Selection and Erection of Cables: Ratings, Colours and Concealed-Cable Protection

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

The rule for overload protection is Ib <= In <= Iz, where Ib is the design current of the circuit, In is the rated current of the protective device, and Iz is the effective current-carrying capacity of the cable in its installed conditions.

Correction (rating) factors

The tabulated rating of a cable must be derated by factors that reflect the installed conditions before it is compared with In:

• Ca for ambient temperature
• Cg for grouping with other circuits
• Ci for thermal insulation around the cable
• a factor of 0.725 where a BS 3036 semi-enclosed (rewirable) fuse protects the circuit

Voltage drop

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

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

Insulation operating temperatures

• Thermoplastic (PVC) insulation: 70 °C maximum conductor temperature
• Thermosetting (XLPE) insulation: 90 °C maximum conductor temperature

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 must be protected by one of:

• an earthed metallic covering
• mechanical protection that resists penetration
• installation in a prescribed safe zone together with 30 mA RCD additional 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. Where old and harmonised colours both appear in one installation, fix a warning notice at or near the consumer unit saying so.
• Green-and-yellow is reserved for protective identification, but a colour or sleeve does not prove that the conductor has been correctly connected. Verify its function and continuity, and prove dead before work. Sleeve the bare protective conductor of flat twin-and-earth green-and-yellow wherever it leaves the sheath, and identify a blue core used as a switched line with brown sleeving or marking at every termination.

How installed conditions change a cable's rating

• A cable's rating depends on how well it sheds heat. Grouped cables warm each other, thermal insulation traps the heat, and conduit in an insulated wall does both, so the same cable reaches its temperature limit at a lower current than when clipped to an open wall.
• A cable totally surrounded by thermal insulation for 0.5 m or more is taken to carry about half its clipped-direct rating; shorter lengths get a less severe factor. Size the whole run on its worst section, or reroute it.
• Where a 90 °C thermosetting cable connects to an accessory rated for 70 °C, base the rating at that termination on 70 °C, so the terminals do not overheat.
• A rewirable BS 3036 fuse needs a larger overload for longer before it melts, so the cable must survive that longer overload: that is what its 0.725 factor allows for.
• Conductor material affects resistance, current rating and which terminations suit it, so it is a design choice, not just a cost.
• To find the minimum tabulated rating, divide the device rating by the product of the factors that apply: It ≥ In ÷ (Ca × Cg × Ci × Cf).

Worked example (illustrative figures). A 20 A device on a circuit with Ca = 0.87 and Cg = 0.70 needs a cable whose tabulated rating is at least 20 ÷ (0.87 × 0.70) = 20 ÷ 0.609 = about 32.8 A.

Working out voltage drop

The 3% and 5% limits 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. Multiply the cable's tabulated millivolts per amp per metre by the design current and the length, then divide by 1,000 for volts.

Worked example (illustrative figures). A 30 m run at 11 mV/A/m carrying 25 A drops 11 × 25 × 30 = 8,250 mV, which is 8.25 V. That is inside the 11.5 V (5% of 230 V) limit for power, but it would fail the 6.9 V (3%) limit for lighting.

Enclosures, routes and fire

• IP2X means the enclosure keeps out the standard test finger and solid objects of 12.5 mm and more; 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 a corner, and horizontally or vertically in line with a point or accessory. In a dwelling, a cable concealed less than 50 mm deep in a zone, without an earthed metallic covering, also needs 30 mA RCD protection. Outside a zone, use an earthed metallic covering, earthed conduit or trunking, or mechanical protection against nails and screws, because later drilling may otherwise hit an unexpected live cable.
• Fit a grommet or bush wherever a cable passes a metal edge, stud or drilled joist, so the insulation cannot be cut or worn through.
• Support wiring so it cannot collapse early in a fire and block escape or firefighting: non-metallic clips or trunking alone are not enough. Where a cable passes through a fire-separating wall or floor, seal the penetration to restore its fire resistance.
• Keep extra-low-voltage and data (Band I) circuits segregated from mains (Band II) circuits unless everything is insulated for the highest voltage present.
• 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.
• A home battery or standby battery that forms part of the installation can energise conductors, deliver very large DC fault currents and stay live when the mains is isolated; its cables need mechanical protection and room to lose heat. Large bundles of powered data cables can also warm up.