Calculator
Cable Sizing and Voltage Drop Calculator
Every metre of cable in a photovoltaic system converts a little of the energy you paid for into heat. On a short residential run that loss is trivial. On a 40 metre string run in 4 mm² cable it is a permanent one to two percent tax on production, paid every sunny day for twenty-five years.
This calculator returns the absolute volt drop, the percentage of nominal voltage, the heat dissipated in the conductor and the cross-section you would need to hit a 1% or 3% target.
Size the run
Distance along the cable route, not straight-line.
String Vmp for DC, 230 or 400 V for AC.
Cables on a hot roof reach 60 to 80 °C.
Voltage drop
2.27 V
Drop as percentage
0.71%
Comfortable
Power lost as heat
25.0 W
At the current entered
Resistivity used
0.02063 Ω·mm²/m
Copper at 70 °C
Cross-section for 1%
4.3 mm²
Round up to the next standard size
Cross-section for 3%
1.4 mm²
How the calculation works
Figure — DC circuit route length
Decision tree
Choosing a DC cable cross-section
Work down the list in order. Ampacity is a safety limit and comes first; voltage drop is an economic limit and is decided afterwards.
Is the cable rated for the design current after every derating factor?
Design current for a PV string is 1.25 × Isc(STC). Derate for ambient temperature, grouping, conduit and solar gain on the roof surface.
- No
- Increase the cross-section until the derated ampacity exceeds design current. Nothing below this is negotiable.
- Yes
- → go to step 2
Is the DC voltage drop at maximum power below 1 percent?
Measure the true routed length, not the plan distance, and double it for the two-wire circuit.
- Above 3%
- Go up at least two sizes. A drop this large costs real yield every sunny hour of the system's life.
- 1–3%
- Acceptable on long ground-mount runs where the copper cost is material; otherwise step up one size.
- Below 1%
- → go to step 3
Does the AC side stay within the total drop allowed by the wiring rules?
Most national rules derived from IEC 60364 allow 3 percent for the final circuit and 5 percent overall from the origin of the installation.
- No
- Enlarge the AC cable or move the inverter closer to the board. Do not solve an AC problem by enlarging DC cable.
- Yes
- → go to step 4
Is the conductor mechanically and environmentally suited to the route?
Rooftop DC cable needs UV-stable, double-insulated, halogen-free construction to EN 50618 with a 90 °C or higher conductor rating.
- No
- Change cable type before changing size. A correctly sized cable in the wrong insulation still fails early.
- Yes
- The size is settled. Record the calculation in the commissioning file.
Voltage drop is Ohm's law applied to the full circuit. Resistance is resistivity times length divided by cross-sectional area, and the current must travel out and back, so a DC or single-phase circuit uses twice the one-way length. Balanced three-phase circuits use a factor of √3 instead, because the return path is shared.
Copper resistivity rises roughly 0.39% per degree, so a cable clipped to a hot roof is noticeably worse than the 20°C figure in a catalogue. Using 70°C conductor temperature for exposed DC runs keeps the answer honest. This calculation sizes for performance; current-carrying capacity, grouping factors and protection coordination are separate checks required by the wiring rules.
The formula
R = ρ(T) × L / A ρ(T) = ρ₂₀ × (1 + α × (T − 20)) ΔV = k × ρ(T) × L × I / A ΔV% = 100 × ΔV / V_nominal P_loss = ΔV × I A_req = k × ρ(T) × L × I / (V_nominal × target%) k = 2 for DC and single-phase AC, √3 for balanced three-phase ρ₂₀ = 0.01724 Ω·mm²/m copper, 0.0282 aluminium α = 0.00393 /°C copper, 0.00403 aluminium L one-way route length · A cross-section · I operating current
Assumptions behind the numbers
- Route length is one-way; the factor k accounts for the return conductor.
- Resistivity is corrected to the conductor temperature you enter, not the 20 °C catalogue value.
- Power factor is assumed to be unity, which is normal for a PV inverter but not for general loads.
- Reactance is ignored. Below about 25 mm² and at these lengths it is negligible; for large commercial feeders use the impedance tables in the cable data.
- This is a performance calculation. Ampacity, derating for grouping and insulation, protective device coordination and earth-fault paths must be checked separately against IEC 60364 and local rules.
- DC string current is taken as operating current, not short-circuit current. Protection sizing uses Isc × 1.25.
How to read the result
Common practice keeps total DC-side drop under 1% at maximum power and the AC side under about 3% from inverter to the point of connection. Those are economic targets rather than safety limits: 1% of a 9,000 kWh annual yield is 90 kWh a year, which over twenty-five years is worth more than the extra copper.
If the required cross-section is larger than the connectors or terminals accept, the answer is usually to shorten the route, raise the string voltage by putting more modules in series, or move the inverter closer to the array.
Worked example
A string of ten modules operating at 11 A and 320 V runs 30 metres from the roof to a garage inverter in 4 mm² copper, with cable temperature reaching 70°C.
Corrected resistivity is 0.01724 × (1 + 0.00393 × 50) = 0.02063 Ω·mm²/m. Drop is 2 × 0.02063 × 30 × 11 ÷ 4 = 3.40 V, or 1.06% of 320 V, dissipating 37 W in the cable at full output. Moving to 6 mm² brings the drop to 2.27 V, or 0.71%, and cuts the heat to 25 W. At roughly €1.50 per metre extra for the larger cable, the upgrade costs about €90 and recovers it in saved energy within a decade — sooner if the string runs at high current most of the year.
Mistakes that skew the answer
- Measuring straight-line distance instead of the real cable route, which is often 30 to 50% longer once trays, bends and drops are counted.
- Using 20 °C resistivity for cables clipped to a dark roof in July.
- Sizing only for ampacity and ignoring drop, then wondering why the inverter reports lower DC voltage than the string should produce.
- Forgetting that the drop applies to both the positive and negative conductor in a DC run.
- Mixing aluminium and copper terminations without bimetallic connectors, which invites corrosion and hot joints.
- Assuming a bigger cable fixes a poor crimp. Most measured losses in the field come from bad terminations, not from cross-section.
Questions readers ask
- What voltage drop is acceptable in a solar installation?
- Design practice keeps the DC array circuit under 1% and the AC circuit from inverter to origin under about 3%, with the whole installation typically limited to 5% by national wiring rules. Those national limits are safety and function limits; the tighter solar targets exist because the loss is continuous and compounds over the system's life.
- Should I use copper or aluminium?
- Copper for almost all residential work: it has about 60% lower resistivity, terminates more reliably and comes in the sizes solar connectors expect. Aluminium becomes attractive on long commercial feeders where the cost of copper dominates, provided the terminations are designed for it and torque is checked on schedule.
- Does voltage drop affect safety or just yield?
- Primarily yield, but not only. The lost voltage becomes heat in the conductor, and a cable that is undersized for drop is often also running warm for its insulation rating. Excessive drop on the AC side can also cause inverters to trip on over-voltage at the point of connection when the grid voltage is already high.
- Can I reduce drop without buying bigger cable?
- Yes. Put more modules in series so the same power travels at higher voltage and lower current — drop scales with current, so halving current halves the drop. Relocating the inverter closer to the array is the other lever. Both are usually cheaper than upgrading a long run by two cable sizes.
- Does this calculator replace a wiring design?
- No. It handles the voltage drop and loss part of the design. A compliant installation also needs ampacity with derating factors for grouping, ambient temperature and installation method, correct protective device selection, earthing and bonding, and DC arc-fault considerations. Have a qualified electrician verify the final design against IEC 60364 and the local rules.