BS 7671 Solar Installation Requirements

BS 7671 is the national standard for electrical installations in the UK. The 18th Edition (BS 7671:2018), updated by Amendment 2 in 2022, is the current version in force. For solar PV, the key section is Part 7, Section 712: Solar photovoltaic power supply systems. This section supplements the general requirements of BS 7671 with PV-specific rules covering DC wiring, isolation, protection, earthing, and labelling.

This guide focuses on the requirements that matter most in practice. It covers what Section 712 actually requires, where solar PV installations differ from standard AC installations, what changed between the 17th and 18th editions, and how Part P compliance works alongside the wiring regulations.

Scope of BS 7671 Section 712

Section 712 applies to all solar PV systems that include a DC circuit — which means all solar PV systems, including off-grid and battery storage. The section covers:

• The DC wiring from PV modules to the inverter (strings, array cables, DC bus)
• DC isolation and protection devices
• Earthing and bonding of the PV array and inverter
• Warning labels and circuit identification
• Testing and commissioning requirements

The AC side of a PV installation (from inverter output to the consumer unit and grid connection) is governed by the general requirements of BS 7671 — the same regulations that apply to any domestic or commercial electrical installation. Section 712 applies specifically to the DC components.

DC Wiring Requirements

Cable Selection and Specification

DC cables in solar PV systems face conditions that standard AC wiring does not. Roof-mounted arrays expose cables to UV radiation, wide temperature swings (-20°C to +90°C in summer on a south-facing dark roof), and potential moisture ingress. Section 712 requires cables suitable for these conditions.

The standard cable type for DC circuits in solar PV is double-insulated solar cable meeting:

• EN 50618 (European standard for DC cables in PV systems)
• TÜV 2 PfG 1169 (German certification widely used as a reference)

These cables have UV-stabilised insulation and sheathing, typically rated at 1,000 V or 1,500 V DC, with a working temperature range of -40°C to +90°C. Standard singles or twin-and-earth AC cable is not appropriate for the DC portions of a PV installation.

String Cables and Array Cables

Within the array, cables connecting individual modules to form strings (string cables) are typically pre-terminated MC4 connectors. The installer’s responsibility is:

• Ensuring the cable cross-section is adequate for the string current (typically 4 mm² or 6 mm² for residential strings)
• Routing cables to minimise mechanical risk (not pinched under mounting rails, not hanging loose)
• Protecting cables where they pass through or near roof structures (conduit or dedicated cable management)
• Avoiding cable loop areas that could cause induced voltages or differential fault detection issues

Array cables (combining outputs of multiple strings at a combiner box, or running from the array to the inverter) must be sized for the combined current of all strings they carry.

Cable Routing and Protection

Section 712 and the general requirements of BS 7671 require cables to be protected against mechanical damage and thermal effects. For solar PV, this means:

• Cables in roof voids or loft spaces must be protected against rodent damage (conduit, armoured cable, or cable management systems)
• Cables passing through insulation must not cause overheating (derate or use conduit to allow heat dissipation)
• External DC cables exposed to UV must be UV-rated (as above)
• Cables must not be routed where they will be walked on or subject to regular mechanical stress

DC Isolation Devices

Inverter DC Isolator

Section 712 requires a means of isolating the PV array from the inverter. In practice, this is a DC isolator switch located adjacent to the inverter (within 3 metres, and visible from the inverter location). The isolator must be:

• Rated for DC switching duty (AC isolators are not suitable — they do not break DC arcs the same way)
• Rated for the maximum system voltage (including temperature correction)
• Rated for the maximum short-circuit current of the array
• Clearly labelled as a DC isolator with system voltage
• Accessible for maintenance without requiring tools

Most modern string inverters have an integrated DC isolator. Where a separate isolator is required, use a device with a DC voltage rating matching or exceeding the system Voc at minimum temperature.

Roof-Level Isolator (Emergency Disconnection)

Section 712 also requires a means of isolating the array accessible to emergency services — in practice, a roof-level DC isolator or, in some designs, a rapid shutdown device. The requirement is that DC voltage can be removed from the roof cables without entering the building. This reduces electrocution risk for fire service personnel attacking a roof fire.

The roof-level isolator must be:

• Accessible from the roof or at the roof edge without requiring roof access
• Clearly labelled (often with a label at ground level showing its location)
• Suitable for outdoor use (IP65 minimum)

String Fuses and DC Circuit Breakers

Where multiple strings are combined (parallel connection), there is a risk of reverse current flowing from other strings into a faulted string. If the combined short-circuit current of the parallel strings exceeds the module’s rated reverse current tolerance, string-level overcurrent protection is required.

For most residential installations with two or fewer strings per MPPT input, the inverter’s internal protection is sufficient. For systems with three or more parallel strings, or where the combined short-circuit current exceeds the module’s rated tolerance, string fuses or DC circuit breakers must be fitted at the combiner box or at the inverter.

