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BS7671 Comprehensive Cable Calculator

BS7671 Comprehensive Cable Calculator

Reference (BS 7671:2018+A2:2022)
• Appendix 4: Current-carrying capacity and voltage drop for cables

Circuit Parameters

ⓘ Help

Cable Selection

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Installation Method

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Above ground: 30°C reference. Underground: 20°C reference.

Derating Factors

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% of 3rd harmonic or THD of line current

Results

Current Capacity

Base Current Rating (It):
-- A
Derated Current Capacity (Iz):
-- A
Design Current (Ib):
-- A
Protection Rating (In):
-- A

Voltage Drop

Voltage Drop (mV/A/m):
-- mV/A/m
Total Voltage Drop:
-- V
Percentage Drop:
-- %
Maximum Permitted:
-- %
Conductor Resistance:
-- Ω
Conductor Reactance:
-- Ω
Impedance:
-- Ω

Applied Derating Factors

Ambient Temperature (Ca):
--
Grouping (Cg):
--
Thermal Insulation (Ci):
--

Compliance Status

Checking current capacity compliance...
Checking voltage drop compliance...
Checking overall compliance...
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Circuit Parameters

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.

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Cable Selection

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

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Installation Methods

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.

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Derating Factors

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.