BS 7671 Section 715 Extra-Low Voltage Lighting Installations (A4:2026)

Extra-low voltage (ELV) is defined in Part 2 of BS 7671:2018+A4:2026 as a nominal voltage not exceeding 50 V AC RMS or 120 V DC ripple-free, measured between conductors or between any conductor and earth. This is the voltage ceiling that determines whether a circuit falls under Section 715 of the Wiring Regulations or under the general low-voltage rules.

The phrase "ripple-free" matters — for DC supplies, the ripple component must not exceed 10% RMS of the nominal value. A nominally 120 V DC supply with excessive ripple is not ELV for the purposes of Section 715. In practice, modern switched-mode drivers and PoE injectors used in ELV lighting circuits produce well-regulated DC outputs that sit comfortably below the ripple limit.

ELV circuits come in three operational categories: SELV (Separated Extra-Low Voltage), PELV (Protective Extra-Low Voltage) and FELV (Functional Extra-Low Voltage). They look similar on a single-line drawing — but their protective status, and therefore their installation rules under Section 715, differ significantly.

1. SELV — Separated Extra-Low Voltage. The source is electrically separated from earth and from any other circuit by double or reinforced insulation. The output is not earthed at any point. A failure of the LV side cannot raise the ELV side above 50 V AC / 120 V DC. Used where any earth reference must be excluded — for example bathroom Zone 0/1 luminaires.
2. PELV — Protective Extra-Low Voltage. Identical to SELV in source isolation requirements, but the ELV side IS connected to earth at one point. Used where exposed-conductive-parts must be earth-referenced for functional reasons or where a Class I luminaire body is involved.
3. FELV — Functional Extra-Low Voltage. The voltage is in the ELV band, but the source is NOT a safety-isolating transformer. The LV side and the ELV side are not separated by double or reinforced insulation. FELV is NOT a protective measure — a fault on the LV side can put LV onto the ELV conductors. Additional protection (automatic disconnection, basic insulation rated for LV) is required on the ELV wiring.

Section 715 applies to extra-low voltage lighting installations supplied from a source at a voltage not exceeding 50 V AC, or 120 V DC ripple-free. The section sits in Part 7 of BS 7671:2018+A4:2026 alongside other special installations and locations. It applies whether the installation is indoor, outdoor, residential, commercial or display-based — what matters is the supply voltage and the fact that the load is lighting.

Regulation 715.411 sets the protection-against-electric-shock framework — SELV and PELV are recognised; FELV is restricted. Regulation 715.422 deals with protection against fire and thermal effects (driver enclosure, ambient considerations). Regulation 715.512 covers the selection and erection of equipment, including the requirement that ELV wiring be physically separated from LV wiring (or insulated for the higher voltage). Regulation 715.521 covers wiring systems — cable type, cross-sectional area, run length and supports.

Equipment used on a Section 715 circuit will frequently be Class III. Class III equipment is designed for connection to a SELV or PELV source only — it has no provision for protective earthing, and its internal insulation is rated only for the ELV nominal voltage. It carries the Class III symbol (a Roman numeral III inside a diamond) on the equipment marking.

The source for an ELV lighting circuit is almost always a controlgear unit — either a magnetic transformer (now rare) or, far more commonly, an electronic LED driver. These devices are governed by BS EN 61347, the lamp controlgear standard. Driver selection is one of the most important decisions on any Section 715 design — get it wrong and the entire circuit either fails to start, runs hot, or fails prematurely.

Volt drop is the dominant constraint on most ELV lighting circuits, well before current-carrying capacity becomes the limit. The reason is simple arithmetic: at 12 V nominal, a 0.5 V drop is over 4% — already at the BS 7671 Appendix 4 limit for lighting circuits. At 230 V the same 0.5 V drop is below 0.25% and would never be noticed.

Section 715 does not override the Appendix 4 voltage-drop guidance — the same 3% for lighting (and 5% for power) limits apply, expressed as a percentage of the source voltage. For 12 V SELV that means 0.36 V at the furthest luminaire; for 24 V SELV 0.72 V; for 48 V SELV 1.44 V; and for 57 V PoE class 4 it is approximately 1.71 V.

