Choosing the correct DC cable size is an important part of solar PV system design. A properly sized cable must safely carry the required current while keeping voltage drop and power losses within the project limits.
In a solar PV system, DC cable sizing is not based only on current and cable length. The design can also depend on PV module characteristics, string configuration, inverter voltage limits, cable ampacity, temperature, grouping, installation conditions, voltage drop and power loss.
A detailed solar PV DC cable calculation therefore helps determine whether a selected cable is suitable for the actual installation conditions.
This guide explains the key parameters and calculation steps used for solar PV DC cable sizing, including string sizing, inverter compatibility, cable ampacity, derating factors, voltage drop, temperature-corrected resistance and power loss.
What Is DC Cable Sizing?
DC cable sizing is the process of selecting a cable cross-sectional area that can safely carry the required DC current while satisfying voltage-drop and power-loss requirements.
For a solar PV system, the cable selection can be influenced by:
- PV module current
- PV module voltage
- Number of modules in series
- String voltage
- Inverter maximum DC voltage
- Inverter minimum operating voltage
- Cable current-carrying capacity
- Ambient or ground temperature
- Soil thermal resistivity
- Cable grouping
- Depth of laying
- Cable resistance
- Cable length
- Allowable voltage drop
A proper solar PV DC cable sizing calculation should evaluate these parameters together instead of selecting a cable only from the load current.
Solar DC Cable Sizing Parameters
Before calculating the cable size, the main PV system parameters should be established.
1. Maximum and Minimum Site Temperature
Temperature is an important input in solar PV cable sizing.
The design considers:
- Maximum site temperature (Tmax)
- Minimum site temperature (Tmin)
- Standard test temperature (Tstc)
Temperature affects PV module voltage and conductor resistance. Therefore, temperature correction should be considered when calculating string voltage and cable voltage drop.
For the example design:
- Maximum site temperature = 50°C
- Minimum site temperature = 10°C
- Standard temperature = 25°C
2. Allowable Voltage Drop
The allowable voltage drop is another important design parameter.
In the example design:
The calculated voltage drop should remain within this design limit.
3. PV Module Electrical Parameters
The following PV module parameters are important for solar cable size calculation:
- Module rated power
- Open-circuit voltage (Voc)
- Maximum power voltage (Vmpp)
- Short-circuit current (Isc)
- Maximum power current (Imp)
- Temperature coefficient of Isc
- Temperature coefficient of Voc
- Temperature coefficient of power
The example design uses a 550 Wp PV module with:
- Voc = 49.9 V
- Vmpp = 41.93 V
- Isc = 13.98 A
- Imp = 13.12 A
- Temperature coefficient of Isc = 0.045%/°C
- Temperature coefficient of Voc = -0.27%/°C
- Temperature coefficient of power = -0.35%/°C
These parameters are used for string sizing and DC cable design.
Solar PV String Sizing
String sizing determines how many PV modules can be connected in series while remaining within the inverter's operating voltage range.
The main checks are:
- Maximum number of modules in series
- Minimum number of modules in series
- String operating voltage
- Maximum string open-circuit voltage
- Inverter maximum DC voltage
- Inverter minimum operating voltage
For example, if 28 modules are connected in series and each module has a maximum power voltage of 41.93 V:
String Vmpp ≈ 1174.04 V
This string operating voltage must be compatible with the inverter's MPPT operating range.
Maximum Number of Modules in Series
The maximum number of modules in a string is determined by the inverter's maximum DC input voltage and the temperature-corrected module open-circuit voltage.
The temperature-corrected module Voc is calculated using the module voltage temperature coefficient and the minimum site temperature.
The corrected maximum module voltage is then used to determine the maximum allowable number of modules in series.
For the example:
- Inverter maximum DC voltage = 1500 V
- Corrected module Voc ≈ 51.92 V
- Maximum modules in series ≈ 29
Therefore, a 28-module string remains within the calculated maximum voltage limit.
Minimum Number of Modules in Series
The minimum string voltage must also satisfy the inverter's minimum DC operating voltage.
The temperature-corrected Vmpp is used to determine the minimum number of modules required.
For the example design:
- Minimum modules in series ≈ 15
- Maximum modules in series ≈ 29
- Selected modules in series = 28
Therefore, the selected 28-module string is within the calculated operating range.
DC Cable Current Calculation
For array-to-inverter cable sizing, the PV module maximum power current (Imp) can be used as the normal operating current basis.
