Voltage Drop Calculation for Solar DC Cables

A solar array can be producing within expected current and voltage limits yet still leave yield on the roof if the cable run is undersized. A correct voltage drop calculation turns cable selection from a quick estimate into a checkable design decision - before cable is pulled, connectors are crimped and the job is costed.

For installers, the objective is not simply to select the largest cable available. It is to keep losses and voltage performance within the project requirements while controlling material cost, conduit fill, termination size and installation time. The right conductor size depends on the actual circuit, not just the inverter capacity printed on the plans.

What voltage drop means on a solar job

Voltage drop is the reduction in voltage between the source and the load caused by resistance in conductors and connection points. Current flowing through cable creates a voltage loss. That loss becomes heat and, in a solar system, can reduce energy delivered to the inverter or downstream load.

On the DC side, voltage drop affects the voltage presented by the array at the inverter or DC optimiser input. On the AC side, it affects voltage supplied from the inverter to the switchboard or point of connection. Both need to be considered, but they are separate circuits with different currents, voltages, cable routes and operating conditions.

A low-voltage, high-current circuit is generally more sensitive than a high-voltage, low-current string. For example, a battery cable or 48 V DC circuit can require a significantly larger conductor than a high-voltage PV string carrying similar power. This is why using kW alone to choose cable is unreliable.

Voltage drop calculation formula for DC cable

For a two-wire DC circuit, the practical starting formula is:

Voltage drop (V) = 2 × one-way cable length (m) × current (A) × cable resistance (Ω/m)

The factor of two accounts for the positive and negative conductors. Use the one-way route length, then include both conductors through the formula. Do not enter the total loop length and multiply by two again.

To express the result as a percentage:

Voltage drop (%) = voltage drop (V) ÷ circuit operating voltage (V) × 100

Cable resistance must match the conductor size, material and temperature assumptions being used. Copper conductor resistance increases as temperature rises, so values taken at 20°C can understate the loss in a hot roof space, on a black rooftop run or inside a loaded conduit. Manufacturer data and the applicable cable-selection standard are the right references for final design work.

Worked example: PV string cable

Assume a PV string has a maximum operating current of 10 A, a one-way cable run of 25 m and 6 mm² copper solar cable. If the working conductor resistance is 0.0036 Ω/m, the calculation is:

2 × 25 × 10 × 0.0036 = 1.8 V

If the string operates around 360 V at maximum power, the percentage voltage drop is:

1.8 ÷ 360 × 100 = 0.5%

That result may be suitable for the design, subject to the complete system requirements. If the same 1.8 V drop occurred on a 48 V battery circuit, it would equal 3.75%. The same cable and current can therefore have a very different impact depending on circuit voltage.

Use the relevant operating voltage for the question being answered. Maximum power voltage is usually useful when assessing production loss on a PV string. Open-circuit voltage remains critical for equipment voltage ratings and cold-weather string design, but it does not represent normal loaded operation.

Calculate each cable section, not the whole system average

A common mistake is to apply one cable length and one current value to an entire array. Current changes wherever strings combine or branch. The conductor from a string to a combiner carries string current; the main output from that combiner carries the combined current of the parallel strings.

Calculate each distinct section separately. This includes string leads, homeruns, combiner outputs, inverter DC inputs, battery links, inverter AC outputs and submain runs. Add the voltage losses where sections are in series, then assess the total against the design target.

Connection quality also matters. A voltage drop calculation based on conductor resistance assumes sound terminations. Poorly crimped connectors, mixed or unsuitable mating connector systems, loose lugs and damaged cable can add resistance at exactly the points that are hardest to inspect later. Genuine, correctly matched PV connectors and the correct crimp tooling protect the result achieved on paper.

AC cable voltage drop requires different inputs

For single-phase AC circuits, a simplified resistive calculation is often expressed as:

Voltage drop (V) = 2 × one-way cable length × current × resistance

For three-phase circuits, the basic relationship uses:

Voltage drop (V) = √3 × one-way cable length × current × impedance

In practical AC design, impedance, power factor, conductor reactance, installation method and circuit grouping can all matter. Longer three-phase runs and larger cable sizes are particularly worth checking with the appropriate tabulated data rather than relying only on a DC resistance figure.

Consider a single-phase inverter output carrying 32 A over a 30 m one-way run. Using a working resistance of 0.0022 Ω/m for the selected conductor gives:

2 × 30 × 32 × 0.0022 = 4.22 V

At 230 V, that is about 1.84%. Whether it is acceptable depends on the project design, upstream supply voltage, applicable requirements and the voltage drop allocated to the rest of the installation. A submain that looks acceptable by itself can become an issue once the final circuits and supply conditions are included.

Selecting cable size: balance loss, rating and installation conditions

Voltage drop is only one part of cable selection. The selected cable must also have adequate current-carrying capacity after derating for ambient temperature, grouping, thermal insulation, conduit installation and other site conditions. It must suit the system voltage, be appropriate for UV exposure where installed externally, and terminate correctly in the isolator, inverter, breaker or connector being used.

Oversizing cable reduces losses, but it can increase job cost and create practical issues. Larger conductors can be slower to pull, harder to route through tight conduit, unsuitable for some terminals or require larger glands and lugs. Undersizing, however, is false economy when it creates avoidable generation loss, thermal stress or a rectification visit.

For standard roof-mounted PV string work, 4 mm² or 6 mm² solar DC cable may be common, but neither size is automatically correct. A short single-string run may suit one size, while a long run, a combined-string cable or a high-current battery circuit may require a different approach. Select from the calculation and the installation conditions, then confirm component compatibility.

Inputs worth checking before finalising the order

Before ordering cable by the reel or carton, confirm the one-way route length rather than relying on plan scale. Allow for vertical drops, roof routing, changes of direction and a sensible termination allowance. Check maximum current for the particular cable section, including combined current where parallel strings join.

Also verify the actual conductor cross-section, temperature rating, voltage rating and installation method. For PV DC circuits, ensure cable, connectors, isolators and associated protection equipment are rated for the maximum system voltage and the relevant DC duty. On AC circuits, confirm the conductor fits protective-device terminals and matches the required earthing and neutral arrangement.

Australian electrical and solar work must be designed and installed in line with applicable standards, manufacturer instructions and network requirements. A voltage drop result does not replace those checks. It supports them by making losses and circuit performance visible before installation.

Keep the calculation with the job file

A short calculation record is useful for quoting, procurement and commissioning. Record circuit name, cable type and size, one-way length, design current, resistance or impedance source, calculated voltage drop and the operating voltage used for the percentage result. It makes later variations easier to assess when a route changes, an inverter is relocated or additional strings are added.

For crews managing repeat installs, standardising common cable sizes and documenting the limits for typical run lengths can speed up estimating without skipping the engineering check. Solar Products Supply can then be used as a practical procurement point for the matched cable, genuine connector and installation consumables required to finish the run properly.

Treat voltage drop as a design input early, not a commissioning surprise. A few minutes spent checking the longest and highest-current cable sections can prevent lost yield, unnecessary cable upgrades and a return trip to site.