Marina & Boat Shore Power Voltage Drop Calculator
Calculate voltage drop for marina shore power with BS 7671 Section 709 stricter 3% limit
⚠️ STRICTER 3% VOLTAGE DROP LIMIT APPLIES
BS 7671 Section 709 requires maximum 3% voltage drop for marina installations (not the standard 5%). Calculator below is set to "Lighting Circuit" mode which uses the 3% limit.
Marina Shore Power Quick Facts
- BS 7671 Section 709 applies to marinas and shore connections for boats
- STRICTER 3% voltage drop limit applies (not standard 5%) - same as lighting
- Standard shore supply: 16A (3.7kW) or 32A (7.4kW) single-phase outlets
- Pontoon feeder cables often run 100m+ requiring careful voltage drop calculation
- PME earthing prohibited for shore supplies - TT earthing system required
Values from BS 7671 Table 4D1B (PVC copper at 70°C)
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Common Marina Installation Scenarios
16A shore outlet - close berth (30m)
16A with 4mm² (11 mV/A/m). At 30m: (11 × 16 × 30) ÷ 1000 = 5.28V (2.30%) ✓ under 3%
16A shore outlet - far pontoon (60m)
16A with 6mm² (7.3 mV/A/m). At 60m: (7.3 × 16 × 60) ÷ 1000 = 7.01V (3.05%) ⚠ borderline
32A shore outlet - medium run (40m)
32A with 10mm² (4.4 mV/A/m). At 40m: (4.4 × 32 × 40) ÷ 1000 = 5.63V (2.45%) ✓ under 3%
Pontoon feeder - long run (100m)
63A with 25mm² (1.75 mV/A/m). At 100m: (1.75 × 63 × 100) ÷ 1000 = 11.03V (2.76% on 400V 3Ø) ✓
Multiple berths - diversity applied
Apply 10% diversity per additional berth (max 50% reduction). Size feeder for maximum demand scenario.
Related Specialist Installations
Caravan park installations have similar Section 708 requirements with PME restrictions. For standard outdoor installations, see our submain calculator. Use our cable sizing calculator for current capacity checks.
BS 7671 Section 709 Requirements
Section 709 covers marinas and similar locations. Key requirements: maximum 3% voltage drop (stricter than general 5%), PME earthing prohibited (TT system required), 30mA RCD protection mandatory, IP44 minimum for equipment, specific socket outlet types (typically blue CEE), and isolation facilities at each berth. These requirements address the unique hazards of electrical supply near water.
Marina Electrical Installation Costs (2024)
Typical UK costs for marina electrical infrastructure. TT earthing required.
| Installation Scenario | Materials | Labour | Total |
|---|---|---|---|
| Single 16A shore outlet | £200-400 | £300-500 | £500-900 |
| Single 32A shore outlet | £300-550 | £400-650 | £700-1200 |
| Pontoon feeder (per 50m) | £800-1500 | £500-900 | £1300-2400 |
| Shore supply pillar (2-4 outlets) | £1500-3000 | £800-1500 | £2300-4500 |
| TT earth electrode system | £200-500 | £300-600 | £500-1100 |
Prices as of 2024. Marina installations vary significantly by site conditions and scale.
Why Marinas Have Stricter Voltage Drop Limits
The 3% voltage drop limit in marinas exists because boats are sensitive to supply voltage quality. Marine equipment, battery chargers, and navigation systems may malfunction with low voltage. Additionally, long pontoon runs compound with boat cable lengths to the final equipment. The stricter limit ensures adequate voltage reaches equipment even at end of long supply chains.
TT Earthing Requirement
PME (Protective Multiple Earthing) is prohibited for marina shore supplies because a broken PEN conductor could make boat hulls live while in water - potentially fatal. TT earthing uses a local earth electrode with all metal parts bonded. This isolates the shore supply earth from the distribution network, protecting people in contact with water around boats.
Shore Supply Pillar Specifications
Marina supply pillars must comply with BS EN 61439-5 (marina feeder pillars). They contain: IP44 or better enclosure, RCD protection for each outlet, individual MCB/RCBO per outlet, emergency isolation, metering facilities if required, and weatherproof construction. Position pillars to minimise cable lengths to berths while maintaining safe clearances from water.
Frequently Asked Questions
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