In the example:
Imp = 13.12 A
The example also considers a 10% bifacial gain.
Therefore:
Design current = 14.432 A
This design current is then compared with the derated current-carrying capacity of the selected cable.
The bifacial gain should be treated as a design input rather than a permanent fixed value.
DC Cable Ampacity and Derating
Cable ampacity is the current that a cable can safely carry under specified installation conditions.
However, the manufacturer's base current-carrying capacity may need to be reduced using applicable correction or derating factors.
The example design considers:
- Temperature derating factor
- Soil thermal resistivity factor
- Group derating factor
- Depth-of-laying factor
The example uses a 4 mm² copper cable with a base current-carrying capacity of 44 A.
The total derating factor is calculated from the applicable correction factors.
For the example:
Ktotal ≈ 0.569772
The derated cable ampacity is:
Therefore:
44 × 0.569772 ≈ 25.07 A
The design current is:
14.432 A
Since:
25.07 A > 14.432 A
the stated cable passes the ampacity check under the given design assumptions.
Temperature Derating Factor
Cable current-carrying capacity can change with temperature.
The example design uses a temperature derating factor of:
k1 = 0.85
The applicable factor should be selected according to the actual installation temperature and the relevant design method.
A single fixed temperature factor should not be applied to every project.
Soil Thermal Resistivity
For underground or buried solar cables, soil thermal resistivity affects heat dissipation from the cable.
The example design uses:
and:
k2 = 1.14
The actual soil thermal resistivity should be based on the project design conditions or soil test information where applicable.
Cable Grouping and Derating
When several cables are installed together, heat dissipation can be reduced. This can lower the permissible current-carrying capacity of each cable.
The example design considers:
- 12 cables in a single pipe
- Group derating factor k3 = 0.6
The grouping factor depends on the actual cable arrangement and installation conditions.
It is important to distinguish between:
- Physical cable count
- Electrical circuit count
- Parallel conductors
- Grouped circuits
These should not automatically be treated as the same thing.
Depth of Cable Laying
For buried cables, the depth of laying can also influence the applicable correction factor.
The example design uses:
Depth of laying = 1 m
and:
k4 = 0.98
The applicable factor should be selected according to the actual installation arrangement and design reference.
Total Cable Derating Factor
The total derating factor combines the applicable correction factors.
The example calculation uses:
For the example:
Ktotal ≈ 0.569772
This factor is then applied to the manufacturer's base cable ampacity.
This provides the effective current-carrying capacity of the cable under the selected installation conditions.
DC Cable Voltage Drop Calculation
Voltage drop is one of the most important checks in solar DC cable sizing.
For a two-conductor DC circuit, the voltage-drop calculation considers both the positive and negative conductor paths.
The calculation can be represented as:
Where:
- Vd = voltage drop in volts
- I = design current in amperes
- R = cable resistance in ohm/km
- L = one-way cable length in metres
The factor 2 accounts for the positive and negative cable paths.
For example, if the one-way cable length is 100 m, the voltage-drop calculation considers the complete DC loop.
Temperature-Corrected Cable Resistance
Conductor resistance increases as conductor temperature increases.
The example design calculates resistance at the actual conductor temperature using:
Where:
- R = resistance at actual temperature
- Rref = resistance at reference temperature
- α = temperature coefficient of resistance
- T0 = actual conductor temperature
For the example:
- Rref = 5.09 ohm/km
- α = 0.00393
- Conductor temperature = 45°C
The calculated resistance is approximately:
R ≈ 6.090185 ohm/km
Using temperature-corrected resistance provides a more realistic voltage-drop calculation than using only the 20°C resistance value.
Solar DC Cable Voltage Drop Percentage
After calculating the voltage drop in volts, the voltage-drop percentage can be calculated against the total string operating voltage.
For the example:
- Voltage drop ≈ 17.58 V
- String Vmpp ≈ 1173.2 V
- Voltage drop ≈ 1.50%
Therefore:
1.50% < 2%
The example passes the voltage-drop criterion.
DC Cable Power Loss
Cable power loss is another important consideration in solar PV cable sizing.
Cable power loss can be calculated using:
Where:
- P_loss = cable power loss
- I = design current
- R = cable resistance at actual temperature
- L = one-way cable length
The power-loss percentage can then be calculated against the string power.
The example design gives an average power loss of approximately 1.66%.
Reducing unnecessary cable losses can improve the overall energy performance of a solar PV system.
