AC Cable Sizing for Solar PV Systems

AC Cable Sizing for Solar PV Systems: The Engineering Detail That Makes or Breaks Grid Connection Approval

AC cable sizing for solar PV is one of the most technically demanding aspects of any commercial or domestic solar installation in the UK, yet it remains one of the most frequently under-engineered elements on EPC contractor submissions. According to data from Distribution Network Operators (DNOs), a significant proportion of G99 and G98 grid connection applications are delayed or rejected due to incomplete or non-compliant AC-side electrical documentation. That is not a paperwork problem. It is a cable design problem. When the conductor cross-section is wrong, the voltage drop exceeds limits, or the protection coordination cannot be demonstrated on paper, the entire project stalls. This article breaks down exactly what UK EPC contractors must get right when sizing AC cables from the inverter output to the grid connection point, and why cutting corners at this stage costs far more than the time saved.

Why AC Cable Sizing in Solar PV Is More Complex Than Standard LV Wiring

Many engineers familiar with standard LV distribution design assume that sizing an AC cable from an inverter output follows the same straightforward process as sizing any sub-main. In practice, solar PV AC wiring UK involves a layered set of constraints that do not apply to conventional fixed loads. The inverter output current is not simply the rated output current. It is a function of the total DC array capacity, the inverter efficiency curve, the power factor setting, and in some cases the reactive power export capability required by the DNO under Engineering Recommendation G99.

A real-world example from a 99.9 kWp commercial rooftop project in the East Midlands illustrates this clearly. The design team sized the AC trunk cable from three 33 kW three-phase string inverters to the AC distribution panel using manufacturer datasheet current ratings directly, without applying grouping derating or ambient temperature correction. The cable run passed through a south-facing plant room that regularly reached 45 degrees Celsius in summer. The result was a cable that was technically undersized for continuous operation at peak irradiance conditions. The DNO inspection flagged it, the cable had to be re-pulled through the entire conduit run, and the project commissioning was delayed by three weeks. That single oversight cost the contractor more than the entire engineering fee for the AC design stage.

Inverter Output Cable Sizing: Starting From the Right Current Value

The starting point for inverter output cable sizing is establishing the design current (Ib) correctly. For a single-phase string inverter, the design current is the rated AC output current stated on the inverter nameplate, which for a typical 5 kW unit at 230 V is approximately 21.7 A. For a three-phase inverter, this value is per phase. The critical mistake many designers make is treating this value as the final design current without asking whether the inverter can export reactive power or operate at a leading power factor, which can increase the current drawn through the cable even at the same real power output level.

For DNO submissions under G99, inverters above 50 kVA may be required to provide reactive power support within a defined power factor range, often 0.95 leading to 0.95 lagging. At 0.95 power factor, the apparent current is approximately 5.3% higher than the active power current alone. On a large central inverter rated at 100 kW three-phase, this means the cable design current must be calculated on the apparent power basis, not the active power basis. Using the wrong value here produces an undersized cable that fails thermal withstand calculations at the first DNO review.

LV AC Cable Calculations for Solar: Voltage Drop Limits Under BS 7671 and IEC 60364-7-712

LV AC cable calculations solar designers must satisfy two overlapping regulatory frameworks in the UK. BS 7671:2018 (the 18th Edition Wiring Regulations) sets the general voltage drop limit for final circuits at 3% and for distribution circuits at 5% from the origin of the installation. IEC 60364-7-712 applies specifically to PV power supply systems and does not override BS 7671 but adds specific requirements for the PV installation itself. The interaction between these two standards is where many EPC contractor designs fall short.

Under BS 7671, the voltage drop AC solar system calculation must account for the full cable run from the inverter output terminals to the point of connection to the main distribution board or grid connection point. For a string inverter located 40 meters from the main LV panel on a large warehouse roof, a 6 mm2 copper cable carrying 25 A will produce a voltage drop of approximately 3.84% on a single-phase 230 V circuit. That exceeds the 3% distribution circuit limit. Increasing to 10 mm2 reduces the voltage drop to approximately 2.3%, which is compliant. The additional material cost of 10 mm2 versus 6 mm2 over 40 meters is modest. The cost of a failed DNO inspection and re-design is not.