The specific calculation method is set out in BS 7671 Section 712 and referenced in IEC 62446. String fuse sizing must account for both the maximum string current and the reverse current from parallel strings.

Earthing and Bonding

Earthing in solar PV systems is more complex than in standard AC installations because there are two distinct requirements that are often confused: protective earthing and functional earthing.

Protective Earthing and Bonding

All metalwork that could become live in the event of a fault must be earthed. For a solar PV installation, this includes:

• Mounting structures (aluminium rails and brackets)
• Module frames (where not earthed through the mounting structure)
• Inverter enclosure
• Any metal cable management (conduit, trunking)

The mounting structure is typically bonded to the protective earth at one point. Where rails are joined by stainless steel bolts with metal-to-metal contact, continuity can be assumed; plastic rail joiners require a separate earth bond. Check continuity of the bonding path during commissioning (see testing section below).

The inverter enclosure must be connected to the installation earth through the AC supply. This is normally handled by the inverter’s own earth terminal connected to the consumer unit earth.

Functional Earthing for Transformerless Inverters

Most modern solar inverters are transformerless designs. These inverters require one pole of the DC circuit to be connected to the installation earth (functional earth) for the inverter’s internal isolation monitoring (earth fault detection) to operate correctly.

The functional earth is typically the negative DC conductor. The inverter manufacturer will specify:

• Which pole requires the functional earth connection
• The maximum impedance of the functional earth path
• Whether a dedicated functional earth terminal is provided

The functional earth connection is made at the inverter’s FE terminal, which is separate from the protective earth terminal. Do not combine functional earth and protective earth connections unless the inverter manufacturer’s instructions specifically permit this.

Warning Labels and Circuit Identification

Section 712 prescribes specific warning labels for PV installations. These must be:

• Durable (outdoor-grade materials for roof-level and external labels)
• Fixed and not removable under normal conditions
• Present at every point of entry into the building, at all isolators and junction boxes, and at the consumer unit

Required Label Text

The standard wording for DC circuit warning labels (per BS 7671 and IET Guidance Note 7) is:

WARNING — SOLAR PHOTOVOLTAIC SYSTEM
This installation contains live circuits which cannot be isolated by the AC supply isolator

Additional labels must show:

• Maximum system voltage (Voc at minimum temperature, rounded up)
• Short-circuit current (Isc per string or combined)
• The location of the DC isolator (where it is not immediately obvious)

Consumer Unit Label

A label must be fixed to or adjacent to the consumer unit or distribution board indicating:

• That a solar PV system is connected
• The location of the AC and DC isolation points

Cable Identification

DC conductors must be identified at both ends. Standard practice is red for positive and black for negative. Where cables are run in conduit and colour is not visible, label at each accessible point (junction boxes, isolators, inverter terminals).

Insulation Resistance and Commissioning Tests

Section 712 and IEC 62446-1 require specific tests before a PV system is energised and before handover. These tests must be recorded and included in the installation documentation.

Insulation Resistance Test

Purpose: confirm there is no fault between the live DC conductors and earth before the system is commissioned.

Procedure:

1. Disconnect the inverter (disconnect at the DC isolator)
2. Short the positive and negative conductors at the inverter end
3. Apply a DC test voltage of at least twice the system open-circuit voltage (minimum 500 V for most residential systems, up to 1,000 V for higher-voltage systems)
4. Measure resistance between the shorted DC conductors and the installation earth
5. The result must be at least 1 MΩ (BS 7671 specifies minimum values per installation)

If the reading is below 1 MΩ, there is an insulation fault in the DC circuit. The fault must be located and rectified before the system is commissioned.

Open-Circuit Voltage and Short-Circuit Current

For each string, measure:

• Open-circuit voltage (Voc) at the combiner box or inverter input
• Short-circuit current (Isc) where this can be measured safely

Compare measured values to calculated expected values (accounting for module temperature at time of test and irradiance conditions). Significant deviation indicates a wiring error, shading issue, or module fault.

Protective Conductor Continuity

Test continuity of all protective earth bonds: mounting structure earthing, module frame earthing, inverter enclosure earth. The test confirms the bonding path is complete and of sufficiently low impedance.

Functional Tests

After all pre-energisation tests pass:

1. Energise the DC circuit by connecting strings at the combiner or inverter
2. Confirm inverter starts and synchronises with the grid
3. Check for fault or warning codes
4. Verify generation is being recorded (monitoring system if fitted)
5. Check export is visible at the meter (or confirmed by DNO for metered systems)

Record all test results on the MCS commissioning form. These records are required for MCS certificate issuance and must be kept for the duration of the MCS guarantee period.

Part P Building Regulations Compliance

Solar PV installation is notifiable electrical installation work under Part P of the Building Regulations (England and Wales). The equivalent requirements apply in Scotland under the Building (Scotland) Regulations and in Northern Ireland under the Building Regulations (Northern Ireland).