Maximum String Power
The maximum string power can be estimated from:
Where:
- Vmpptotal = total string maximum power voltage
- I0 = design current
For the example, the calculated string power is approximately 15.26 kW.
This value can then be used for evaluating the cable power-loss percentage.
How to Select the Final Solar DC Cable Size
A proper DC cable size calculator should not select a cable using voltage drop alone.
The selected cable should be checked against the applicable design criteria, including:
1. Design current
2. Cable ampacity
3. Temperature derating
4. Soil thermal conditions
5. Grouping derating
6. Installation depth
7. Cable resistance
8. Voltage drop
9. Power loss
The final cable should satisfy the required checks under the selected design assumptions.
For the example design, a 4 mm² copper cable satisfies the stated voltage-drop and power-loss criteria.
Solar DC Cable Sizing Example
The following example summarizes the main parameters used in the design calculation:
| Parameter | Value |
|---|---|
| Module power | 550 Wp |
| Voc | 49.9 V |
| Vmpp | 41.93 V |
| Isc | 13.98 A |
| Imp | 13.12 A |
| Modules in series | 28 |
| String Vmpp | ≈1174 V |
| Bifacial gain | 10% |
| Design current | 14.432 A |
| Cable size | 4 mm² Copper |
| Base ampacity | 44 A |
| Total derating factor | 0.569772 |
| Derated ampacity | ≈25.07 A |
| Cable length | 100 m |
| Voltage drop | ≈17.58 V |
| Voltage drop percentage | ≈1.50% |
| Power loss | ≈1.66% |
These values represent the example calculation and should not be treated as universal values for every solar PV project.
Why Use a DC Cable Size Calculator?
Manual solar cable calculations involve several parameters and formulas. A DC cable size calculator can combine these calculations and make it easier to evaluate different cable sizes.
A useful solar PV cable calculator should consider:
- PV module electrical characteristics
- String configuration
- Inverter voltage limits
- Design current
- Cable ampacity
- Temperature derating
- Grouping derating
- Soil thermal conditions
- Cable resistance
- Cable length
- Voltage drop
- Power loss
This makes the calculation easier to review during solar PV design and EPC engineering.
Frequently Asked Questions
What is a DC cable size calculator?
A DC cable size calculator is a tool used to estimate the required cable cross-sectional area based on electrical current, voltage, cable length, voltage drop, ampacity and installation conditions.
How is solar DC cable size calculated?
Solar DC cable sizing involves determining the design current, checking the cable's derated ampacity, calculating cable resistance at the applicable temperature, checking voltage drop and evaluating power loss.
What is the allowable voltage drop for a solar DC cable?
The allowable voltage drop depends on the project design criteria. In the example design, the allowable voltage drop is 2%.
Why is temperature important in DC cable sizing?
Temperature affects both the current-carrying capacity and electrical resistance of a conductor. Therefore, temperature derating and temperature-corrected resistance can be important parts of solar DC cable sizing.
Why is cable grouping important?
When multiple cables are installed together, heat dissipation can be reduced. This can reduce the effective current-carrying capacity of the cables, so an appropriate grouping factor may need to be applied.
What is the difference between cable voltage drop and cable power loss?
Voltage drop represents the reduction in voltage along the cable, while power loss represents electrical energy dissipated due to cable resistance. Both are useful checks during solar PV cable selection.
Can a cable pass voltage drop but fail ampacity?
Yes. A cable can have sufficiently low resistance for voltage-drop requirements while still having insufficient current-carrying capacity under the actual installation conditions. Therefore, both ampacity and voltage drop should be checked.
Can a cable pass ampacity but fail voltage drop?
Yes. A cable may safely carry the required current but still have excessive voltage drop if the cable is very long or has high resistance.
Conclusion
Correct solar DC cable sizing requires more than simply selecting a cable based on current. A proper calculation should consider PV module characteristics, string voltage, inverter limits, design current, cable ampacity, temperature derating, grouping, installation conditions, temperature-corrected resistance, voltage drop and power loss.
A detailed DC cable sizing calculation helps engineers evaluate whether the selected cable is suitable for the intended solar PV installation.
Use the OneCalcApp DC Cable Size Calculator to perform solar PV DC cable calculations using the relevant module, inverter, cable and installation parameters.
For preliminary design, always verify the final cable selection against applicable standards, manufacturer cable data, actual installation conditions and project-specific engineering requirements.