Cable sizing and voltage drop in PV systems

Cable sizing sits at the intersection of two disciplines that installers often treat separately: electrical safety and energy yield. A cable that is technically compliant with ampacity tables can still be the wrong cable if it bleeds away two percent of annual production through resistive losses. Getting sizing right means working through current, temperature, routing, and economics together rather than picking the smallest conductor that clears a code check.

This article walks through the DC side (string cables, home-run cables) and the AC side (inverter-to-distribution-board, and distribution-board-to-grid) of a typical residential or small commercial system. It covers ampacity derating for real installation conditions, the voltage drop calculation itself, and where the two constraints — thermal limit and voltage drop limit — tend to bind in practice. Readers who want a quick sanity check on an existing design can cross-reference the voltage drop calculator alongside the worked examples below.

The underlying physics is simple; the practical difficulty is in the number of derating factors that stack on top of a base ampacity figure, and in resisting the temptation to skip the voltage drop check once the thermal check has passed. Both checks are independent and both must pass.

Why two separate checks matter

A cable has a current-carrying capacity (ampacity) set by insulation temperature limits, and it has a resistance per unit length that causes a voltage drop proportional to current and length. A short, heavily loaded cable can pass the thermal check easily while still causing several percent of voltage drop if the load is far from the source. Conversely, a very long but lightly loaded cable can satisfy the voltage drop rule while being oversized for ampacity. Both DC string cabling and AC output cabling need both checks run independently, and the larger of the two resulting cross-sections is the one specified.

For DC strings, IEC 62548 sets requirements for string and array wiring, including cable selection, overcurrent protection, and rapid shutdown provisions in some jurisdictions. For AC low-voltage wiring, IEC 60364-7-712 (the section of the wiring regulations dealing specifically with PV supply systems) governs cable and protection selection, and national wiring regulations (e.g., BS 7671 in the UK, VDE 0100-712 in Germany) layer on top.

Ampacity and derating

Base ampacity tables (for example those in IEC 60364-5-52 or manufacturer datasheets for single-core PV cable to EN 50618) specify current capacity for a defined installation method, ambient temperature, and grouping. Real installations rarely match the reference condition exactly, so several correction factors are applied multiplicatively:

• Ambient temperature factor — rooftop conduit routing under a dark membrane can see local ambient temperatures well above the 30°C or 40°C reference used in tables, especially for cables run close to the module frame.
• Grouping factor — multiple circuits bundled in the same conduit or cable tray reduce each cable's ability to shed heat.
• Insulation and installation method factor — free air, conduit, buried, or clipped-direct methods each have different reference ampacities.
• Solar/roof-mounted correction — some national codes add a specific addition to ambient temperature for cables run in conduit on or above a roof, reflecting measured temperature rises under membrane roofing.

A cable rated 30 A at reference conditions might realistically carry only 21–24 A once ambient temperature and grouping factors are applied for a rooftop conduit run with three or four DC circuits bundled together. Skipping derating is one of the most common reasons a system passes initial inspection but shows insulation degradation or connector discoloration years later.

DC string cable sizing

DC string cable (typically EN 50618 or UL 4703 rated, double-insulated, UV- and weather-resistant) is sized first for the maximum string current — normally 1.25 × the module's short-circuit current (Isc) at STC, to account for irradiance enhancement effects above 1000 W/m² that can push actual current above the nameplate figure. A further 1.25 factor is sometimes applied for continuous-duty derating in some codes, giving a combined 1.56× multiplier from module Isc to minimum overcurrent device and cable rating, though many contemporary designs use 1.25× alone with a properly rated fuse or the inverter's inherent overcurrent tolerance where no fuse is required.

Worked example: DC home-run cable

Consider a string with module Isc = 11.5 A, home-run cable length (one-way) of 35 m from the array to the combiner/inverter, system voltage around 600 V DC, and 4 mm² copper PV cable (resistance approximately 4.61 mΩ/m at 20°C, rising with temperature).

Design current: 11.5 A × 1.25 = 14.4 A. A 4 mm² PV cable derated for rooftop conduit conditions typically retains ampacity well above this, so the thermal check passes comfortably.

Voltage drop check: voltage drop (V) = 2 × I × L × R, where the factor of 2 accounts for the outbound and return conductor, I is design current, L is one-way length, and R is resistance per metre.

V_drop = 2 × 14.4 A × 35 m × 0.00461 Ω/m = 4.65 V

As a percentage of a typical string operating voltage of 400 V: 4.65 / 400 = 1.16%. This sits comfortably under the commonly used 1–2% DC voltage drop target for string cabling, so 4 mm² is adequate for this run length. Extending the run to 70 m would double the drop to roughly 2.3%, at which point stepping up to 6 mm² (resistance approximately 3.08 mΩ/m) would bring the figure back to about 1.5%.

AC cable sizing

AC output cable runs from the inverter to the main distribution board, and from there to the point of common coupling. The relevant current is the inverter's rated AC output current (or the sum of currents for multiple inverters sharing a feeder), and the same ampacity-then-voltage-drop sequence applies, using three-phase or single-phase formulas as appropriate.

Worked example: single-phase AC run

Inverter rated output: 25 A at 230 V, cable run 20 m one-way, 6 mm² copper cable (resistance approximately 3.08 mΩ/m, reactance typically small enough to neglect at this scale for a first-pass check).

