Solar DC String Cable Sizing & Voltage Drop Calculator
Size PV string cable for voltage drop and rooftop temperature derating to AS/NZS 5033.
Last reviewed by the myCalculator Editorial Team
Input Parameters
Measure array to inverter one way; the calculator doubles it for the return path.
AS/NZS 5033 recommends keeping DC string voltage drop at or below 3%; 1% to 1.5% is good practice.
Calculation Results
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Formula & How Solar DC String Cable Sizing & Voltage Drop Calculator Works
DC string cable is sized for voltage drop rather than thermal capacity, because a long roof run at low current can still lose significant energy. The drop is calculated over twice the one-way length to account for the return conductor. Rooftop cable also runs far hotter than the 30 °C reference ambient — surfaces regularly reach 70 °C in an Australian summer — so the conductor's current rating must be derated before checking compliance. Worked example: with a short-circuit current of 10.5 A, a 30 m one-way run and an allowable drop of 1.5% of 400 V (6 V), copper at roof temperature of 70°C has a resistivity of about 0.0209 Ω·mm²/m. The minimum area is 2 × 30 × 10.5 × 0.0209 ÷ 6 = about 2.2 mm², so the next standard size up, 4 mm², would be chosen. Always follow AS/NZS 5033 and use a licensed installer for real systems.
- Enter Module Short-Circuit Current Isc (A) (A): Input your target parameter value.
- Enter String Open-Circuit Voltage Voc (V) (V): Input your target parameter value.
- Enter One-Way Cable Run (m) (m): Input your target parameter value.
- Enter Allowable Voltage Drop (%) (%): Input your target parameter value.
- Enter Rooftop Ambient Temperature (°C) (°C): Input your target parameter value.
- Apply Formula: Calculate using
Vdrop = 2 × L × I × ρ / A | A_min = 2 × L × I × ρ / (Vdrop_allowable). - Review Results: View exact computed answers, metrics, and worked mathematical proofs.
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MyCalculator.com.au. (2026). Solar DC String Cable Sizing & Voltage Drop Calculator. Retrieved October 24, 2026, from https://www.mycalculator.com.au/calculators/engineering/solar-dc-cable-sizing
MyCalculator.com.au. (2026) 'Solar DC String Cable Sizing & Voltage Drop Calculator'. Available at: https://www.mycalculator.com.au/calculators/engineering/solar-dc-cable-sizing (Accessed: 24 October 2026).
Frequently Asked Questions (FAQs)
What voltage drop is allowed on a solar DC string?
AS/NZS 5033 recommends a maximum 3% drop on the DC side, but most designers target 1% to 1.5% because the lost energy compounds across every hour of generation for the system's life.
Why derate cable for rooftop temperature?
Cable current ratings are published at 30 °C ambient. A roof cavity can reach 70 °C, which can cut usable current capacity by roughly 40% and requires a larger conductor.
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Authoritative Standards & Verified Reference Citations
Calculations and mathematical logic on this page are grounded in published statutory rules, regulatory standards, and official government data published by the following bodies:
Australian Standards including AS/NZS 3000 (Wiring Rules), AS 3600 (Concrete Structures), and AS 4100 (Steel Structures).
Accreditation and technical guidelines for civil, structural, mechanical, and electrical engineering.
Dedicated Australian home loans, borrowing capacity, mortgage repayments & refinance modeling.