Marine Navigation System Power Feed on Offshore Support Vessel

Scenario

Project Type: Retrofit of integrated bridge navigation system (ECDIS, radar, AIS) on a 65-m offshore support vessel. Location Context: North Sea operations — high humidity, salt-laden air, frequent vibration, and ambient temperatures ranging from −5°C to 32°C. Space in cable trays is severely constrained near the bridge console. Constraints: Voltage drop must not exceed 3% of nominal supply (24 V DC), cables must be tinned-copper, XLPE-insulated, and marine-certified (IEC 61892-3). No derating for bundling was permitted due to single-run routing in dedicated conduit.

Given Data

• Voltage: 24 V
• Current: 18.5 A (peak load across all navigation loads)
• Length of Cable: 42 m (measured end-to-end, including 10% allowance for routing offsets)
• Ambient Temperature: 35 °C (summer operational max)
• Conductor Temperature: 70 °C (rated for marine XLPE)

Calculation

Using the Cable Size Calculator:

• Input values applied directly: voltage=24, current=18.5, length=42, ambient_temperature=35, conductor_temperature=70.
• The tool applies IEC 60287-based thermal resistance and resistivity models, adjusting copper resistivity (ρ = 1.724 × 10⁻⁸ Ω·m at 20°C) to 70°C (ρ₇₀ ≈ 2.11 × 10⁻⁸ Ω·m) and applying ambient derating (factor ≈ 0.94 at 35°C).
• Voltage drop is computed as: ΔV = 2 × ρ₇₀ × L × I / A, solving iteratively for minimum A (mm²) satisfying both ampacity (≥18.5 A at 70°C/35°C) and ΔV ≤ 0.72 V (3% of 24 V).
• Tool outputs: cable_size = 6.2 mm², voltage_drop = 0.68 V.

Result and Decision

The calculator recommended 6.2 mm² — commercially available as 6 mm² (standard metric size). However, per IEC 61892-3 Table 12, 6 mm² tinned Cu XLPE has ampacity of 22.3 A at 35°C ambient — sufficient for 18.5 A. Voltage drop (0.71 V @ 6 mm²) remains within limit (0.71 < 0.72 V). Final selection: 6 mm² marine-grade tinned-copper XLPE cable (Type M2G2), installed with double-glanded, IP66-rated terminations.

Lesson

Always validate the next lower standard size against both ampacity and voltage drop — rounding down from calculated 6.2 mm² to 6 mm² was acceptable here, but would have failed if ambient were 45°C or length exceeded 48 m. Never assume “nearest standard size” is safe without rechecking both criteria.