Every UK cable colour code in one reference — current harmonised colours, old UK colours, three-phase identification, flex colours, and BS 7671 requirements. Includes EICR observation guidance for common identification defects.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
Under the current harmonised code (BS 7671 Table 51, identification per Reg 514.3): single-phase — Line = brown, Neutral = blue, earth (CPC) = green-and-yellow. Three-phase — L1 = brown, L2 = black, L3 = grey, with blue neutral and green-and-yellow earth. The old pre-2004 colours (red line, black neutral; red/yellow/blue three-phase) are still found in existing installations — the critical trap is that blue was a line colour in the old system but is neutral now.
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The current cable colour code used in the UK was introduced by Amendment 2 to BS 7671:2001 in 2004 and became mandatory for all new installations from 31 March 2006. These colours are "harmonised" — meaning they align with the colour code used across Europe under the CENELEC harmonisation documents.
These colours are used in all types of fixed wiring cable — twin and earth (flat cable), SWA (steel wire armoured), MICC (mineral insulated copper clad), and singles in conduit or trunking. The earth conductor in twin and earth cable is bare copper and must be sleeved with green/yellow sleeving at every termination point and at every accessible position.
The harmonised colours solved a long-standing problem with international trade and cross-border electrical work. Before harmonisation, each country had its own colour code, creating potential confusion for imported equipment and for electricians working across European borders.
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The old UK cable colour code was used for decades before the harmonised colours were introduced. Many existing installations still use these colours, and every electrician must be able to identify them correctly during inspection and testing.
Black is the neutral colour in the old system but is the L2 phase colour in the new three-phase system. Blue is the neutral colour in the new system but was the L3 phase colour in the old three-phase system. This overlap creates a serious safety risk in mixed-colour installations and is the primary reason warning notices are mandatory.
Three-phase colour codes changed along with single-phase colours during the 2004 harmonisation. The change in three-phase colours is more significant because it introduces new colours (Black and Grey) and reassigns existing colours (Blue) to different functions.
The three-phase colour change is particularly important for distribution board wiring and three-phase installations. Incorrect identification of three-phase conductors can result in incorrect phase rotation, which can damage three-phase motors and other equipment. It can also create dangerous voltage conditions if a conductor assumed to be neutral is actually a phase.
Flexible cables used for appliance connections, extension leads, and portable equipment have always used a slightly different colour convention from fixed wiring. The current harmonised flex colours are the same as fixed wiring — Brown (line), Blue (neutral), and Green/Yellow (earth).
Older flex cables manufactured before the 2004 harmonisation used a different old flex colour code: Brown (line), Blue (neutral), and Green/Yellow (earth). Note that old flex already used Brown and Blue — the old flex colours happened to match the future harmonised colours for single-phase. The difference was in fixed wiring, where the old UK colours (Red, Black, Green) applied.
BS 7671 Section 514 sets out the requirements for conductor identification. Regulation 514.3.1 requires that cable cores shall be identified by colour as required by Regulation 514.4 (except where Regulation 514.6 exempts identification). The specific colours, application points, and bi-colour ratios are then set out in Regulation 514.4. This is not optional — it is a regulatory requirement.
Key identification requirements include:
During an EICR, missing or incorrect conductor identification is recorded as an observation. Missing earth sleeving is typically C3. Missing switch wire sleeving where it creates a polarity identification risk may be C2.
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Many UK installations contain wiring from both the old and new colour codes. This is perfectly legal — the old colours were compliant when installed and do not need to be replaced. However, the presence of both colour codes in the same installation creates a significant identification risk that must be managed.
BS 7671 Regulation 514.14, which previously required a mandatory warning notice for mixed-colour installations, has been deleted in BS 7671:2018+A4:2026. A replacement regulation is forthcoming. When citing a regulatory basis on EICR documentation, do not reference Reg 514.14 — there is no current text at that clause number.
The On-Site Guide (OSG Reg 6.12) continues to recommend affixing a caution notice at or near the appropriate distribution board in a mixed-colour installation:
"Caution — This installation has wiring colours to two versions of BS 7671. Great care should be taken before undertaking extension, alteration or repair that all conductors are correctly identified."
