Introduction:
Selecting the correct DC cable is one of the most important tasks in solar PV system design. An undersized cable increases voltage drop, power loss, and overheating, while an oversized cable unnecessarily increases project cost.In this article, we'll learn how to calculate solar DC cable size step by step . The cable selection is based on three engineering criteria: Short Circuit Withstand Capacity Current Carrying Capacity (Ampacity) Voltage DropAt the end of this tutorial, you'll understand how EPC companies select the final DC cable size.
PROJECT DATA
ParameterValue
Plant Capacity18 MWModule Rating590 WModule
Vmp-42.89 V
Module Imp1-4.45 A
Module Isc-15.21 A
Modules per String-26
Maximum Strings per SMB-21
Cable TypeCu XLPO Solar CableSystem Voltage-1500 V DC
Ste p 1– Calculate Full Load CurrentThe current flowing from the strings into the SMB is:
Full Load Current = Number of Strings × Module Imp
Using our project:
Full Load Current = 2 × 14.45 = 28.896 A
Therefore,Design Current = 28.896 AThis current will be used in the remaining calculations.
Step 2 – Short Circuit Withstand CalculationSuppose a fault occurs
.The cable should not melt before the protection device disconnects the fault.
According to IEC standards, the required conductor area is calculated using:
A = (Isc × √t) / K
Where: Isc = 30.43 A(15.21X2),Two strings are connected in parallel using a Y-Connector
Fault Duration = 1 second
Copper Constant K = 143
Substituting the values:
A = (30.43 × 1) / 143A = 0.2128 mm²
Although the minimum theoretical requirement is only 0.2128 mm², this does not mean a 1 mm² cable can be used.The cable must still satisfy current carrying capacity and voltage drop requirements.
Step 3 – Current Carrying Capacity
The cable should continuously carry load current without exceeding its permissible conductor temperature.
However, cable ratings provided by manufacturers are valid only under standard conditions.
Actual site conditions require derating.
For this project, the following derating factors were considered:
Ground Temperature = 35°C
Cable Grouping = 12
cables Soil Thermal Resistivity = 1.5 K·m/W
Buried InstallationThe combined derating factor becomes:0.534
Therefore,Required Current Capacity= 28.9 ÷ 0.534= 54.11 A
The selected 6 mm² Cu XLPO Solar Cable has a current carrying capacity of 70 A, which is higher than the required 54.11 A.Hence, it satisfies the ampacity requirement.
Step 4 – Voltage Drop Calculation
Next, verify that voltage loss remains within acceptable limits.
Project Data:
Current = 28.896 A
Cable Length = 65 m
Cable Resistance = 3.39 Ω/km
Operating Temperature = 50°C
After temperature correction, the resistance becomes:4.056 Ω/km
The calculated voltage drop is:7.62 VPercentage voltage drop:0.68%Since this is below the commonly accepted design limit, the selected cable is acceptable.
Step 5 – Power Loss
Every cable has resistance, so some energy is lost as heat.
Power Loss:0.2201 kW
Percentage Power Loss:0.07%
This confirms that the selected cable provides excellent efficiency with negligible energy loss.
Conclusion:
DC cable sizing is not just a mathematical calculation—it is a critical engineering process that directly impacts the safety, efficiency, and long-term performance of a solar PV system. By considering short-circuit withstand capacity, current carrying capacity, voltage drop, and power loss together, engineers can confidently select the most suitable cable for any solar installation.If you're a solar design engineer or EPC professional, mastering DC cable sizing will help you design safer, more efficient, and standards-compliant solar projects.