BS 7671, also known as the IET Wiring Regulations, provides guidelines for cable sizing to ensure electrical safety and performance in the United Kingdom. Proper cable sizing is crucial to prevent overheating, voltage drop, and other electrical issues. The cable size is determined based on several factors, including the maximum current the cable will carry, the voltage, the installation method, and the ambient temperature. Here is a general procedure for cable sizing according to BS 7671:
Determine the Maximum Current (Ib):
Identify the maximum current the cable will carry. This is typically based on the connected load and the type of equipment it supplies. For single-phase loads, Ib is the current rating of the equipment. For three-phase loads, Ib is the current rating of the largest phase.
Correction Factors:
Apply correction factors for various installation conditions:
Temperature: Consider the ambient temperature where the cable will be installed. Correct for high or low temperatures using the appropriate factor.
Grouping: If multiple cables are installed in close proximity, apply a grouping factor.
Thermal insulation: Consider the thermal insulation of the cable, if applicable.
Depth of burial: If the cable is buried underground, consider the depth and apply the appropriate correction factor.
Diversity and Load Factor:
If multiple circuits are supplied by a single cable or conductor, consider diversity (the likelihood of all circuits operating simultaneously) and apply load factors as necessary.
Voltage Drop:
Calculate the allowable voltage drop for the installation. This is typically specified in the relevant regulations or standards, such as a maximum of 3% for lighting circuits.
Select the Cable Type:
Choose the appropriate type of cable based on the application (e.g., PVC, XLPE, SWA for different environments and conditions).
Cable Sizing Calculation:
Calculate the cable size using the following formula:
Cable Size (in mm²) = (Ib × Correction Factors) / (Current-Carrying Capacity Factor × Voltage Drop Factor)
Where:
Ib is the maximum current.
Correction Factors account for ambient temperature, grouping, insulation, depth of burial, etc.
Current-Carrying Capacity Factor represents the derating factor for the cable type and installation method (provided in tables in BS 7671).
Voltage Drop Factor accounts for the allowable voltage drop.
Select the Nearest Standard Cable Size:
Choose a cable size from the standard available sizes that meets or exceeds the calculated size.
Check for Short-Circuit Protection:
Ensure that the cable is adequately protected against short-circuit currents by selecting appropriate protective devices (e.g., circuit breakers or fuses).
Documentation:
Maintain records of the cable sizing calculations, including cable types, sizes, and installation details.
It's important to note that the actual cable sizing calculations may vary depending on the specific circumstances and the type of installation. Therefore, it's crucial to refer to BS 7671 for detailed guidance and consult with a qualified electrical engineer or professional experienced in electrical design and installation when determining cable sizes for specific projects.