This notice should be durable, legible, and positioned at or near the consumer unit or distribution board where circuits of both colour codes are present. Self-adhesive labels are available from electrical wholesalers, or the notice can be printed and laminated.
During a EICR inspection, check for the presence of this notice at every distribution board in a mixed-colour installation. Following the OSG recommendation to affix such a notice remains good practice, though the deleted Reg 514.14 no longer provides a regulatory basis for the observation.
UK cable colour codes explained: the new harmonised colours vs the old red and black, for single, twin & earth and three-phase, with a comparison.
Try it free for 7 daysBS 7671:2018+A4:2026 introduced two new notice requirements in Section 514 that apply to new installations and affect EICR inspections.
Where an installation includes alternative or additional sources of supply (such as solar PV, battery storage, or a generator), warning notices shall be durably affixed at: (a) the origin of the installation; (b) the meter position, if remote from the origin; (c) the consumer unit or distribution board to which the source is connected; and (d) all points of isolation of all sources of supply. Each notice must identify the relevant isolation point(s). An example notice format is given in Figure 11E of Appendix 11 of BS 7671.
Where SPDs are installed, an information notice shall be placed at or near the relevant distribution board(s) to indicate their presence. This ensures maintenance personnel are aware of the surge protection before working on associated circuitry. There is an exception: domestic (household) premises are exempt from this labelling requirement.
Both obligations are live under A4:2026. During an EICR on an installation with solar PV, battery storage, or SPDs, check for these notices and record their absence as an observation where applicable.
Sleeving is the primary method of conductor identification where the factory-applied insulation colour does not match the conductor's function. The two most common applications are earth sleeving and switch wire sleeving.
The bare copper earth conductor in twin and earth cable must be sleeved with green/yellow sleeving at every termination point — at the consumer unit, at every junction box, at every switch, socket, and light fitting. The sleeving must cover the conductor from the point where it emerges from the cable sheath to the termination. Regulation 514.4.2 requires that when the bi-colour is applied, one colour shall cover at least 30% and at most 70% of the surface, with the other colour covering the remainder. Poor-quality or wrongly-proportioned sleeving can therefore fail on its own even if the correct colours are present. Missing earth sleeving is one of the most common EICR findings.
In switch circuits using two-core and earth cable, the blue (or black in old wiring) conductor is used as the switched line return from the switch to the light fitting. This conductor carries line voltage when the switch is on, so it must be sleeved with brown (or red in old wiring) sleeving at both ends to identify it as a line conductor. Missing switch wire sleeving creates a polarity identification risk and is commonly raised as a C2 or C3 finding on an EICR.
Sleeving must be the correct colour, the correct size for the conductor, and securely fitted so it cannot slide or fall off. Heat-shrink sleeving provides a more secure fit than push-on PVC sleeving.
Conductor identification errors are among the most common findings during EICR inspections. Being aware of these common mistakes helps both during installation and inspection.
Bare copper earth conductors without green/yellow sleeving at termination points. This is extremely common in older installations and in DIY work. It creates a risk of the earth conductor being mistaken for a line conductor or being accidentally contacted. Typically raised as C3 on an EICR.
Blue (or black) conductors used as switched lines without brown (or red) sleeving. This means a conductor that carries line voltage is identified as a neutral, creating a serious risk for anyone working on the circuit. Typically C2 or C3 depending on circumstances.
Installations with both old and new colour codes but no warning notice at the consumer unit. The On-Site Guide (OSG Reg 6.12) recommends this notice. Note: BS 7671 Reg 514.14 — which previously mandated the notice — was deleted in A4:2026, so citing it on EICR documentation is not appropriate. A replacement regulation is forthcoming.
In three-phase installations, incorrect or missing phase identification on individual conductors. This can lead to phase rotation errors, incorrect neutral identification, and dangerous voltage conditions.
20 pair cable colour codes explained for UK electricians. BS 7671:2018+A4:2026 compliant identification. Quick reference chart and installation rules.
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