The voltage drop formula used for three-phase AC circuits in BS 7671 Appendix 4 is based on the millivolt per ampere per metre (mV/A/m) values tabulated for each conductor cross-section. For three-phase balanced loads, the formula is: Vd (%) = (mV/A/m value x Ib x L) / (1000 x Vn) x 100. For IEC 60364 solar AC cabling compliance, designers must also verify that the voltage at the inverter terminals under maximum output conditions does not exceed the inverter’s maximum AC output voltage tolerance, typically plus or minus 10% of nominal, which is a separate check from the BS 7671 voltage drop limit.

Short-Circuit Current Calculations and Protection Coordination on the AC Side

Protection coordination is an area where solar PV electrical design diverges significantly from standard LV work. On the AC side of a grid-connected inverter, the prospective short-circuit current (PSCC) at the inverter output terminals is determined by two sources: the grid fault contribution flowing back through the cable from the upstream distribution board, and the inverter’s own fault current contribution.

Modern grid-tied inverters are designed to disconnect from the grid within 200 milliseconds of detecting a fault condition, as required by G99 and G98. This means the inverter’s contribution to a sustained fault current is minimal. However, the grid-sourced fault current at the inverter terminals can still be substantial, particularly on large commercial sites with high fault level upstream. A contractor working on a 250 kWp array connected to an 11 kV/LV substation with a 350 MVA fault level must calculate the PSCC at each inverter output and verify that the cable’s thermal withstand capacity is not exceeded during the fault clearance time of the upstream protection device.

The thermal withstand check uses the adiabatic equation from BS 7671: S = I x sqrt(t) / k, where S is the minimum conductor cross-section in mm2, I is the fault current in amps, t is the clearance time in seconds, and k is the conductor material constant (115 for PVC-insulated copper, 143 for XLPE-insulated copper). A 100 A PSCC clearing in 0.4 seconds through a PVC cable requires a minimum conductor of: S = 100 x sqrt(0.4) / 115 = 0.55 mm2. That value seems low, but at higher fault levels, for example 3 kA at 0.4 seconds, the required minimum section becomes 16.5 mm2, which may govern the cable selection over the current-carrying capacity requirement.

Single-Phase Versus Three-Phase Inverter AC Runs: Neutral Sizing Under Unbalanced Conditions

The distinction between single-phase string inverter AC runs and three-phase trunk cable design is one of the most practically important considerations for EPC contractor solar design UK projects involving mixed-orientation rooftop arrays. A south-facing array with east-facing and west-facing sections will typically use multiple string inverters, some single-phase and some three-phase, depending on the array geometry and the available phase connections at the distribution board.

When single-phase inverters are distributed across a three-phase supply in an attempt to balance the phases, the actual balance at any given moment depends entirely on irradiance conditions. In the morning, the east-facing strings produce significantly more power than the west-facing strings, creating a phase imbalance that can be 30 to 40% in some configurations. This is not a theoretical concern. It has direct consequences for neutral conductor sizing in the AC trunk cable feeding back to the main distribution board.

Under balanced three-phase conditions, the neutral carries only the residual unbalanced current, which is theoretically zero for a perfectly balanced linear load. Under the unbalanced conditions typical of mixed-orientation solar arrays, the neutral current can be a significant fraction of the phase current. BS 7671 Regulation 524.2 requires that where the neutral current is expected to exceed the phase current, for example in circuits with significant harmonic distortion from inverter outputs, the neutral conductor must be sized accordingly. For conservative EPC design, neutral conductors on three-phase AC runs from solar arrays should be sized equal to the phase conductors, not reduced, unless a detailed harmonic and imbalance analysis has been completed and documented.

Derating Factors: The Most Common Cause of Undersized AC Cables That Fail DNO Inspection

The impact of cable grouping derating factors (Cf) and ambient temperature correction on real-world conductor sizing cannot be overstated for UK solar installations. Using uncorrected current ratings from manufacturer datasheets is the single most common cause of undersized AC cables that fail DNO inspection, and it is entirely avoidable.

