DC cable sizing is the process of selecting a conductor cross-section that can carry the design current safely under the actual installation conditions, while keeping voltage drop and power loss inside the limits set by the project design basis.
It is not a single formula. A cable that passes the heating check can still be the wrong cable, because the run is long enough for voltage drop to decide the size. Equally, a cable that satisfies voltage drop on paper can be undersized once twelve circuits share one buried duct in warm ground.
Use the DC Cable Sizing Calculator to run the whole workflow — design current, base ampacity, derating, derated ampacity check, temperature-corrected resistance, voltage drop, power loss and a candidate size comparison — on your own numbers.
What DC Cable Sizing Actually Means
A correct DC cable selection satisfies three independent criteria at the same time:
1. Current-carrying capacity. The design current must not exceed the cable's tabulated ampacity after every applicable correction factor has been applied.
2. Voltage drop. The drop across the complete positive-plus-negative path must stay within the allowable percentage.
3. Power loss. The I²R loss must stay within the loss budget, because on solar and battery circuits that loss is paid for every operating hour.
The smallest standard size that passes all three is the minimum acceptable cable.
Why Ampacity Alone Is Insufficient
Ampacity answers one question: will the conductor overheat? On short runs that is usually the binding constraint. On long runs it almost never is.
A 4 mm² PV cable in free air is rated around 55 A. A single string draws roughly 14 A, so thermally it is comfortable by a factor of four. But over a 100 m one-way run at 70 °C conductor temperature, that same cable drops about 14.4 V and dissipates about 202 W. Whether that is acceptable depends entirely on the string voltage and the loss budget — not on the ampacity table.
Design Current
Design current is a decision, not a measurement. The calculator therefore asks you to choose the basis explicitly:
| Basis | When to use it |
|---|---|
| User-entered value | Generic DC circuits, battery feeders, DC loads |
| Module Impp × parallel strings | Energy-loss and voltage-drop evaluation of a PV string |
| Module Isc × parallel strings | Thermal sizing and protection coordination |
| Temperature-corrected Isc | Hot sites, where Isc rises with cell temperature |
PV String Current: Isc vs Impp
Isc is the short-circuit current — the maximum the module can source. It is the correct basis for thermal sizing and for coordinating fuses and disconnects. Design codes commonly apply a 1.25 factor to Isc for continuous-duty PV string cable.
Impp is the current at maximum power. That is what the array actually delivers, so it is the honest basis for voltage-drop and annual energy-loss calculations.
Both matter. A cable sized only on Impp may be thermally marginal under fault-adjacent conditions; a cable sized only on 1.25 × Isc may be needlessly heavy on a short run.
Bifacial Current Allowance
Bifacial modules can produce a rear-side current gain, and some design bases add an allowance — 5%, 10%, whatever the project's yield study supports. This is a project assumption and nothing more. The calculator defaults the allowance to 0% and applies exactly the value you enter, showing the base current and the adjusted current separately.
Safety Factor
Any design multiplier is shown as its own line. If you set 1.25, you will see:
```text
Base current 14.00 A
Bifacial allowance 0% 14.00 A
Safety factor × 1.25 17.50 A -> design current
```
Nothing is applied silently.
Cable Material and Resistance
| Property | Copper | Aluminium |
|---|---|---|
| Resistivity at 20 °C (reference) | 0.01724 Ω·mm²/m | 0.02826 Ω·mm²/m |
| IEC 60364 Annex G normal service | 0.0225 Ω·mm²/m | 0.036 Ω·mm²/m |
| Temperature coefficient α | 0.00393 /K | 0.00403 /K |
| Relative ampacity | 1.00 | ≈ 0.78 |
The Annex G values are deliberately higher than pure-metal resistivity: they represent conductors in normal service, including stranding and operating effects. Which basis you use changes the answer, so the calculator makes it a visible input rather than an internal assumption. When you have the manufacturer's maximum DC resistance at 20 °C, use it — it beats any generic value.
Temperature Effect on Resistance
A copper cable at 70 °C has about 20% more resistance than the same cable at 20 °C. Since voltage drop and loss are both proportional to resistance, the operating conductor temperature is a first-order input — and it is not the same as ambient temperature. PV cable clipped to a module frame in strong sun regularly reaches 70–90 °C while the air is at 40 °C.
Current-Carrying Capacity and Derating
Tabulated ampacity applies at reference conditions only. IEC 60364-5-52 uses:
- 30 °C ambient air for cables in air
- 20 °C ground temperature for buried cables
- 2.5 K·m/W soil thermal resistivity
- 0.8 m depth of laying
- one circuit, no grouping
Real installations differ, and each difference contributes a correction factor:
$$I_z = I_{table} \times k_{temp} \times k_{group} \times k_{soil} \times k_{depth}$$
Crucially, a factor that does not apply is not applied. A tray cable gets no soil factor. A buried cable gets no ambient air factor. Read the full breakdown in DC Cable Derating Factors.
