Solar PV Detailed Electrical Design: DC, AC, Cable Sizing and Single-Line Diagrams

Solar PV detailed electrical design converts datasheets and layout into checked cable sizes, protection settings and a consistent single-line diagram (SLD). The logic is the same on every project: confirm the equipment limits, size each conductor for current, derating, voltage drop and fault duty, coordinate protection, design earthing, and make sure the SLD, equipment schedule, cable schedule and physical layout all say the same thing. Final sizing always depends on project codes, installation method, ambient conditions and manufacturer data.

Design inputs

Collect these before starting:

•             Module datasheet: Voc, Vmp, Isc, Imp, temperature coefficients, maximum system voltage and maximum series fuse rating.

•             Inverter datasheet: MPPT range, maximum DC voltage and current, AC rating, efficiency, protection functions and grid-code features.

•             Layout and stringing drawings with string counts and approximate cable lengths.

•             Site temperature range, soil thermal resistivity and installation method.

•             Utility technical requirements, fault level at the point of connection and metering rules.

•             Governing standards and project specification.

String voltage and current checks

Verify that string open-circuit voltage at the lowest temperature stays below the inverter and module limits, and that string voltage at the highest temperature stays within the MPPT window. Verify the current per MPPT input, including any manufacturer rules on overloading. Check protection requirements for parallel strings, since reverse current limits depend on the module’s maximum series fuse rating and the number of parallel strings.

DC cable sizing

A DC string cable must satisfy three checks:

1.          Current-carrying capacity. Design current is based on module short-circuit current with the factor required by the governing standard. The cable’s tabulated rating is reduced for ambient temperature, grouping and installation method.

2.          Voltage drop. The design target is set by the project loss budget, since DC ohmic loss reduces energy.

3.          Fault and insulation. Cable insulation, voltage rating and short-circuit withstand must suit PV application and UV exposure.

Conceptually:

I design ≤ I rated × k temperature × k grouping × k installation

ΔV (DC) = 2 × I × L × ρ ÷ A

Here I is current, L is one-way length, ρ is conductor resistivity at operating temperature and A is cross-sectional area. Use the project standard’s formulas and tables for the final selection.

AC cable sizing and inverter output

AC cables from the inverter must carry rated inverter output current, including any overload capability, with derating for temperature, grouping and installation. For three-phase circuits a common approximation for voltage drop is:

ΔV (AC, three-phase) = √3 × I × L × (R cosφ + X sinφ)

Check short-circuit withstand of the cable and the protective device’s ability to clear faults within the cable’s thermal limit. If the inverter output is limited by reactive power requirements, the apparent power determines the current, not just the active power.

Transformer sizing and MV cables

Size the transformer for inverter apparent power, ambient temperature, harmonic content and loading profile. Specify impedance, vector group, tap range, cooling and loss class considering both the grid code and protection design. MV cable sizing follows the same principles as AC: current rating with derating, voltage drop, and thermal short-circuit withstand, plus screen earthing arrangements and termination type. MV cable laying method and soil thermal resistivity often govern the size more than load current does.

Short-circuit and protection coordination

A short-circuit study provides maximum and minimum fault levels at each bus. These determine breaker ratings, cable withstand and relay settings. A protection coordination study then sets time-current characteristics so the device nearest the fault clears first. Include inverter fault contribution, which is limited and differs from rotating machines, as per manufacturer data. IEC 60909 is a common reference for fault calculations, but the utility may require its own method.

Earthing and lightning protection

Earthing design uses soil resistivity, the fault current the grid must carry and the clearing time to limit step and touch voltages. Include equipotential bonding of structures, equipment frames and metallic fences according to the standard and project specification. Lightning protection follows a risk assessment and covers buildings, masts and surge protective devices on DC, AC and communication circuits.

Auxiliary supply, metering and SCADA

Auxiliary supply needs its own sizing for loads such as control building, HVAC, trackers, lighting and security, with backup where specified. Metering design must meet the utility’s accuracy class, CT/PT ratios, and sealing requirements. SCADA design defines data points, communication protocols and network topology, and should match the inverter and meter interfaces.

Developing the SLD

The SLD should show:

•             All major equipment with ratings and tags.

•             Cable sizes and types between equipment.

•             Protective devices, their ratings and settings references.

•             CT/PT and metering points.

•             Earthing connections.

•             Interfaces to the grid and auxiliary supply.

Build the SLD in levels, from a plant overview to block-level details, so readers can follow the system without clutter.

Reconcile the SLD with schedules and layout

Before issue, check that:

•             Every item on the SLD appears in the equipment schedule with matching rating and tag.

•             Every cable on the SLD appears in the cable schedule with the same size and from-to.

•             Cable lengths in the schedule match the routed layout.

•             Protection ratings and settings match the coordination study.

•             Equipment positions and clearances on layout match what the SLD assumes.

A mismatch discovered on site means rework, delay or an incorrect purchase.

Electrical design review checklist

•             ☐ Module and inverter data confirmed from current datasheets.

•             ☐ String voltage and current checked at temperature extremes.

•             ☐ DC cable sizes verified for current, derating, voltage drop and fault.

•             ☐ AC and MV cable sizes verified for current, derating, voltage drop and fault.

•             ☐ Transformer specification consistent with grid code and protection.

•             ☐ Short-circuit study and coordination study complete.

•             ☐ Earthing and lightning protection designed from site data.

•             ☐ Metering and SCADA consistent with utility requirements.

•             ☐ SLD, equipment schedule, cable schedule and layout reconciled.

•             ☐ Revision control applied to all electrical documents.

Common mistakes

•             Applying standard cable tables without derating for site conditions.

•             Using a voltage-drop limit from another project without checking the loss budget.

•             Ignoring reactive power when sizing AC components.

•             Forgetting that fault levels vary with grid conditions.

•             Treating the SLD as a drawing instead of a controlled design record.

When to involve a specialist

Bring in a specialist when the project involves a weak grid, an unusual voltage level, a complex substation or third-party review. Solyug Energy provides solar design consultancy for utility-scale solar projects, and publishes more guides in the resource center.

Standards and references

•             IEC 62548: PV array design requirements.

•             IEC 60364-7-712: electrical installations, PV power supply systems.

•             IEC 60364-5-52 and IEC 60287: cable selection and current rating calculation.

•             IEC 60909: short-circuit current calculation.

•             IEC 62305: lightning protection.

•             IS 3043 (India): earthing practice.

•             Utility technical requirements and grid code.

Frequently Asked Questions

How do you size DC cables in a solar plant?

Check current-carrying capacity with derating, voltage drop against the loss budget and fault or insulation requirements. Use the governing standard for design current and factors.

What voltage drop is acceptable?

There is no universal figure. The limit comes from the project specification, utility requirements and the loss budget, balanced against cable cost.

Do I need a short-circuit study for a solar plant?

Usually yes for MV plants, to select equipment ratings and set protection. Inverters contribute limited fault current, but the grid contribution can be large.

What is the purpose of an SLD?

It documents the electrical design, showing equipment, ratings, protection and connections, and is the reference for schedules, procurement and construction.

Why must the SLD match the cable schedule?

Because the cable schedule drives procurement and installation. A mismatch causes wrong cable orders or terminations.

Are combiner boxes always required?

No. String-inverter designs often connect strings directly to the inverter, while central inverter designs commonly use combiner boxes. The choice depends on architecture.