BS 7671 Appendix 4 provides the base current-carrying capacity (It) for cables under reference conditions: single cable in free air at 30 degrees Celsius ambient for thermoplastic (PVC) insulation, or 35 degrees Celsius for thermosetting (XLPE) insulation. Real-world solar installations routinely deviate from these reference conditions in three specific ways:

• Grouping: Multiple AC cables from adjacent string inverters are often bundled together in the same conduit or cable tray on the roof. A group of three single-core cables in a single conduit has a grouping derating factor of approximately 0.7 per BS 7671 Table 4C1. A group of six cables in the same conduit reduces this further to approximately 0.57.
• Ambient temperature: Cables routed across south-facing metal-decked roofs in direct sunlight can experience ambient temperatures of 50 to 60 degrees Celsius in summer. The temperature correction factor for a PVC cable at 50 degrees Celsius ambient is 0.71, and at 60 degrees Celsius it drops to 0.5. Ignoring this correction can halve the effective current-carrying capacity of the cable.
• Installation method: Cables clipped directly to a hot metal roof structure have a lower current-carrying capacity than cables in free air. The reference installation method must match the actual installation method used on site.

The combined effect of these derating factors means that a 4 mm2 copper cable with a base current-carrying capacity of 32 A (clipped direct, single cable) may have a derated capacity of only 32 x 0.7 x 0.71 = 15.9 A when grouped with two other cables in a hot rooftop environment. A string inverter with a rated output current of 21 A would overload that cable at full output. The correct cable for that circuit is 10 mm2, not 4 mm2. The difference between these two selections is the difference between a compliant installation and a failed DNO inspection.

For EPC contractors looking to get this right from the outset, the solar design preparation checklist provides a structured framework for documenting every site condition that affects cable derating before the design is even started.

Solar Grid Connection Cable Sizing and the G99/G98 Documentation Requirements

Solar grid connection cable sizing must satisfy not only the electrical engineering requirements of BS 7671 but also the documentation standards required by the DNO for G99 or G98 grid connection applications. The IEC 62446-1 standard for grid-connected PV systems defines the minimum documentation requirements for the AC-side electrical design, and DNOs in the UK routinely use this standard as the benchmark for assessing whether a submitted design package is complete.

Under IEC 62446-1, the AC cable schedule must document the following for every AC circuit in the installation:

• Circuit reference and description
• Conductor cross-section in mm2
• Conductor material (copper or aluminium)
• Insulation type and temperature rating (e.g., XLPE 90 degrees Celsius or PVC 70 degrees Celsius)
• Cable length in meters
• Installation method reference (from BS 7671 Appendix 4)
• Base current-carrying capacity (It) from the relevant table
• Applied derating factors and corrected current-carrying capacity (Iz)
• Design current (Ib)
• Protection device type, rating, and breaking capacity
• Voltage drop in volts and as a percentage of nominal voltage
• PSCC at the circuit origin and verification of thermal withstand

This level of documentation is not optional for projects requiring G99 connection. DNO engineers will check these values, and any inconsistency between the cable schedule, the single-line diagram, and the AutoCAD electrical schematics will trigger a request for clarification that delays the connection offer. Understanding how to pass grid connection approval requires getting this documentation right the first time, not as an afterthought after the cables have already been pulled.

How AutoCAD Electrical Schematics Must Document AC Cable Schedules for EPC Submissions

The role of AutoCAD in EPC contractor solar design UK submissions has evolved from a drawing tool to a documentation compliance engine. For G99 and G98 applications, the AutoCAD electrical schematics must not only show the single-line diagram of the AC distribution but must also link to a fully attributed cable schedule that is consistent with the engineering calculations.

A well-structured AutoCAD package for a commercial solar PV installation will include the following AC-side documents:

1. A single-line diagram showing all inverters, AC isolators, surge protection devices, generation meters, and the point of connection to the DNO network, with all cable sizes annotated
2. A detailed AC cable schedule as described under IEC 62446-1, either embedded as a table in the drawing set or produced as a linked spreadsheet with a clear revision history
3. A protection coordination diagram showing the time-current characteristics of all overcurrent protection devices from the inverter output back to the DNO fuse, demonstrating discrimination
4. Conduit and cable routing drawings showing the physical routing of all AC cables, with identified sections that experience elevated ambient temperatures or grouping effects
5. A completed voltage drop calculation sheet for each AC circuit, referenced to the cable schedule

The AutoCAD solar planning process at Lion Solar integrates all of these elements into a single coordinated drawing package that satisfies both G99/G98 submission requirements and IEC 62446-1 documentation standards. The value of this approach is not just compliance. It is the reduction in RFI cycles with the DNO, which on large projects can represent weeks of programme delay.

