Selection and Sizing of DC String Cables for Solar PV Rooftop Projects

Sizing of solar DC string cables – Current Carrying Capacity (CCC)

Required Current Carrying Capacity (CCC) of the cable can be calculated as below:

• If Overcurrent Protection Device is provided: Minimum Current Carrying Capacity (CCC) of cable should be equal to current rating of Overcurrent Protection Device
• If Overcurrent Protection Device is not provided:
  • Single String Array – CCC = 1.25 x Isc
  • Multi String Array – CCC = 1.25 x Isc x (N-1)

Where,

Isc = Short Circuit Current of Solar PV Modules at STC (To be determined from Datasheet)

N = Number of strings connected in parallel (To be determined from design considerations)

Sizing of solar DC string cables – Voltage Drop

Voltage Drop (%) = ( I x R x 100 ) / ( Vmp, array )

Where,

I (A) = Maximum Power Point Current (Imp) of Solar PV module at STC (To be determined from Datasheet)

R (ohms, at 90 degree celsius) = Resistance for Total Cable Circuit (To be calculated)

Vmp, array (V) = Vmp of Module x Number of Modules in a String (To be calculated)

Most of the DC cable manufacturers provide DC resistance values in “Ohm/km” and at 20 degree celsius. But for solar applications, these DC cables experience very high temperatures. So, it is recommended to correct the resistance value as given by the manufacturer to a higher temperature resistance (90 degree celsius).

The below-mentioned formula can be used for the same:

R(ohms/km, at Final Temp) = R(ohms/km, at Initial Temp) x [ 1 + Temp Coefficient of Resistance x (Final Temp – Initial Temp) ]

The temperature coefficient of resistance for copper is “0.0040“

Also, the Resistance (R) to be used in the Voltage drop formula is in “ohms” units. While the above-corrected Resistance is in “ohms/km” units. So, the corrected Resistance value needs to be multiplied with Total Cable Length.

To calculate Total Cable Length, the below points need to be considered:

• If route length is provided by the site team (such as distance between module and inverter), multiply the distance with “2”. Since we need to calculate the circuit cable length (Positive & Negative cables)
• If individual cable lengths of Positive and Negative cabes are provided. Do not multiply by “2”. Just add both the lenghts.
• Legth of cable leads as attached with Solar PV Modules also need to be considered. Multiply the number of modules in a string with cabe lead legth for one module.