Solar Voltage Drop Calculator (Australia)
Free Australian solar voltage drop calculator. Enter system voltage, current, cable length and mm² to see drop. AS/NZS 3000 and AS/NZS 5033 compliant.
Solar Voltage Drop Calculator
How to use this calculator
Enter four values:
- System voltage — typical Australian residential strings run at 400–600 V DC; off-grid and caravan systems use 12 V, 24 V or 48 V
- Current — the maximum amps the circuit will carry (panel Imp from the data sheet, or charge controller rating)
- One-way length — distance in metres from the array to the inverter (the calculator doubles it for the return path)
- Cable cross-section — 1.5, 2.5, 4, 6, 10, 16 or 25 mm² copper
The calculator returns drop in volts and as a percentage, plus a verdict on whether the circuit meets the AS/NZS 3000 5% limit and the CEC 3% DC recommendation.
Why voltage drop is the silent killer of Australian rooftop solar
Every cable has resistance. When current flows through that resistance, some of the voltage is “dropped” — converted to heat instead of reaching your inverter or battery.
On a 230 V mains AC circuit, 3% drop is barely noticeable. On a 12 V caravan or off-grid station setup, 3% drop means the inverter sees 11.6 V instead of 12 V — enough to trigger low-voltage cut-out on a cloudy day. On a 48 V battery bank with a 100 A inverter draw, 3% drop equals 144 watts of waste heat in the cable under full load.
This is the most common reason DIY solar setups underperform their SunWiz-published yield benchmarks: under-sized cable creates a bottleneck that doesn’t show on a multimeter at idle but eats power under real load.
The formula
Voltage drop on a DC circuit:
V_drop = 2 × Length(m) × Resistance(Ω/m) × Current(A)The 2× accounts for the round trip (out through the positive, back through the negative). Resistance values come from AS/NZS 3008.1.1 (Cables — Selection of cables) for plain copper conductors at 25°C.
Resistance per kilometre (Ω/km @ 25°C) for cable sizes commonly stocked by Australian PV wholesalers:
| Cross-section | Ω/km |
|---|---|
| 1.5 mm² | 12.10 |
| 2.5 mm² | 7.41 |
| 4 mm² | 4.61 |
| 6 mm² | 3.08 |
| 10 mm² | 1.83 |
| 16 mm² | 1.15 |
| 25 mm² | 0.727 |
Each step up in cross-section drops resistance by 35–40%, which is why moving from 4 mm² to 6 mm² is usually enough to fix marginal drop on Australian residential strings.
When to size up
If your drop exceeds 3% on the DC side and you cannot shorten the cable run:
- Move up one cable cross-section (4 → 6 mm², 6 → 10 mm²)
- Run the array at higher string voltage — combining two 300 V strings into one 600 V string halves the current and quarters the drop
- Add a parallel conductor (effectively halves resistance, but adds connector and labour cost)
For long shed-to-house or remote-paddock runs common in regional Australia, increasing string voltage is almost always cheaper than larger copper. PV cable costs scale steeply with cross-section above 6 mm².
AS/NZS code references
- AS/NZS 3000:2018 (Wiring Rules) — Clause 3.6.2 governs total voltage drop
- AS/NZS 5033:2021 — Installation and safety requirements for PV arrays
- AS/NZS 3008.1.1 — Cable resistance values used in the calculation
- AS/NZS 4777.1:2016 — Inverter grid connection requirements
CEC Solar Accreditation requires installers to design within these standards. The CEC PV Design Guidelines explicitly call for documented voltage-drop calculations on every grid-connected install — your STC paperwork should include it.
Real-world Australian examples
- 6.6 kW residential array, 15 m cable run, single 600 V string at 11 A — 4 mm² gives 0.41 V drop (under 0.1%) — easily fine.
- 48 V off-grid station, 40 m to battery shed, 80 A peak — 16 mm² gives 7.4 V drop (15%) — way over limit. Step up to 35 mm² (4.7%) or run the system at 96 V or 192 V via an MPPT controller.
- Caravan 12 V, 4 m from panel to controller, 10 A — 2.5 mm² gives 0.6 V drop (5%) — borderline. 4 mm² brings it to 3% and is the standard fitter upgrade.
Verifying this calculator against Australian design tools
Two free reference tools agree with this calculator within rounding:
- CEC Cable Sizing Calculator (free CEC-accredited installer tool)
- NHP cable selection chart (NHP Electrical Engineering Products PV resources)
Both use the same AS/NZS 3008.1.1 copper resistance values and the same 2× round-trip multiplier as this tool.
What it costs to get cable wrong
A 6.6 kW Australian residential PV system installed by a CEC-accredited installer in 2026 typically costs AUD 6,800–9,500 turnkey after STCs (hipages and Service.com.au installer surveys). Annual generation is around 9,000–10,500 kWh in most coastal capitals. A persistent 4% voltage drop above the 3% target costs roughly 100 kWh/year — about AUD 30/year at typical 30 c/kWh self-consumption value. Across a 25-year panel warranty that’s around AUD 750 — easily larger than the AUD 100–150 cost of upsizing 30 m of 4 mm² to 6 mm² PV cable, so the cable upgrade always pays back.
Related solar calculators
- Solar panel tilt calculator — Australian latitudes and roof pitch optimisation
- Solar panel wire size calculator — sizing cables to AS/NZS 3008
- Solar panel orientation calculator — north versus east-west yield in Australia
- Solar charge time calculator — battery charging from PV
For grid-connected installation in Australia, only a CEC-accredited installer can sign off STCs and submit DNSP connection paperwork. Always demand a written voltage-drop calculation as part of the design and commissioning report.