How to identify conductors and coordinate a cable with its protective device, including derating, voltage drop, insulation temperatures, IP ratings 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