Submain Voltage Drop Calculator for Outbuildings
Check voltage drop compliance for submain cables to garden offices, garages, workshops, and outbuildings
Submain Voltage Drop Quick Facts
- Submains must comply with 5% voltage drop limit from origin to furthest point
- Combined voltage drop (submain + final circuit) must stay under 5%
- Typical submain sizes: 10mm² (4.4 mV/A/m), 16mm² (2.8 mV/A/m), 25mm² (1.75 mV/A/m)
- 32A or 40A supply covers most outbuilding needs
- Voltage drop often determines cable size, not current capacity
Values from BS 7671 Table 4D1B (PVC copper at 70°C)
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Common Submain Installations
Garden office (20-30m from house)
32A submain with 10mm² SWA. At 25m: (4.4 × 32 × 25) ÷ 1000 = 3.52V (1.53%) ✓
Detached garage/workshop (30-40m)
32A or 40A with 10mm² or 16mm². 40A at 35m with 16mm²: 3.92V (1.70%) ✓
Granny annexe/self-contained unit
63A submain with 16mm² or 25mm² SWA. 63A at 30m with 25mm²: 3.31V (1.44%) ✓
Shed with lighting and sockets
16A or 20A radial with 4mm² or 6mm² SWA. 16A at 20m with 4mm²: 3.52V (1.53%) ✓
Agricultural building (100m+)
Consider three-phase or 25mm²+. 63A at 100m with 25mm²: 11.03V (4.79%) - just compliant
Complete Your Outbuilding Setup
Planning an EV charger in your garage? Check voltage drop for the charging circuit too. For garden offices with extensive lighting, use our garden lighting calculator with the stricter 3% limit. Need to size the cable? Try our cable sizing calculator.
Submain Voltage Drop
Submain voltage drop is the reduction in voltage along the cable feeding an outbuilding from your main consumer unit. BS 7671 limits total voltage drop to 5% for power circuits (11.5V on 230V supply). This includes both the submain AND any final circuits in the outbuilding. Long cable runs require larger conductors to keep voltage drop within limits.
Submain Installation Costs (2024)
Typical UK installation costs including SWA cable, consumer unit, and labour.
| Installation Scenario | Materials | Labour | Total |
|---|---|---|---|
| Short run submain (<15m) | £180-300 | £250-350 | £430-650 |
| Medium run (15-30m) 10mm² SWA | £300-500 | £350-500 | £650-1000 |
| Long run (30-50m) 16mm² SWA | £500-800 | £450-600 | £950-1400 |
| Very long run (50m+) 25mm² SWA | £700-1200 | £500-750 | £1200-1950 |
| Consumer unit in outbuilding | £150-350 | £200-350 | £350-700 |
Prices as of 2024. Actual costs vary by region and ground conditions for cable burial.
Why Submain Voltage Drop Matters
Excessive voltage drop causes equipment problems: motors run hot and inefficient, lights dim noticeably, and sensitive electronics may malfunction. For outbuildings with power tools, EV chargers, or workshop equipment, adequate voltage is essential. Voltage drop compounds - if your submain uses 3% of the allowance, final circuits only have 2% remaining.
Voltage Drop Allocation Strategy
Best practice allocates voltage drop between submain and final circuits. A common approach: 3% for submain, 2% for final circuits. This ensures the total stays under 5%. For lighting circuits in outbuildings, remember lighting has a stricter 3% limit - plan submain voltage drop accordingly.
Frequently Asked Questions
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