How does temperature affect DC cable resistance?

Conductor resistance increases as temperature increases. A common calculation is R_T = R₂₀ × [1 + α(T − 20)]. Therefore, a cable operating at a higher temperature can have greater voltage drop and power loss than the same cable at 20 °C.

What is DC cable derating?

DC cable derating means reducing the reference current-carrying capacity to account for actual installation conditions. Depending on the installation, temperature, cable grouping, soil thermal resistivity and burial depth can affect the allowable current.

Why does cable grouping reduce ampacity?

Grouped cables generate heat close to one another and have less ability to dissipate that heat. As cable grouping increases or spacing decreases, the allowable current-carrying capacity can decrease according to the applicable correction factor.

How do I calculate DC cable power loss?

DC cable power loss can be calculated using P_loss = I² × R_loop, where I is the current and R_loop is the complete positive-plus-negative cable loop resistance. Lower resistance reduces resistive cable losses.

Should I use copper or aluminium DC cable?

Copper has lower resistance for a given cross-sectional area and is widely used for PV strings and smaller DC circuits. Aluminium can be economical for larger DC feeders when the cable construction, termination system and installation method are suitable.

Does cable length affect solar PV cable sizing?

Yes. Longer PV cable routes increase resistance and voltage drop. A cable that is adequate for a short string run may require a larger cross-sectional area when the same current is carried over a much longer distance.

Can bifacial solar panels affect DC cable sizing?

They can when the project design explicitly includes additional rear-side generation in the design current. If the design current increases, the cable ampacity and protection checks should be reviewed using the updated design basis.

Can this DC cable calculator replace detailed electrical design?

No. It is a preliminary sizing and verification tool. Final DC cable selection should be checked against manufacturer data, applicable standards, installation conditions, protection coordination and the project's approved electrical design basis.

DC Cable Size Calculator

SolarInstant resultsFormula shown100% in your browser

Calculate the correct DC cable size for solar PV and DC electrical systems using current, voltage, cable length, conductor material, installation conditions, cable ampacity, voltage drop and power loss. Compare standard cable sizes and identify a suitable minimum cable size.

Inputs

Mode

Calculation mode

A — Basic DC circuit

Design limits are editable; nothing is forced to 1% or 2%.

DC operating voltage (V)

Nominal operating voltage of the circuit. In PV mode this is replaced by the calculated string Vmpp.

One-way cable length (m)

Route length from source to load. The engine adds the return conductor internally — do not double it yourself.

Allowable voltage drop (%)

Editable design limit — 1% is common for PV string cable, 2% for array/battery circuits.

Allowable power loss (%)

B — Design current basis

Every multiplier is displayed and applied only when you set it.

Design current basis

Design current (A)

Bifacial gain allowance (%)

Project assumption only. Default 0% — enter 10% only if your design basis says so.

Design safety factor (×)

Shown explicitly and never applied silently. IEC 62548 / NEC practice for PV string cable is 1.25.

D — Cable data

Conductor material

Cable construction

Parallel cables per polarity

Operating conductor temperature (°C)

Not the same as ambient temperature. Editable — PV cable in sunlight commonly runs 70–90 °C.

Resistance basis

Cable size

Preferred cable size

E — Installation & derating

Only the factors relevant to the selected installation method are applied.

Installation method

Ambient air temperature (°C)

Reference condition is 30 °C.

Cable arrangement

Number of circuits in the group

Additional project factor (×)

How to use this calculator: DC Cable Size Calculator

1Confirm that the DC Cable Size Calculator matches the quantity or design check you need.
2Enter DC operating voltage, One-way cable length, and Allowable voltage drop using the units printed beside each field.
3Select the applicable Calculation mode, Design current basis, and Conductor material options; these choices change the calculation method or factors.
4Calculate, then follow the substituted equations in the worked example and compare the result with any stated limit.
5Read the assumptions, warnings and cited references before using the result for a financial, medical or engineering decision.

Input guide and example values

Use values from the same measurement basis and time period. Conditional fields appear only when the related option is selected.

