Domestic Lighting Voltage Drop Calculator
Calculate voltage drop for indoor lighting circuits - stricter 3% limit applies
💡 LIGHTING = 3% VOLTAGE DROP LIMIT (NOT 5%)
BS 7671 specifies stricter limits for lighting circuits to prevent visible dimming and LED flicker. Maximum 6.9V drop on 230V supply.
Lighting Circuit Quick Facts
- Lighting circuits have STRICTER 3% voltage drop limit (vs 5% for power)
- Standard domestic lighting: 1.5mm² cable, 6A MCB, radial circuit
- LED lighting draws much less current than traditional - often <1A per circuit
- Voltage drop affects LED dimming performance and can cause flicker
- Maximum 12 lighting points per circuit (guidance, not regulation)
Values from BS 7671 Table 4D1B (PVC copper at 70°C)
📐 3% Voltage Drop Calculation
Lighting (25m, 6A, 1.5mm²)
= (29 × 6 × 25) ÷ 1000
= 4.35V (1.89%)
✓ Under 3% limit
Maximum length at 3%
6.9V = (29 × 6 × L) ÷ 1000
L = 6900 ÷ 174 = 39.7m
Max ~40m with 1.5mm² at 6A
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Common Lighting Circuit Scenarios
Single floor lighting (20m) - 6A
6A with 1.5mm² (29 mV/A/m). At 20m: (29 × 6 × 20) ÷ 1000 = 3.48V (1.51%) ✓
Two-storey house circuit (35m)
6A with 1.5mm² at 35m: (29 × 6 × 35) ÷ 1000 = 6.09V (2.65%) ✓ within 3%
Long run to garage lighting (40m)
6A with 2.5mm² (18 mV/A/m). At 40m: (18 × 6 × 40) ÷ 1000 = 4.32V (1.88%) ✓
LED circuit - actual load 2A (25m)
2A with 1.5mm² at 25m: (29 × 2 × 25) ÷ 1000 = 1.45V (0.63%) ✓ plenty of margin
Kitchen downlights (30m total run)
6A with 1.5mm² at 30m: (29 × 6 × 30) ÷ 1000 = 5.22V (2.27%) ✓
Related Lighting Calculations
For outdoor lighting, see garden lighting. Commercial installations: commercial lighting. For 12V systems: 12V DC voltage drop. Use our cable sizing calculator for current capacity.
Why 3% for Lighting?
BS 7671 specifies a stricter 3% voltage drop limit for lighting circuits (vs 5% for power). This protects against visible dimming, flicker, and reduced light output. Historically set for incandescent lamps which dim noticeably with voltage drop, the limit remains relevant for LED driver performance and dimming stability.
Lighting Circuit Costs (2024)
Typical UK costs for domestic lighting work.
| Work Type | Materials | Labour | Total |
|---|---|---|---|
| New lighting circuit - single room | £50-100 | £150-280 | £200-380 |
| Full floor lighting rewire | £150-300 | £350-600 | £500-900 |
| Kitchen downlights (6 lights) | £200-400 | £300-500 | £500-900 |
| Dimmer switch installation | £30-80 | £60-120 | £90-200 |
| Emergency lighting circuit | £200-400 | £250-450 | £450-850 |
Prices as of 2024. LED fitting costs are additional to circuit installation.
LED Lighting and Voltage Drop
LED drivers typically accept a voltage range (e.g., 200-240V) and regulate output to the LEDs. However, excessive voltage drop can cause: reduced efficiency, dimmer incompatibility, flickering at low voltage, and premature driver failure. Quality LED drivers tolerate more variation, but budget fittings may be sensitive to voltage drop.
Lighting Circuit Design
Domestic lighting circuits typically use 1.5mm² cable with 6A MCB protection. The circuit feeds multiple lighting points in a radial configuration using loop-in, loop-out wiring. Switch drops use 1mm² or 1.5mm² cable. Modern practice increasingly uses LED-compatible dimmers and separate circuits for high-power areas like kitchens.
Calculating Lighting Load
For voltage drop, use actual or expected load, not the circuit rating. A 6A lighting circuit might only draw 2A with LED lighting. However, design for potential future load - someone might replace LEDs with higher-power fittings. For new installations, calculate at circuit rating; for verification, actual load is acceptable.
Frequently Asked Questions
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