DC and AC cables are an important part of Solar PV rooftop system, used in solar power plant for generating solar energy for home/industrial/commercial roof top or ground mounted. The 25 years life of Solar PV rooftop system can be expected only when the cables used in solar power plant for generating solar energy for home/industrial/commercial roof top or ground mounted are of appropriate size and quality. In this blog we will be talking on DC and AC cables size and quality.
To reduce the installation cost and increase their profit margin, many of the installers use very bad quality of cables. Even some of them do not size the cables correctly. The money saved by wrong sizing of cables and using low quality cables is not much, but the effect to doing this to the Solar PV system is adverse.
If bad quality cables are used in solar power plant for generating solar energy for home/industrial/commercial roof top or ground mounted, then there may be a possibility of breakage of outer insulation cover. This will reduce the system performance and make system inefficient.
Cable sizing should also take into account the ambient temperature on the roof. And the cable copper conductor size should be adjusted accordingly.
There is a drop of voltage in DC and AC cables. As per the standards, 3% of Voltage drop at DC side and 2% of Voltage drop at AC side is permissible, and the cable sizing should be done accordingly.
As the length of cable increases, the voltage drop also increases, to compensate this, the cross sectional area (in Sq. mm) should be increased. But with the increase in cross sectional area of cable the cost also increases. So when the point of cost optimisation or profit making, cross sectional area is kept minimum and voltage drop is not seriously considered.
For example, if for a particular case, for 25 meters long cable, considering 2% DC voltage drop, cross sectional area comes to be around 4 sq.mm. In other case if the length is increased to 40 meters, then there is a possibility that with 4 sq.mm cable the voltage drop is more than 2%, in this case one needs to increase the cross sectional area to may be 6 sq. mm. So if such a situation comes, one need to increase the cross sectional area, and properly size the cable. If not, the output of the system will not be optimum. The same goes for AC cables.
Also last but not the least , cables are packed in a conduit and at times multiple cables are packed which also needs to be taken into consideration while determining the conductor size. This is called conductor fill adjustment for sizing properly .As an example the following table is used to adjust conductor size to take into consideration the cable fill factor.