Cable Selection and Erection: Ratings, Colours, Concealed Cables

Harmonised conductor identification

• Line (single phase): brown
• Neutral: blue
• Circuit protective conductor (earth): green-and-yellow
• Three-phase lines L1, L2, L3: brown, black, grey

Coordinating cable and protective device

For overload protection, check Ib <= In <= Iz:

• Ib is the circuit's design current.
• In is the protective device's rated current.
• Iz is the cable's effective current-carrying capacity in its installed conditions.

Also check I2 <= 1.45 Iz. I2 is the current that makes the device operate within its conventional time. In is a rating: the current can rise above In for a while before the device operates.

Correction (rating) factors

Use the rating table and factors for the cable's construction and installed conditions. Where the chosen rating already allows for a condition, do not apply that condition again. Common factors are:

• Ca for ambient temperature
• Cg for grouping with other circuits
• Ci for thermal insulation around the cable
• Cc = 0.725 where a BS 3036 semi-enclosed (rewirable) fuse gives overload protection. Cc is also used in buried-cable rating calculations, so follow the applicable table.

Voltage drop

For an installation supplied from the public low voltage network, the recommended maximum voltage drop from the origin to the point of use is:

• 3% for lighting circuits
• 5% for other uses such as power

Insulation operating temperatures

• General-purpose thermoplastic (PVC) insulation: 70 °C maximum continuous conductor temperature
• Standard thermosetting (XLPE) insulation: 90 °C maximum continuous conductor temperature. The terminals it connects to may set a lower limit.

IP ratings

The two-digit IP code describes enclosure protection:

• the first numeral covers solid objects and access to live parts
• the second numeral covers ingress of water

Cables concealed in walls

A cable buried less than 50 mm deep in a wall needs one of these:

• a qualifying earthed metallic covering, or metallic conduit/trunking that meets the protective-conductor requirements
• mechanical protection that resists penetration
• a route in a prescribed safe zone, plus 30 mA RCD additional protection

Where a wall or partition has metal parts inside it, such as metal studs, the concealed-cable protection rules apply at any depth. Metal fixings such as nails and screws do not count as metal parts here. For ordinary low-voltage wiring, provide the required 30 mA RCD protection or a permitted protective wiring arrangement. An edge grommet stops abrasion, but it gives no electrical protection.

Identification in practice

• Older installations use the colours in force before 2004: red for line and black for neutral, with red, yellow and blue for the three phases.
• A notice describing mixed colour codes can help later workers. The notice used to be mandatory, but Amendment 2 deleted that requirement. Mixed colours alone are no reason to rewire.
• Where a core may be used for another function, it needs the correct identification at its terminations. Never use green-and-yellow as a line conductor.
• Green-and-yellow is reserved for protective conductors. A colour or sleeve shows what a conductor should be, not that it is connected correctly: verify its function and continuity, and prove dead before you work.
• Sleeve the bare protective conductor of flat twin-and-earth in green-and-yellow wherever it leaves the sheath.
• Where a blue core is used as a switched line, identify it with brown sleeving or marking at every termination.

How installed conditions change a cable's rating

• A cable's rating depends on how easily its heat can escape. Loaded cables next to each other heat each other, and thermal insulation around a cable slows its cooling. Choose the matching installation method; do not assume the clipped-direct rating applies.
• Without more precise information, rate a single cable totally surrounded by thermal insulation for 0.5 m or more at half its clipped-direct capacity. Short lengths have their own factors, with their own conductor-size and insulation conditions. Design for the worst section, or change the route.
• Where a 90 °C thermosetting cable connects to an accessory rated for 70 °C, base the rating at that termination on 70 °C. Otherwise the terminals overheat.
• Where a BS 3036 fuse gives overload protection, its operating characteristic needs the 0.725 allowance in cable selection.
• Conductor material affects resistance, current rating and which terminations suit it. It is a design choice, not just a cost.
• For simple overload sizing, divide In by the product of the factors that apply: It ≥ In ÷ (Ca × Cg × Ci × Cc), where It is the cable's tabulated rating. A factor that does not apply is 1. Never count a condition the table already includes.

Worked example. A 20 A device protects a circuit with Ca = 0.87 and Cg = 0.70. The cable's tabulated rating must be at least 20 ÷ (0.87 × 0.70) = 20 ÷ 0.609 = about 32.8 A.

Working out voltage drop

For a public LV supply, the usual 3% and 5% recommendations run from the origin of the installation to the terminals of the fixed current-using equipment. So they include the distribution circuit as well as the final circuit. Too much drop leaves equipment short of voltage under load.

To work it out, multiply the cable's tabulated millivolts per amp per metre by the design current and the length. Divide by 1,000 to get volts.

Worked example. A 30 m run from the installation origin, with no drop upstream, uses cable at 11 mV/A/m and carries 25 A. It drops 11 × 25 × 30 = 8,250 mV, which is 8.25 V. That is inside the 11.5 V (5% of 230 V) allowance for non-lighting use. It is over the 6.9 V (3%) allowance for lighting. This checks voltage drop only: you still need to check equipment requirements, current capacity and fault protection.

Enclosures, routes and fire

• IP2X means fingers cannot reach hazardous parts and solid objects of 12.5 mm or larger cannot get in, with a separate test for each. A first numeral of 4 keeps out objects of 1.0 mm and more, such as a wire. Cable entries, glands and grommets must keep the enclosure's rating.
• A prescribed zone runs within 150 mm of the top of a wall or partition, or of an internal corner. Other zones run horizontally or vertically in line with a visible accessory or connection point. They are not a 150 mm border around the accessory.
• In a dwelling, a cable concealed less than 50 mm deep in a zone also needs 30 mA RCD protection, unless it has an earthed metallic covering.
• Outside a zone, use a qualifying earthed metallic covering, metallic conduit/trunking that meets the protective-conductor requirements, or protection strong enough to stop nails and screws. Otherwise later drilling may hit a live cable nobody expected.
• Protect cables from sharp edges where they pass through enclosures, studs or joists. Use grommets, bushes or other protection that suits the material and wiring method.
• Support wiring throughout the installation so it cannot collapse early in a fire. The supports need suitable fixings and spacing, and non-metallic clips or trunking alone are not enough. Where a cable passes through a fire-separating wall or floor, seal the hole to restore its fire resistance.
• Where Band I and Band II circuits share a wiring system, check the required segregation, barriers or insulation. SELV and PELV protective-separation requirements still apply: check them specifically, because a general highest-voltage statement does not cover them.
• Choose cables and accessories for their surroundings: heat, water, sunlight, chemicals and mechanical damage. Buried cables need a suitable route, depth, marking and mechanical protection.
• Battery systems can keep conductors live after the grid supply is isolated. Work out fault current and protection from the actual source, circuit and equipment data. Provide suitable cable support, mechanical protection, terminations and cooling.
• Powered data cables also heat up through their resistance, especially in bundles.