Input	Example value	Why it matters
Calculation mode	Generic DC cable mode	Select the option that matches the real installation or scenario.
DC operating voltage	600 V	Nominal operating voltage of the circuit. In PV mode this is replaced by the calculated string Vmpp.
One-way cable length	40 m	Route length from source to load. The engine adds the return conductor internally — do not double it yourself.
Allowable voltage drop	2 %	Editable design limit — 1% is common for PV string cable, 2% for array/battery circuits.
Allowable power loss	2 %	Measured or known allowable power loss used by the calculation engine.
Design current basis	User-entered design current	Select the option that matches the real installation or scenario.
Design current	12 A	Measured or known design current used by the calculation engine.
Bifacial gain allowance	0 %	Project assumption only. Default 0% — enter 10% only if your design basis says so.
Design safety factor	1 ×	Shown explicitly and never applied silently. IEC 62548 / NEC practice for PV string cable is 1.25.
Conductor material	Copper	Select the option that matches the real installation or scenario.
Cable construction	1C positive + 1C negative (single-core)	Select the option that matches the real installation or scenario.
Parallel cables per polarity	1	Measured or known parallel cables per polarity used by the calculation engine.
Operating conductor temperature	70 °C	Not the same as ambient temperature. Editable — PV cable in sunlight commonly runs 70–90 °C.
Resistance basis	Reference resistivity (Cu 0.01724 / Al 0.02826 Ω·mm²/m at 20 °C)	Select the option that matches the real installation or scenario.
Cable size	Let the calculator select the minimum acceptable size	Select the option that matches the real installation or scenario.
Preferred cable size	4 mm²	Select the option that matches the real installation or scenario.
Installation method	PV cable in free air (module frame, tray, messenger)	Select the option that matches the real installation or scenario.
Ambient air temperature	40 °C	Reference condition is 30 °C.
Cable arrangement	Bunched / touching	Select the option that matches the real installation or scenario.
Number of circuits in the group	1	Measured or known number of circuits in the group used by the calculation engine.
Additional project factor	1 ×	Optional input; leave the supplied default only when it matches your case.

Formula, derivation and worked example

DC cable sizing should be checked using more than one electrical criterion. The conductor must carry the required design current under the actual installation conditions, while the voltage drop and resistive power loss should remain within the project's design limits. This DC Cable Size Calculator follows that engineering workflow for solar PV strings, battery DC circuits and other two-wire DC applications. It considers design current, cable ampacity, temperature, grouping, soil and burial conditions where applicable, conductor resistance at operating temperature, voltage drop and I²R power loss. The calculator compares standard cable cross-sections and helps identify a suitable minimum cable size for the entered conditions.

I_design = I_base × (1 + bifacial allowance) × safety factor

I_z = I_table × k_temp × k_group × k_soil × k_depth × parallel-cable factor

R₂₀ = ρ₂₀ × 1000 / A

R_T = R₂₀ × [1 + α × (T − 20)]

R_loop = 2 × R_T × L / 1000 / n_parallel

Voltage Drop = I_design × R_loop

Voltage Drop % = (Voltage Drop / System Voltage) × 100

Power Loss = I_design² × R_loop

Power Loss % = [Power Loss / (V × I_design)] × 100

Substitution steps

1. Base current
User-entered value
= 12 A
2. Ambient air temperature factor
40 °C air vs the 30 °C reference (IEC 60364-5-52 Table B.52.14).
= × 0.91
3. Grouping factor
1 circuit(s), bunched / touching.
= × 1
4. Combined derating factor
0.91 × 1
= 0.91
5. Conductor resistance at 20 °C (2.5 mm²)
ρ₂₀ × 1000 / A = 0.01724 × 1000 / 2.5
= 6.896 Ω/km
6. Resistance at operating temperature
R₂₀ × [1 + α (T − 20)] = 6.896 × [1 + 0.00393 × (70 − 20)]
= 8.2511 Ω/km
7. Loop resistance (positive + negative)
2 × R_T × L / 1000 / n_par = 2 × 8.2511 × 40 / 1000 / 1
= 660.1 mΩ
8. Voltage drop
I × R_loop = 12 × 0.66009
= 7.92 V
9. Voltage drop percentage
ΔV / V × 100 = 7.92 / 600 × 100
= 1.32%
10. Power loss
I² × R_loop = 12² × 0.66009
= 95.1 W
11. Power loss percentage
P_loss / (V × I) × 100 = 95.1 / 7,200 × 100
= 1.32%