Common Design Errors That EPC Contractors Must Eliminate From Solar PV Electrical Design

Across multiple commercial solar PV projects reviewed for DNO submission compliance, the same categories of error appear repeatedly. Understanding these failure modes is the fastest way for EPC contractors to improve their solar PV electrical design quality and reduce DNO rejection rates.

The first and most damaging error is the use of manufacturer-provided cable sizing recommendations without verification against site-specific derating conditions. Inverter manufacturers typically include a recommended cable size in the installation manual, derived from the inverter’s rated output current under standard reference conditions. These recommendations are a starting point, not a final design. They do not account for cable grouping, elevated ambient temperatures, or the specific cable length on a given project. A recommendation of 6 mm2 in the manual can legitimately become a requirement for 16 mm2 on a real site, and using the manufacturer’s figure without verification is not an engineering calculation. It is a guess.

The second major error is failing to carry the voltage drop calculation through the entire cable chain from the inverter to the DNO connection point. Many designers calculate voltage drop for the longest individual cable run but fail to sum the voltage drops across multiple series segments, for example from the inverter output to the AC combiner panel, and then from the AC combiner panel to the main distribution board. Each segment contributes to the total voltage drop, and the cumulative figure must remain within the BS 7671 limit.

The third error is inconsistency between the cable sizes shown on the single-line diagram and the cable sizes specified in the cable schedule. This is a documentation error rather than an engineering error, but it is treated as an engineering deficiency by DNO reviewers. AutoCAD drawings with manually entered cable annotations are particularly prone to this error when the engineering calculations are revised after the drawings are produced. Linked attributes and parametric cable schedules in AutoCAD Electrical eliminate this risk entirely.

For a comprehensive understanding of how these electrical design elements fit into the broader process of designing a PV system from site survey to connection, the guide on how a PV solar system is designed provides context for where AC cable sizing sits within the full design workflow.

Practical AC Cable Sizing Workflow for UK EPC Contractors

A repeatable, defensible AC cable sizing methodology for UK solar PV installations follows a clear sequence that ensures compliance with BS 7671, IEC 60364-7-712, and IEC 62446-1 simultaneously. The steps below represent the minimum required process for any commercial solar PV AC circuit design:

1. Establish the design current (Ib): Use the inverter’s rated apparent output current (kVA / Vn) rather than the active power current if reactive power export is required by the DNO connection agreement.
2. Identify the installation method: Determine the actual installation method for each cable run (clipped direct, in conduit, on cable tray, buried, etc.) and assign the corresponding BS 7671 reference installation method.
3. Apply derating factors: Calculate the grouping correction factor (Cg) from BS 7671 Table 4C1, the temperature correction factor (Ca) from Table 4B1, and any additional factors for thermal insulation or buried cables. The combined derating factor is the product of all applicable factors.
4. Select the cable cross-section: Choose the minimum conductor size where the corrected current-carrying capacity (Iz = It x combined derating factor) is greater than or equal to the design current (Ib).
5. Verify voltage drop: Calculate the voltage drop for the selected conductor using the mV/A/m method from BS 7671 Appendix 4 and verify that the cumulative voltage drop from the inverter to the DNO connection point does not exceed 3% for distribution circuits or 5% for final circuits.
6. Check thermal withstand: Calculate the PSCC at the cable origin and verify that the adiabatic equation is satisfied for the minimum conductor cross-section selected.
7. Verify protection coordination: Confirm that the upstream overcurrent protection device rating (In) satisfies the conditions: Ib is less than or equal to In, and In is less than or equal to Iz. Also verify that the protection device will clear any fault current within the cable’s thermal withstand time.
8. Document in the cable schedule: Record all of the above values in the IEC 62446-1 compliant cable schedule and cross-reference to the AutoCAD single-line diagram.

This workflow is not burdensome when embedded in a standard project design template. The time investment is measured in hours for a typical commercial installation. The cost of not following it is measured in weeks of programme delay and potential rework costs that can reach tens of thousands of pounds on larger projects.