V_drop = 2 × 25 A × 20 m × 0.00308 Ω/m = 3.08 V

As a percentage of 230 V: 3.08 / 230 = 1.34%. Many grid codes and IEC 60364 guidance recommend keeping total voltage drop from origin to any point of use under about 3–5%, with the PV-specific portion often targeted at 1–3% to leave headroom for the rest of the installation's wiring. This run passes with margin; a 40 m run at the same current would approach 2.7%, prompting consideration of 10 mm² cable.

Voltage drop targets by circuit segment

Conductor and installation comparison

Overcurrent protection coordination

Cable sizing cannot be decided independently of overcurrent protection. The cable's ampacity, once derated, must exceed the rating of the fuse or circuit breaker protecting it, and that protective device rating must in turn be compatible with the source's maximum fault current contribution — which for a PV string is limited and close to Isc, unlike a grid-fed circuit where fault current can be very high. This is one reason DC fusing philosophy differs from AC fusing philosophy: many two-string arrays do not require string fuses at all, because the maximum fault current one string can push backward into another under a fault condition is inherently limited to about 1.25× Isc, often within the cable's rating without added protection. Multi-string combiners with three or more parallel strings usually do require fusing, since a faulted string could see fault current contributions from all the healthy strings combined.

Temperature effects on copper resistance

Cable resistance rises with conductor temperature at roughly 0.4% per °C for copper relative to the 20°C reference figures used in most tables. A cable operating at 60°C conductor temperature (a realistic figure for a derated, loaded rooftop conduit run) has resistance around 16% higher than its 20°C rated value. For long runs operating near their thermal limit, using the 20°C resistance figure without a temperature correction can understate voltage drop by a meaningful margin, so designs with tight voltage drop margins should recalculate using expected operating temperature rather than the nameplate 20°C value.

Practical sizing workflow

1. Determine design current: module/string Isc × safety factor for DC, inverter rated output current for AC.
2. Select an installation method and estimate ambient/grouping derating factors specific to the actual routing.
3. Choose a trial cross-section meeting the derated ampacity requirement with margin.
4. Calculate voltage drop at design current over the actual one-way run length, using resistance at expected operating temperature.
5. If voltage drop exceeds the target, step up to the next standard cross-section and repeat.
6. Confirm overcurrent protection rating is compatible with both the cable's ampacity and the source's fault current characteristics.
7. Document the final selection with calculations retained for commissioning records, referenced against the inverter sizing calculator outputs used earlier in design.

This workflow mirrors the broader design sequence discussed in roof mounting done right, where mechanical and electrical routing decisions are made together rather than as an afterthought.

Standards and references

• IEC 60364-5-52 — Selection and erection of electrical equipment: wiring systems, including ampacity tables and correction factors for installation method, grouping, and ambient temperature.
• IEC 60364-7-712 — Requirements for PV power supply systems within low-voltage installations, covering DC and AC wiring, protection, and isolation specific to PV.
• IEC 62548 — Design requirements for PV arrays, including string and array wiring, overcurrent protection, and earthing.
• EN 50618 — Specification for single-core cables for use in photovoltaic systems (DC string and home-run cable).
• IEC 62109 — Safety requirements for power converters used in PV systems, relevant to inverter output terminal ratings that determine AC cable design current.

Frequently asked questions

Why is DC string cable rated differently from ordinary building wire?

PV string cable (EN 50618) is designed for continuous outdoor exposure, UV radiation, wide temperature swings, and mechanical stress from clips and connectors over a 25-30 year service life, conditions ordinary PVC building wire is not rated for. It typically uses cross-linked, halogen-free insulation with a higher temperature rating, often 90°C or higher continuous, than standard building cable. Using non-PV-rated cable for exposed DC runs risks premature insulation failure, which is both a safety and a fire risk given the cable often cannot be de-energised by simply switching off a breaker while the array is illuminated.

How much does voltage drop actually cost in lost energy?

A cable run with 2% voltage drop at rated current loses roughly 2% of the power delivered through that segment whenever the system operates near that current, though actual losses scale with the square of current relative to the design condition, so annual energy loss is usually somewhat less than the peak percentage figure. For a small residential system, 1-2% avoidable loss might represent 50-150 kWh per year — modest per system but worth eliminating at design time since the incremental cable cost is small compared to total losses over 25 years.

Does aluminium conductor make sense for PV cabling?

Aluminium is occasionally used for longer AC feeders in commercial and utility-scale systems where cost and weight favour it over copper, but it requires roughly 1.6 times the cross-sectional area of copper for equivalent ampacity and demands terminations rated for aluminium to avoid galvanic corrosion and loosening over time. For DC string cabling, copper remains near-universal because cable diameters are small and the added complexity of aluminium terminations is not justified at those currents.

What happens if I oversize the cable well beyond the calculated requirement?

Oversizing reduces voltage drop and resistive losses further and adds thermal margin, at the cost of higher material cost, more difficult routing due to larger bend radius and weight, and sometimes incompatibility with connector or gland sizes. Moderate oversizing by one standard size is common practice and inexpensive insurance; excessive oversizing rarely pays back given diminishing returns on loss reduction once drop is already well under 1%.

How does conduit fill affect cable choice beyond simple derating?

Conduit fill limits (the proportion of a conduit's cross-sectional area that may be occupied by cables) affect both installation practicality and heat dissipation, and are set out in wiring regulations and cable manufacturer guidance. High fill ratios increase mutual heating and reduce effective ampacity beyond what a simple grouping factor captures, and can make future cable additions or replacements impractical, so many designers deliberately keep fill below the regulatory maximum with headroom for maintenance access.

Sources

• NREL — PV system design resources
• PVEducation — Wiring and cabling losses
• IEC — Standards for photovoltaic energy systems
• Fraunhofer ISE — Photovoltaics report