Computed example results

Recommended minimum cable size: 2.5 mm² Copper; 1C positive + 1C negative
Design current: 12 A; User-entered design current
Base ampacity: 41 A; Single-core PV cable in free air, on the module frame / tray
Combined derating factor: × 0.91
Derated ampacity: 37.3 A
Resistance at operating temperature: 8.2511 Ω/km; 70 °C conductor
Loop resistance: 660.1 mΩ; 40 m one-way = 80 m electrical path
Voltage drop: 7.92 V; 1.32% of 600 V
Receiving-end voltage: 592.1 V
Power loss: 95.1 W; 0.095 kW · 1.32%
Power transmitted: 7.2 kW
Cable section efficiency: 98.68%

Understanding the result

Read the main result together with supporting checks, assumptions, limits and intermediate values.

For a manual check, repeat the first equation, confirm the units and change one input at a time.

Common mistakes when using DC Cable Size Calculator

Do not mix units for DC operating voltage (V), One-way cable length (m), Allowable voltage drop (%). A unit mismatch changes the input magnitude even when the typed number looks reasonable.
Do not leave Calculation mode on the default choice unless that choice matches the real scenario; the selected option can change the calculation path or factor.
Do not replace the displayed I_design = I_base × (1 + bifacial allowance) × safety factor relationship with a different convention without also changing the underlying assumptions; compare like-for-like methods when checking the result.
Do not treat Recommended minimum cable size = 2.5 mm² Copper from the worked example as a universal answer. It belongs to the displayed example inputs and must be recalculated for the actual case.

When the DC Cable Size Calculator is useful

DC Cable Size Calculator is designed for cases where Calculation mode, DC operating voltage, One-way cable length, Allowable voltage drop are known and you need Recommended minimum cable size, Design current, Base ampacity. The page keeps the live calculator, calculation method and worked example together so the result can be checked instead of treated as a black-box number.

Use the calculator for the scope described by its inputs and notes. The displayed method is I_design = I_base × (1 + bifacial allowance) × safety factor. If the real project or decision needs factors that are not represented here, treat the result as an estimate and add the missing checks separately.

Calculation mode and DC operating voltage: what changes the answer

The worked example uses Calculation mode = Generic DC cable mode, DC operating voltage = 600 V, One-way cable length = 40 m, Allowable voltage drop = 2 %. With those values, Recommended minimum cable size is 2.5 mm² Copper. Changing an input should be interpreted according to that field's unit, range, option and hint rather than by the number alone.

For this calculator, the main input roles are: Calculation mode: Select the option that matches the real installation or scenario. Available choices include Generic DC cable mode, PV string / array cable mode. DC operating voltage (V): Nominal operating voltage of the circuit. In PV mode this is replaced by the calculated string Vmpp. One-way cable length (m): Route length from source to load. The engine adds the return conductor internally — do not double it yourself. Allowable voltage drop (%): Editable design limit — 1% is common for PV string cable, 2% for array/battery circuits.

How to sanity-check a DC Cable Size Calculator result

Start by confirming the entered values and units, then compare the substituted working with the displayed formula or calculation steps. Pay particular attention to Recommended minimum cable size, because it is the first worked-example output shown by the live engine.

Finally, compare the result with the assumptions, warnings and related calculators on this page. A nearby calculator can be useful as a cross-check when it measures the same workflow from a different input or output direction.

Next logical calculator

Continue with DC Cable Size Calculator (12V / 24V / 48V)

Useful next check because both tools use Allowable voltage drop and Conductor material, while DC Cable Size Calculator (12V / 24V / 48V) answers a different part of the same workflow.

Open DC Cable Size Calculator (12V / 24V / 48V)

Standards, source trail and limitations

References show the method used. Check the current local edition, amendments and project specification before a regulated decision.

Formula

I_design = I_base × (1 + bifacial allowance) × safety factor
I_z = I_table × k_temp × k_group × k_soil × k_depth × parallel-cable factor
R₂₀ = ρ₂₀ × 1000 / A
R_T = R₂₀ × [1 + α × (T − 20)]
R_loop = 2 × R_T × L / 1000 / n_parallel
Voltage Drop = I_design × R_loop
Voltage Drop % = (Voltage Drop / System Voltage) × 100
Power Loss = I_design² × R_loop
Power Loss % = [Power Loss / (V × I_design)] × 100

Engineering notes

The calculator is intended for preliminary DC cable sizing and verification.
Use manufacturer-specific cable resistance and ampacity data for final engineering design.
The entered cable length is one-way route length; the return conductor is included automatically in the DC loop calculation.
Cable selection should consider current-carrying capacity, voltage drop, power loss, temperature, grouping, installation method and protection requirements.
For solar PV applications, also verify cable voltage rating, insulation type, UV resistance, connector compatibility and applicable PV installation requirements.

