BS7671 Comprehensive Cable Calculator
• Appendix 4: Current-carrying capacity and voltage drop for cables
Circuit Parameters
ⓘ HelpCable Selection
ⓘ HelpInstallation Method
ⓘ HelpDerating Factors
ⓘ HelpResults
Current Capacity
Voltage Drop
Applied Derating Factors
Compliance Status
Circuit Type: Determines the maximum permitted voltage drop:
• Lighting circuits: max 3% drop
• Other final circuits: max 5% drop
• Distribution circuits: varies based on installation requirements
Design Current (Ib): The current intended to be carried by the circuit in normal service. This is used to determine the required cable size and protection device.
Power Factor: The ratio of real power to apparent power. Affects voltage drop calculations for larger cables.
Conductor Material:
• Copper: Higher conductivity, smaller size, more expensive
• Aluminum: Lower cost, larger size for same current, special terminations required
Insulation Type:
• Thermoplastic 70°C (PVC): Standard for general wiring
• Thermosetting 90°C (XLPE): Higher temperature rating allows higher currents
• Mineral: Fireproof, high temperature resistance
Cable Type: Affects current-carrying capacity and installation options.
• Single-core: Flexible installation, higher ratings for larger sizes
• Multicore: Easier installation, built-in phase grouping
• Armored: Mechanical protection, suitable for harsh environments
BS7671 defines several reference methods that determine the base current-carrying capacity:
Method A: Enclosed in conduit in thermally insulating wall
Method B: Enclosed in conduit on a wall or in trunking
Method C: Clipped direct to a surface
Method D: Direct burial in ground or in underground ducts
Method E: In free air or on perforated cable tray (multicore)
Method F: In free air or on perforated cable tray (single-core)
Ambient Temperature: Reference temperatures are 30°C for above ground and 20°C for buried cables. Deviations require applying the Ca factor.
Ambient Temperature (Ca): Cables in hotter environments can carry less current as they dissipate heat less effectively.
Grouping (Cg): When multiple cables are installed together, they heat each other, reducing their current-carrying capacity.
Thermal Insulation (Ci): Cables surrounded by thermal insulation cannot dissipate heat effectively and must be derated.
Soil Conditions (Cs): For buried cables, soil with higher thermal resistivity reduces current-carrying capacity.
Depth of Burial (Cd): Deeper buried cables have reduced current-carrying capacity due to poorer heat dissipation.
Harmonics: Harmonic currents cause additional heating, particularly in the neutral conductor of three-phase systems.