BTR Enerji · Solar TrackerWhy Accurate DC Cable Sizing Matters in Utility-Scale Solar
DC cabling looks like a small line in a PV budget, but the wrong cross-section or an unrealistic cable quantity sets both the capital cost and an energy loss repeated every day for 25–30 years. This article explains how string cables are sized and how the layout changes the cable quantity.
Three criteria for DC cable selection
- Current-carrying capacity: the cable must carry the maximum string current with its capacity corrected for ambient temperature, grouping and installation method. Standards such as IEC 62548 base the calculation on 1.25 times the module short-circuit current; for bifacial modules the rear-side gain is added.
- Voltage drop and losses: power dissipated in the cable resistance is lost from production. Many EPCs target roughly 1–1.5% on the DC side.
- Voltage class and durability: EN 50618 / H1Z2Z2-K solar cable for 1500 V systems; UV, ozone and temperature resistance; proper protection for buried sections.
The voltage-drop formula
The DC voltage drop of one string, in percent:
ΔU% = 2 × L × I × R′ / U × 100
L is the one-way cable length (m), I the string current (A), R′ the resistance per metre at operating temperature (Ω/m) and U the string voltage (V). The factor 2 accounts for the outgoing and return conductors. Copper resistance rises with temperature (about 0.39%/°C): a 6 mm² solar cable with roughly 3.39 Ω/km at 20 °C exceeds 4 Ω/km at 70 °C.
Worked example
A 28-module string operates at 41.7 V × 28 ≈ 1,168 V and 13.3 A on a 6 mm² cable at 70 °C (≈ 4.06 Ω/km):
| One-way distance | Voltage drop | Comment |
|---|---|---|
| 60 m | ≈ 0.55% | Inverter next to its table group |
| 150 m | ≈ 1.39% | At the design target |
| 300 m | ≈ 2.8% | Change the cross-section or the topology |
The annual energy loss is lower than this full-power figure, because the plant runs at part load most of the time and the loss falls with the square of the current. Long string cables still add up to a significant loss over the project lifetime.
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Topology sets the quantity: distributed or central?
- Distributed string inverters: inverters are placed within the table groups; string cables stay short and no DC combiner boxes are needed. AC cabling increases, but losses stay low at the higher AC voltage. A common choice for tracker plants.
- Central inverters: strings are combined in combiner boxes and heavy DC feeders (often aluminium) run to the inverter station. Equipment cost falls on large sites, and DC-side design becomes more critical.
A common error in cable estimates
Assuming at feasibility stage that every string runs to a single central point overstates the DC quantity several times over for a distributed design, and overstates the voltage drop with it. Our simulator clusters the tables, places inverters at the cluster centres and inverters into transformer blocks, then measures cables along orthogonal routes following rows and roads. The cable quantities in the BOM and the voltage drop are therefore based on the actual site layout.
Practical checklist
- Define the string current including bifacial gain and high-irradiance conditions.
- Take correction factors (temperature, grouping, buried installation) from the manufacturer's tables.
- Check the voltage drop for the longest string as well, not only the average.
- Before increasing the cross-section, check whether moving the inverter is cheaper.
Frequently asked questions
What is the maximum DC voltage drop in a PV plant?
There is no single binding limit; many EPCs target about 1–1.5% on the DC side and treat 3% as an upper bound. The right target comes from comparing cable cost with the energy lost over the project lifetime.
Which DC cable is used in tracker PV plants?
EN 50618 H1Z2Z2-K solar cable is common in 1500 V systems: 4–6 mm² for string cables and larger cross-sections for feeders in central-inverter designs.
Is a larger cable cross-section always better?
No. With long string cables, moving the inverter closer to the table group is often cheaper and more efficient than increasing the cross-section.
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The ratios on this page are typical industry ranges and are not binding; exact values are determined by a site-specific analysis.