Formulas explained

I_z = I_table × k1 × k2 × k3 × …

Cable ampacity tables are based on reference installation conditions. Actual installation conditions can reduce the allowable current-carrying capacity. Temperature, grouping, soil thermal resistivity and burial depth are considered where applicable.

I_table: Base current-carrying capacity of the selected cable and installation method, A
k_temp: Temperature correction factor
k_group: Grouping or number-of-circuits correction factor
k_soil: Soil thermal resistivity correction factor for buried cables
k_depth: Depth-of-laying correction factor for buried cables

Only correction factors applicable to the selected installation method should be applied.

R_T = R₂₀ × [1 + α × (T − 20)]

Conductor resistance increases as the conductor temperature increases. Using the operating-temperature resistance gives a more realistic voltage-drop and power-loss calculation.

R₂₀: Conductor DC resistance at 20 °C, Ω/km
α: Temperature coefficient of resistance; approximately 0.00393/K for copper and 0.00403/K for aluminium
T: Operating conductor temperature, °C

R_loop = 2 × R_T × L

A normal two-wire DC circuit has a positive conductor and a negative return conductor. Because the entered cable length is the one-way route length, the electrical loop includes both conductors.

R_loop: Complete positive-plus-negative DC circuit resistance, Ω
R_T: Conductor resistance at operating temperature
L: One-way cable route length

Parallel cables per polarity reduce the effective loop resistance.

ΔV = I × R_loop

DC voltage drop increases with current and cable resistance. Longer cable runs and smaller conductor cross-sectional areas generally produce greater voltage drop.

ΔV: DC cable voltage drop, V
I: Operating or design current, A
R_loop: Complete DC loop resistance, Ω

ΔV% = (ΔV / V) × 100

Voltage-drop percentage expresses the voltage lost in the cable relative to the DC system operating voltage.

ΔV: Calculated voltage drop, V
V: Operating DC system voltage, V

P_loss = I² × R_loop

Cable power loss is resistive I²R loss. Because current is squared, reducing current or reducing conductor resistance can significantly reduce cable losses.

P_loss: Power dissipated in the complete DC cable loop, W
I: Current through the cable, A
R_loop: Complete DC loop resistance, Ω

Why DC cable size matters

Choosing the correct DC cable size helps control conductor heating, voltage drop and electrical losses. A cable that is too small can have excessive voltage drop, higher I²R losses and insufficient current-carrying capacity under actual installation conditions.

DC cable ampacity

Ampacity is the current a conductor can carry continuously under specified installation conditions. The allowable current can change with ambient temperature, grouping, installation method, soil conditions and burial depth.

DC cable voltage drop

Voltage drop occurs because every cable has electrical resistance. For a two-wire DC circuit, both the positive and negative conductors contribute to the total loop resistance. Longer cables, higher current and smaller cable sizes increase voltage drop.

Solar PV cable sizing

Solar PV cable selection depends on string current, operating voltage, cable route length, conductor material, installation method, temperature and the required voltage-drop limit. The cable should also satisfy the applicable PV system voltage and installation requirements.

Copper vs aluminium DC cable

Copper provides lower resistance for the same cross-sectional area and is commonly used for PV strings and smaller DC circuits. Aluminium can be useful for larger and longer DC feeders when suitable cable construction and termination methods are available.

Cable length and voltage drop

Cable length directly affects resistance. If current, conductor size and temperature remain the same, increasing the one-way cable length increases the total DC loop resistance and therefore increases voltage drop and power loss.

Temperature and cable resistance

Conductor resistance increases with temperature. A cable operating at a higher conductor temperature therefore experiences higher resistance, which increases both voltage drop and resistive power loss.

Why ampacity and voltage drop are both required

A cable can pass a voltage-drop calculation but still be unsuitable because of insufficient current-carrying capacity. Conversely, a large enough cable for ampacity may be selected unnecessarily if voltage drop is not considered. Both checks should be reviewed.