1. Calculate the load current at the furthest luminaire — sum the wattage of all downstream fittings on that branch and divide by the nominal voltage.
2. Choose a candidate cross-sectional area (1.0 mm², 1.5 mm², 2.5 mm², 4.0 mm² etc.) from Appendix 4 Table 4D2A or equivalent for ELV.
3. Calculate the volt drop using mV/A/m values for that conductor, then multiply by the run length in metres.
4. Compare the calculated volt drop to the 3% limit (or stricter, if the luminaire data sheet specifies a tighter window).
5. If the calculated volt drop exceeds the limit, increase the cross-sectional area or shorten the run.
6. For long runs at 12 V or 24 V, consider relocating the driver closer to the load rather than upsizing the cable — every doubled cross-sectional area roughly halves the volt drop, but is significantly more expensive and harder to install.

Cable cross-sectional area for ELV lighting is also affected by the wiring system. ELV cable installed alongside or sharing containment with LV cable must either be physically separated, or insulated for the higher voltage present — Regulation 715.512 is explicit about this, and the same rule appears in Section 528. The practical implication for an electrician retrofitting ELV feature lighting into an existing LV containment system is that ELV-only cable cannot simply be dropped into the same trunking — it must be either rated for the higher voltage or run in its own segregated route.

BS 7671:2018+A4:2026 (the 18th Edition Amendment 4, published 15 April 2026) refreshed Section 715 in step with developments in LED lighting, PoE infrastructure, and the wider Part 7 alignment work.

For a full summary of the changes introduced by Amendment 4, see our BS 7671 A4:2026 summary guide. The AFDD requirements (Section 421.1.7), TN-C-S (PNB) earthing rules and new schedule columns all sit on the LV side and indirectly affect any final circuit that feeds an ELV driver.

Power over Ethernet (PoE) lighting is the fastest-growing class of ELV lighting installation in UK commercial buildings, and it sits squarely inside the scope of Section 715. PoE delivers both data and DC power to a luminaire over a single Cat 6 or Cat 6A cable from a network switch. The voltage is below the ELV ceiling, the load is lighting, and the circuit is therefore an extra-low voltage lighting installation by the Section 715 definition.

For a deep dive into PoE lighting installation — switch selection, cable routing, luminaire commissioning and the convergence with the building IT infrastructure — see our PoE lighting installation guide. The two guides are complementary: this Section 715 page anchors the regulatory framework; the PoE guide walks through the practical specification and install.

ELV lighting is sometimes assumed to be inherently fire-safe because of the low voltage — that assumption is wrong. The current in a 12 V or 24 V circuit can be substantial (a 60 W load at 12 V draws 5 A), and a short circuit in poorly designed ELV wiring can develop significant heat before any upstream protective device operates.

ELV circuits still need to be inspected and tested before energisation and at periodic intervals afterwards. The test sequence differs from a standard LV final circuit because the protective measures and the source characteristics are different, but the principle of structured verification is identical.

1. Continuity — verify the continuity of every ELV conductor from source (driver / PoE port) to luminaire. For PELV circuits, also verify continuity of the bonding from the ELV-side reference earth.
2. Insulation resistance — test between live conductors and between live conductors and earth on the ELV wiring. The test voltage is reduced for ELV (250 V DC test voltage for circuits ≤ 50 V) to avoid damaging connected electronics.
3. Polarity — confirm correct polarity on DC ELV circuits, especially LED downlighters and LED tape, where reverse polarity will simply prevent operation and is easy to miss without a methodical check.
4. Functional test — energise the circuit through the driver / PoE source, confirm each luminaire operates, confirm dimming or scene control responds as commissioned, confirm any addressable luminaires (DALI, DMX, Zigbee, PoE-managed) are correctly addressed and report status.
5. SELV / PELV verification — confirm by inspection that the source is the declared type (look for the safety-isolating transformer symbol on the driver, or the SELV / PELV declaration in the data sheet).
6. Record on the Schedule of Inspections — note the source type, the nominal voltage, the protective measure (SELV / PELV), and any specific Section 715 considerations for the inspector who follows on the next periodic.

Where the ELV lighting forms part of an EIC submission, the Schedule of Inspections should record the source location, the voltage, the protective measure, and any limitations on access for maintenance. Where it forms part of an EICR, the same items should be examined and recorded, with an observation code raised against any deficiency (typically C3 for missing labelling, C2 for missing isolation or a damaged driver enclosure that exposes LV conductors).