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Lighting Circuit Calculator

Cable sizing and protection for lighting circuits

Reference Info & Formulas
Quick Reference

Standard cable: 1.5mm²

Standard MCB: 6A or 10A

Max VD: 3%

Typical max: 12 points

LED Inrush

LED drivers can have very high inrush current (up to 20× steady state).

Consider Type C MCB for large LED installations.

Circuit Details
Enter lighting circuit parameters
W
m

To furthest point

Data: BS 7671:2018+A2:2022 — Table 4D5, Regulation 521

For guidance only. The responsibility for any electrical installation lies with the qualified person carrying out the work. Always verify calculations independently and apply professional judgement.

How This Calculator Works

Lighting circuit design accounts for inrush current from LED drivers and electronic ballasts, not just steady-state load.

Inrush current from LED drivers can be 40-100 times the steady-state current for less than 1ms at switch-on. While the design current is calculated from the steady-state load, the MCB type must be chosen to tolerate cumulative inrush peaks without nuisance tripping. The calculator sizes the cable based on running load and checks that the selected MCB type can handle the total inrush from all fittings on the circuit.

BS 7671 assumes a minimum of 100W per lampholder for design purposes, regardless of the actual LED wattage. This provides a built-in safety margin and simplifies load estimation where final fitting choices have not been confirmed.

Quick Reference — Lighting Loads and MCB Selection

Typical lighting types and MCB recommendations

BS 7671 Section 559
Fitting TypeWatts EachInrushMCB Type
LED fittings5–15W40–60× for <1msType B or C
Fluorescent T5/T814–58W10–20× for <5msType B
Halogen20–50WNo significant inrushType B
HID (Metal Halide/SON)70–400WHigh starting currentType C

Inrush multiples are approximate and vary by manufacturer. Always check product data sheets.

Practical Notes

Cumulative LED Inrush

LED inrush is cumulative across all fittings on the circuit. Twenty LED downlights each drawing a 40A inrush peak can produce a combined peak of 800A, far exceeding the instantaneous trip of a 6A Type B MCB (30A). Check manufacturer C-value ratings or switch to Type C protection.

Minimum Assumed Load

BS 7671 allows a minimum assumed load of 100W per lampholder for design purposes, even when the actual LED load is much lower. This provides a generous safety margin and accounts for future lamp replacements with higher-wattage fittings.

Voltage Drop Limit for Lighting

Lighting circuits are limited to 3% voltage drop (6.9V at 230V), stricter than the 5% limit for power circuits. On long domestic runs this can be the governing factor in cable selection, particularly with 1.0mm² cables.

Frequently Asked Questions

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