Calculate circuit capacity, cable sizing, voltage drop, and diversity for LED, CFL, fluorescent, and halogen lighting installations. Professional BS 7671 compliant tool for UK electricians.
Apply BS 7671 diversity factors (domestic 66%, commercial tiered) and automatic control gear loss calculations for LED (10%), CFL/fluorescent (15%).
Automatic cable size recommendations from Table 4D5. Voltage drop compliance checking against 3% limit (Regulation 525) with detailed mV/A/m calculations.
Verify Ib ≤ In ≤ Iz compliance (Regulation 433.1). Real-time circuit utilization monitoring with warnings at 80%, 90%, 95% thresholds. MCB type recommendations.
Total: 8 fittings
This calculator is provided for informational and educational purposes only. While we strive for accuracy and compliance with BS 7671:2018+A2:2022 regulations, calculations should be verified by a qualified electrician.
Professional Verification Required: All electrical work must be designed, installed, tested, and certified by a competent person in accordance with BS 7671 (18th Edition) and Building Regulations Part P.
No Liability: London Electrical Distributors and its contributors accept no responsibility for any loss, damage, or injury arising from the use of this calculator. Results are estimates and may vary based on specific installation conditions, environmental factors, and local regulations.
Use at Your Own Risk: By using this tool, you acknowledge that electrical design and installation carries inherent risks and should only be undertaken by qualified professionals with appropriate insurance and certification.
Every circuit must satisfy the fundamental protection equation: Ib ≤ In ≤ Iz
This ensures the circuit can safely carry the design load, the MCB will protect against overload, and the cable won't overheat.
Diversity acknowledges that not all lighting circuits operate at full load simultaneously:
Applying diversity reduces the design current, allowing more efficient MCB and cable sizing while maintaining safety.
Modern lighting uses electronic drivers/ballasts that consume additional power:
Always calculate circuit load using the total power drawn from the circuit, not just the lamp wattage.
BS 7671 limits voltage drop to 3% of nominal voltage for lighting circuits (6.9V at 230V):
MCB type determines the magnetic trip threshold:
Using Type B with >20 fittings may cause nuisance tripping during switch-on due to cumulative inrush current. Consider Type C or stagger switching.
BS 5266 requires:
Lighting circuit capacity is calculated by multiplying the MCB rating (In) by the supply voltage (230V) and applying diversity factors if applicable. For example, a 10A MCB provides 2,300W capacity. With domestic diversity (66% per BS 7671), the design capacity would be approximately 1,518W. The calculator considers total connected load, control gear losses (10-15% for LED/CFL), and diversity factors to determine if the circuit is within safe operating limits.
BS 7671 Appendix 1, Section 1 specifies diversity factors for lighting installations. Domestic: 66% (assume only 66% of lights on simultaneously). Commercial: Tiered approach - first 10 fittings at 100%, next 10 at 75%, next 10 at 50%, remaining at 25%. Industrial/Emergency: No diversity (100%) - all lights must be able to operate simultaneously. Diversity reduces the design load and allows more efficient circuit design while maintaining compliance.
Control gear losses refer to power consumed by drivers, ballasts, and control equipment in addition to the lamp itself. LED fittings have approximately 10% losses (multiply by 1.1), while CFL and fluorescent fittings have 15% losses (multiply by 1.15). Halogen has no control gear (multiply by 1.0). For example, a 50W LED panel actually draws 55W from the circuit. This calculator automatically applies appropriate control gear factors based on fitting type to ensure accurate load calculations.
Cable sizing must satisfy two criteria per BS 7671: (1) Current-carrying capacity (Iz) must be ≥ MCB rating (In) and design current (Ib), and (2) Voltage drop must be ≤ 3% (6.9V) of supply voltage. Typical sizes: 1.0mm² for domestic lighting (up to 13.5A), 1.5mm² for commercial (up to 17.5A), 2.5mm² for industrial/long runs (up to 24A). The calculator recommends the minimum size meeting both requirements and checks voltage drop compliance if cable length is provided.
MCB type determines the magnetic trip threshold for protection against short circuits. Type B (3-5× In) is standard for general lighting with low inrush. Type C (5-10× In) is recommended for LED and fluorescent circuits with moderate inrush from control gear. Type D (10-20× In) is for halogen circuits with very high cold-filament inrush (up to 12× running current). Using Type B with >20 fittings may cause nuisance tripping during switch-on. This calculator warns when MCB type may not suit the installation.