross Sectional Area of the cable ( CSA )
· The Current Carrying Capacity of the cable ( CCC )
· The Temperature of the area ( highest temperature )
· The Grouping Factor ( where cables are grouped together )
· The Volt Drop across the cable
Tables which take these factors into consideration can be found in the ETCI Rules.
Heat Generated in Cables
Current flowing through a cable causes heat to be produced within the cable. This heat will have a damaging effect on the cable insulation. Therefore it must be limited to a value, which the insulation can withstand. Also heat dissipated in a cable is power wasted or lost. This power must be paid for. Correct cable size reduces this power loss to an acceptable level. The greater the power loss the greater the amount of heat generated.
Power Loss equals Current squared multiplied by Cable Resistance Assume that a current of 40 Amps is flowing through a cable of resistance 0.125 Ohms.
P = I2 x R
P = 402 x 0.125
P = 200 Watts.