Construction Site Voltage Drop Calculator
Calculate voltage drop for construction site temporary electrical installations
⚠️ BS 7671 SECTION 704 APPLIES - TEMPORARY INSTALLATIONS
Construction sites require 110V CTE for portable tools, 30mA RCD on all sockets, IP44 minimum equipment rating, and 3-monthly inspection intervals.
Construction Site Quick Facts
- BS 7671 Section 704 covers construction and demolition site installations
- Reduced low voltage (110V CTE) required for portable tools in most situations
- Site distribution boards must be robust (IP44 minimum) and clearly identified
- All socket outlets require 30mA RCD protection - no exceptions
- Cable routes must be protected against mechanical damage from site activities
Values from BS 7671 Table 4D1B (PVC copper at 70°C)
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Common Construction Site Scenarios
Main site supply to welfare (30m 3Ø)
63A with 16mm² (2.8 mV/A/m). At 30m 3Ø: (2.8 × 63 × 30 × 0.866) ÷ 1000 = 4.58V (1.15% on 400V) ✓
Tower crane supply (60m 3Ø)
100A with 35mm² (1.25 mV/A/m). At 60m 3Ø: (1.25 × 100 × 60 × 0.866) ÷ 1000 = 6.50V (1.63% on 400V) ✓
110V transformer feed (40m)
32A with 6mm² (7.3 mV/A/m). At 40m: (7.3 × 32 × 40) ÷ 1000 = 9.34V (4.06%) ✓
Site hoist supply (50m 3Ø)
63A with 25mm² (1.75 mV/A/m). At 50m 3Ø: (1.75 × 63 × 50 × 0.866) ÷ 1000 = 4.78V (1.19% on 400V) ✓
Portable distribution board (80m run)
32A with 10mm² (4.4 mV/A/m). At 80m: (4.4 × 32 × 80) ÷ 1000 = 11.26V (4.90%) ⚠ borderline
Related Industrial Calculations
For three-phase motor circuits powering cranes and hoists. Warehouse distribution principles apply to large site compounds. Use our cable sizing calculator for current capacity.
BS 7671 Section 704 Requirements
Section 704 applies to temporary installations for construction and demolition work. Key requirements include: all socket outlets protected by 30mA RCDs, reduced low voltage (110V CTE) for portable hand tools, robust equipment suitable for site conditions (IP44 minimum), and cables protected against mechanical damage. Supplies are temporary but must still meet full safety standards.
Construction Site Electrical Costs (2024)
Typical UK costs for construction site temporary installations.
| Installation Scenario | Materials | Labour | Total |
|---|---|---|---|
| Temporary site supply connection | £500-1500 | £600-1200 | £1100-2700 |
| Main site distribution board | £800-2000 | £500-1000 | £1300-3000 |
| Sub-distribution board (per unit) | £300-700 | £200-400 | £500-1100 |
| 110V transformer (5kVA) | £400-800 | £150-300 | £550-1100 |
| Site rewire/relocation | £200-600 | £300-600 | £500-1200 |
Prices as of 2024. DNO connection charges are additional. Costs vary significantly by site size.
110V Centre-Tapped Earth (CTE) System
The 110V CTE system provides 55V to earth from any conductor - significantly safer than 230V if contact occurs. Yellow 110V equipment is standard for UK construction sites. The transformer has a centre-tapped secondary with 110V across the outer conductors. 16A (yellow) and 32A (yellow) plugs/sockets have different configurations to prevent cross-connection with 230V.
Site Distribution Board Requirements
Site distribution boards must be: robustly constructed for site conditions, rated IP44 minimum (higher for exposed locations), clearly marked with voltage and purpose, equipped with main isolation, protected by RCDs on all socket circuits, and positioned to minimise cable runs while remaining accessible. Boards should be secured against theft and vandalism.
Cable Protection on Sites
Construction sites present multiple cable damage risks: vehicles, excavation, moving materials, and general site activity. Cables must be protected by: burial (600mm min with markers), overhead routing clear of traffic, robust conduit or trunking at ground level, or routing away from high-risk areas. Cables across access routes need heavy-duty protection or ramped covers.
Frequently Asked Questions
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