Frequently Asked Questions About AC Cable Sizing for Solar PV Systems

What is the maximum voltage drop allowed for AC cables in a solar PV installation in the UK?

Under BS 7671:2018, the maximum permitted voltage drop for distribution circuits is 5% from the origin of the installation, and 3% for final circuits. For solar PV AC circuits, many DNOs and designers target 3% as a conservative design standard to maintain inverter operating voltage within its specified tolerance range. IEC 60364-7-712 does not set a different voltage drop limit but requires compliance with BS 7671 as the applicable UK national standard.

Do I need to use XLPE or PVC cables for solar PV AC wiring in the UK?

Both PVC and XLPE insulated cables can be used for solar PV AC wiring UK installations, provided they are suitable for the environment. XLPE cables are generally preferred for rooftop solar work because their higher operating temperature rating (90 degrees Celsius versus 70 degrees Celsius for PVC) means their current-carrying capacity is less severely derated at the elevated ambient temperatures found on south-facing rooftops. The insulation type must be specified in the cable schedule submitted with G99/G98 applications.

How does cable grouping affect AC cable sizing for string inverters on a commercial rooftop?

Cable grouping has a significant impact on conductor sizing. When multiple AC cables from adjacent string inverters share the same conduit or tray, each cable’s current-carrying capacity must be derated using the grouping correction factor from BS 7671 Table 4C1. For two cables in a conduit, the factor is approximately 0.8. For six or more cables, it can drop to 0.57. A cable selected on its uncorrected rating may be insufficient when installed alongside other cables, requiring a larger cross-section to compensate for reduced heat dissipation.

What documentation must the AC cable schedule include for a G99 grid connection submission?

The AC cable schedule for a G99 submission under IEC 62446-1 must include: circuit reference, conductor cross-section, conductor material, insulation type and temperature rating, cable length, installation method, base and derated current-carrying capacity, design current, protection device type and rating, voltage drop percentage, and PSCC verification. This schedule must be fully consistent with the single-line diagram and protection coordination documents in the submission package.

How is the neutral conductor sized for three-phase solar PV AC cable runs?

For three-phase AC runs from solar arrays, the neutral conductor must be sized to carry the expected neutral current under the actual operating conditions of the installation. Because mixed-orientation rooftop arrays produce phase imbalances throughout the day, BS 7671 Regulation 524.2 requires that the neutral conductor reflect the anticipated unbalanced current. For conservative EPC design without a detailed harmonic analysis, the neutral conductor should be sized equal to the phase conductors.

Why do manufacturer cable recommendations sometimes differ from BS 7671 calculations?

Manufacturer cable recommendations are based on standard reference conditions and do not account for site-specific derating. They are a starting point, not a final design. Using these values without verifying against actual site conditions using BS 7671 Appendix 4 methodology is the most common cause of undersized AC cables that fail DNO inspection.

Conclusion: Getting AC Cable Sizing Right Is a Commercial Imperative for EPC Contractors

The technical requirements for AC cable sizing solar PV systems in the UK are well-defined across BS 7671, IEC 60364-7-712, and IEC 62446-1. The engineering methodology is established and repeatable. What separates high-performing EPC contractors from those who face repeated DNO rejections and costly rework is the discipline to apply that methodology fully, document it completely, and embed it in every project design from day one.

Correct inverter output cable sizing begins with the right design current, accounts for every applicable derating factor, verifies voltage drop across the complete cable chain, confirms thermal withstand against prospective fault currents, and is documented in a cable schedule that is fully consistent with the AutoCAD single-line diagram submitted to the DNO. None of these steps is optional. All of them are auditable. And all of them can be systematised into a repeatable design process that adds minimal time to each project while eliminating the programme risk associated with non-compliant submissions.

For EPC contractors building that systematic capability, starting with the right foundational documents and processes makes all the difference. The solar design preparation checklist and the structured approach to AutoCAD solar planning provide the framework within which compliant AC cable designs can be produced consistently, across every project, regardless of size or complexity. When those designs feed into a well-prepared grid connection approval submission, the result is faster DNO approvals, fewer RFI cycles, and projects that connect on time and within budget.

Heval Arcan