Installation Method
Installation method changes the base ampacity before any factor is applied:
| Method | Character | Relative capacity |
|---|---|---|
| Single-core PV cable in free air | Best heat rejection | Highest |
| Perforated tray, touching | Good | High |
| In conduit or pipe in air | Trapped air | Reduced |
| Buried in duct | Soil plus air gap | Reduced |
| Direct buried | Soil contact | Moderate |
Grouping
Cables in a group heat each other. Two spaced cables on a tray stay near 0.9; twelve cables bunched inside one pipe can fall to roughly 0.5. There is no universal 0.6 factor — a value like that is a specific project's result, not a rule.
Soil Thermal Resistivity and Depth
For buried runs, soil thermal resistivity (K·m/W) governs how quickly heat escapes. IEC tables assume a precautionary 2.5 K·m/W. A soil thermal survey showing 1.0 K·m/W can raise the rating by roughly 50%; dry sand at 3.0 K·m/W lowers it. Depth of laying corrects against the 0.8 m reference — deeper is hotter, hence a factor below 1.0.
Voltage Drop
A DC circuit carries current out on the positive conductor and back on the negative, so the resistive path is twice the route length:
$$\Delta V = I \times 2 R_T L$$
Enter the one-way length. The engine adds the return path internally — doubling it yourself double-counts the run and doubles the answer.
Typical design limits: 1% for PV string and array DC cable, 2% for array-to-inverter and battery circuits, 3% as an outer bound. These are design decisions, which is why the calculator makes the limit editable instead of forcing one.
Power Loss and Cable Efficiency
$$P_{loss} = I^2 R_{loop}$$
For a purely resistive DC circuit at a given current, the loss percentage equals the voltage-drop percentage. That equality is a useful sanity check: if your loss percentage and drop percentage disagree, one of the two calculations has used a different resistance or a different length.
Cable section efficiency is simply 100% − loss%.
Worked Example: PV String Cable
Inputs: 14 A design current, 1170 V string Vmpp, 100 m one-way, copper, 70 °C conductor, single-core PV cable in free air, 40 °C ambient, one circuit, 2% voltage-drop and power-loss limits.
Step 1 — Base ampacity, 4 mm²: 55 A.
Step 2 — Derating: ambient factor at 40 °C = 0.91, grouping = 1.00. Combined 0.91, so I_z = 50.1 A. 14 A ≤ 50.1 A → PASS.
Step 3 — Resistance: R₂₀ = 0.01724 × 1000 / 4 = 4.31 Ω/km; R₇₀ = 4.31 × [1 + 0.00393 × 50] = 5.157 Ω/km.
Step 4 — Loop resistance: 2 × 5.157 × 0.1 km = 1.031 Ω.
Step 5 — Voltage drop: 14 × 1.031 = 14.44 V = 1.23% → PASS.
Step 6 — Power loss: 14² × 1.031 = 202.2 W = 1.23% → PASS.
Conclusion: 4 mm² copper satisfies ampacity, voltage drop and power loss together. 2.5 mm² also passes at 1.97%, but with no margin against a 2% limit.
Cable Size Comparison
A single recommended number is not enough for an engineering review. The calculator evaluates every standard size and shows the candidates around the selection:
| Cable size | Derated ampacity | Voltage drop | VD % | Power loss | Status |
|---|---|---|---|---|---|
| 1.5 mm² | 27.3 A | 38.5 V | 3.29% | 539 W | FAIL — voltage drop, power loss |
| 2.5 mm² | 37.3 A | 23.1 V | 1.97% | 323 W | PASS |
| 4 mm² | 50.1 A | 14.44 V | 1.23% | 202 W | PASS |
| 6 mm² | 63.7 A | 9.63 V | 0.82% | 135 W | PASS |
| 10 mm² | 89.2 A | 5.78 V | 0.49% | 81 W | PASS |
Array-to-Inverter Cable Sizing
Array-to-inverter (main DC) cable carries the combined current of all strings on the combiner box, over longer routes, often buried in duct. That combination usually makes derating and voltage drop both binding. Size it on the combined design current, apply the buried factors honestly, and check whether copper or aluminium is more economical at the resulting cross-section.
Common DC Cable Sizing Mistakes
- Doubling the entered cable length when the engine already accounts for the return conductor.
- Applying a soil factor to a cable on a tray, or an air-temperature factor to a buried cable.
- Reusing one project's grouping factor as a universal value.
- Using 20 °C catalogue resistance for a conductor that runs at 70–90 °C.
- Selecting the cable on ampacity alone on a long run.
- Treating medium-voltage IEC 60502 rating tables as generic 1.5 kV DC PV cable data.
Why Manufacturer Datasheets Matter
Reference tables are generic. Real cables differ in conductor class, insulation, sheath and permitted operating temperature. For the issued design, take the maximum DC resistance at 20 °C and the current-carrying capacity from the manufacturer's datasheet for the actual installation condition, and state in the calculation which source you used.