Worked example

1Example DC circuit: 600 V system voltage, 12 A operating current, 40 m one-way cable length, copper conductor and 4 mm² cable.
2Using copper resistivity of 0.01724 Ω·mm²/m, the complete two-conductor loop resistance is R = 2 × 0.01724 × 40 / 4 = 0.3448 Ω.
3Voltage drop = I × R = 12 × 0.3448 = 4.138 V.
4Voltage-drop percentage = 4.138 / 600 × 100 = 0.69%.
5Receiving-end voltage = 600 − 4.138 = 595.862 V.
6Power loss = I² × R = 12² × 0.3448 ≈ 49.65 W.
7For a selected 2% voltage-drop limit, the calculated 0.69% voltage drop is within the selected limit.

Assumptions

The calculator represents a two-wire DC circuit with positive and negative conductors following the same route.
The entered cable length is one-way route length.
The return conductor is included automatically in the loop resistance.
Reference conductor resistivity values may be used for preliminary calculations.
Generic ampacity information should not replace manufacturer-specific cable datasheets.
Final cable selection must consider the complete project design, installation environment and applicable standards.

Tips

If a cable passes ampacity but fails voltage drop, increasing conductor cross-sectional area can reduce resistance and voltage loss.
Long cable routes can require larger conductor sizes even when the operating current is relatively low.
Reducing cable route length can reduce voltage drop and power loss without increasing conductor size.
Higher DC system voltage can reduce current for the same transmitted power, which can reduce resistive cable losses.
For long solar PV cable runs, compare cable cost with the expected lifetime energy loss before finalizing the conductor size.
Use a DC voltage-drop calculation when the cable size is already known and you only need to evaluate voltage loss.
Use this DC Cable Size Calculator when you want to compare cable sizes and identify a suitable conductor cross-section.

Warnings

This calculator provides preliminary DC cable sizing and verification results and should not be treated as a substitute for a complete electrical design.
Do not select a DC cable using voltage drop alone. Verify ampacity, installation conditions, protection coordination, insulation rating and short-circuit requirements.
For buried cables, actual soil thermal resistivity and burial depth should be considered where applicable.
Always verify the final cable size using the selected cable manufacturer's datasheet and project design requirements.

Standards & references

IEC 60364-5-52
IEC 60364-7-712
IEC 62548
IEC 60502 where the applicable cable voltage class and installation conditions require it

Frequently asked questions

What is a DC cable size calculator?

A DC cable size calculator helps determine a suitable conductor cross-sectional area for a DC circuit by considering current, voltage, cable length, conductor material, voltage drop and installation conditions. For solar PV systems, it can be used to evaluate the suitability of common PV cable sizes.

How do I calculate DC cable size?

Start with the design current and identify the cable installation method. Determine the applicable current-carrying capacity and correction factors, then check voltage drop and power loss. The smallest standard cable size that satisfies the applicable design requirements can then be considered for selection.

How do I calculate solar PV cable size?

For solar PV cable sizing, determine the design current, operating voltage and one-way cable length. Select the conductor material and installation conditions, then check ampacity and voltage drop. The final cable should also meet the PV system voltage, protection and installation requirements.

What is the difference between DC cable sizing and voltage drop calculation?

DC cable sizing determines a suitable conductor size for the circuit. DC voltage-drop calculation determines the voltage lost through a particular cable size and length. A complete cable selection normally considers both ampacity and voltage drop.

How do I calculate DC cable voltage drop?

For a two-wire DC circuit, calculate the complete positive-and-negative loop resistance and multiply it by the operating current. The basic relationship is ΔV = I × R_loop. When the cable length entered is one-way, the return conductor is included in the loop calculation.

How does cable length affect DC voltage drop?

Voltage drop increases approximately linearly with cable length when current, conductor size and temperature remain constant. Doubling the cable route length approximately doubles the resistance, voltage drop and resistive power loss.

What cable size is commonly used for solar PV strings?

Common solar PV string cable sizes include 4 mm² and 6 mm², but the correct cable size depends on string current, cable length, installation method, temperature, grouping, voltage-drop requirements and cable ampacity. There is no single cable size suitable for every solar installation.

What voltage drop should be used for solar DC cables?

The acceptable voltage-drop limit is a project design decision and depends on the particular circuit and system architecture. A lower voltage-drop target generally requires a larger conductor and can reduce electrical losses.

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