What Notifiable Means

Notifiable work must be either:

1. Self-certified by a registered competent person: an installer registered with an approved competent person scheme (NICEIC, NAPIT, ELECSA, or similar) can certify their own work without involving building control. This is the standard approach for MCS-certified solar installers — most are already registered under a competent person scheme.

2. Notified to the local authority: if the installer is not registered under a competent person scheme, they must submit a building regulations application to the local authority before starting work. Building control will inspect the installation. This route is rarely used for residential solar.

After self-certification, the installer must issue an Electrical Installation Certificate (EIC) to the customer. A copy is submitted to the certification body’s database, which notifies building control automatically.

EICR Relevance

An Electrical Installation Condition Report (EICR) assesses the condition of an existing electrical installation. Solar PV installations should be included in the scope of an EICR inspection. For installers carrying out surveys of properties with existing solar systems, the EICR process requires checking:

• Original installation certificate (EIC) is present
• DC circuit insulation resistance is within acceptable limits
• Labelling is present and legible
• Isolators are functioning and accessible
• No visible cable damage or degradation

Properties with solar systems that lack an original EIC are a common finding at EICR. The absence of certification does not necessarily mean the installation is unsafe, but it must be coded accordingly and the deficiency noted.

Differences: 17th vs 18th Edition

The 17th Edition of BS 7671 (BS 7671:2008) was replaced by the 18th Edition in 2018. Amendment 2 of the 18th Edition (2022) made further changes. For solar PV, the key differences are:

All solar PV installations designed and installed today must comply with the 18th Edition including Amendment 2. Systems installed under the 17th Edition are not automatically required to be upgraded unless an EICR identifies code C1 or C2 deficiencies.

Arc Fault Detection (Regulation 712.443)

The 18th Edition introduced consideration of arc fault detection devices (AFDDs) for DC circuits. Arc faults in DC circuits (unlike AC) do not self-extinguish at current zero crossings, which makes them a significant fire risk. Regulation 712.443 does not mandate AFDDs for all systems, but it requires the designer to consider arc fault protection as part of the risk assessment.

In practice, arc fault detection is increasingly specified for commercial PV systems with long DC cable runs. For residential installations with short string runs and modern inverters that include arc fault detection in their internal monitoring, the risk is managed without separate AFDDs in most cases. The design records should document the consideration.

Using Solar Design Software for BS 7671 Compliance

Solar design software should generate the documentation BS 7671 and MCS 012 require. This includes:

• String voltage and current calculations (for isolator and fuse sizing)
• Maximum system voltage at minimum temperature (for cable and equipment rating)
• Design drawings showing DC circuit layout, isolator positions, and earthing

SurgePV’s solar designing workflow produces these calculations as part of the standard design output. Design records that include voltage/current calculations, cable schedules, and equipment ratings are exactly what MCS assessors expect to see in your installation files — and they are the starting point for any commissioning test verification.

Frequently Asked Questions

Does solar PV installation qualify as notifiable work under Part P?

Yes. Solar PV installation is classified as notifiable electrical installation work under Part P of the Building Regulations in England and Wales. This means the work must either be carried out by a registered competent person (such as an NICEIC or NAPIT-registered electrician) who self-certifies the work, or a building regulations application must be made to the local authority before work begins. In practice, nearly all MCS-certified solar installers are registered under a competent person scheme, which handles Part P compliance as part of the certification process.

What is the maximum open-circuit voltage allowed for a PV string in the UK?

BS 7671 Section 712 does not prescribe a single maximum voltage for all systems, but it requires that all components (cables, connectors, isolators, inverters) are rated for the maximum system voltage including temperature correction. In practice, most residential string inverter systems operate at array voltages of 200–600 V DC. Commercial systems using 1,000 V or 1,500 V DC components must use equipment rated accordingly, with all wiring and protection devices rated to the same level.

What changed between the 17th and 18th editions of BS 7671 for solar PV?

The 18th Edition (BS 7671:2018) introduced several changes relevant to solar PV. Amendment 2 (2022) updated Section 712 to align with IEC 62446-1:2016 for testing and documentation requirements. Key changes included: updated requirements for arc fault detection in DC circuits (Regulation 712.443), clarification of earthing requirements for transformerless inverters, updated labelling requirements, and stronger requirements for documentation handover.

Is functional earthing the same as protective earthing for solar PV systems?

No. These are distinct concepts. Protective earthing (PE) connects metal enclosures and frames to earth to provide shock protection — standard across all electrical installations. Functional earthing in solar PV applies to transformerless inverters, which require one pole of the DC circuit (typically the negative) to be connected to earth for the inverter’s internal protection systems to work correctly. The functional earth connection carries no fault current under normal conditions and is separate from the protective earth bonding of the array frame and mounting structure.

Part of the UK Solar Compliance hub. See also: MCS certification · G98 vs G99 · G99 application guide