Refitting the 12/24 V electrical system
Refitting a 12/24 V system — choosing cable, protection, crimped joints, corrosion and safety rules.
A yacht's electrical installation works in exceptionally hard conditions: damp, salt, vibration and movement accelerate corrosion and loosen connections. An old installation extended over the years is often a tangle of cables of different sections, with makeshift joints and no documentation. The refit starts with an inventory of the circuits and drawing a diagram, because without that it is hard to modernise the system safely. The aim is an installation that is clear, properly protected and resistant to corrosion.
Cables on a boat should be stranded, not solid, because vibration breaks rigid conductors. Best are cables with tinned strands, which resist corrosion in a salty environment far better than bare copper. The cross-section is chosen to suit the current and the length of the circuit, allowing for the permissible voltage drop — at 12 V even a small resistance causes a noticeable drop, so longer circuits need heavier cable. Under-sizing the cable risks overheating and fire.
Every circuit must be protected by a fuse or circuit breaker matched to the cable's cross-section, placed as close as possible to the source of supply (the battery positive). The protection protects the cable, not just the appliance — it is the cable that can catch fire in a short circuit. The battery should have a main switch and a main fuse on the positive lead. The battery terminals and connections must be secure, clean and protected, because that is where the largest currents flow.
Connections are made with good-quality crimp terminals, ideally with adhesive-lined heat-shrink, which seals them and takes the mechanical strain. Soldering as the only joint is generally discouraged where there is vibration, because it stiffens the conductor and encourages breaks just beyond the joint; if you do solder, provide strain relief. Crimping calls for a good crimping tool and terminals matched to the cable section. A good joint is mechanically secure and has low resistance — loose contacts heat up and oxidise.
Cable runs should be tidy: looms secured every 10–20 centimetres, away from sources of heat and sharp edges, in places where water will not collect. Passages through bulkheads are protected with grommets, so the insulation cannot chafe through. It is worth labelling every cable (a tag or a number) to match the diagram, which makes fault-finding easier. Earth (negative) connections must be secure, because a corroded earth is the cause of many faults that are hard to trace.
Corrosion is the main enemy of a marine installation, which is why panels and connections have to be sited in dry places and exposed contacts protected with moisture-repelling products. Galvanic corrosion and stray currents also have to be managed deliberately, especially in installations with shore power (230 V) and where sacrificial anodes are involved. Batteries need ventilation (especially lead-acid ones, which give off hydrogen) and secure mounting so that they cannot move when the boat heels. The choice and placement of components has a real effect on durability and safety.
Although a 12/24 V system is low voltage, it is not without risk: a short across the battery gives an enormous fault current capable of melting tools, setting a cable alight and burning you. Work with the supply disconnected, take off jewellery, use insulated tools and cover the terminals. Working on a 230 V shore installation requires knowledge and qualifications — that voltage kills. It is worth carrying out the refit to recognised industry standards and, in case of doubt, consulting a marine electrician.
Material prices — products and options
Indicative prices in EUR (incl. VAT), as of July 2026. Products widely available in Europe; prices depend on the shop, the pack size and consumption. The links lead to example shops that ship across the EU. Dollar amounts are converted at 1 EUR = 1.1622 USD (ECB, 7 September 2026) and rounded — treat them as a rough guide, not a price in a US shop. Imperial values in brackets are conversions of the metric figure.
Stranded cable with tinned conductors
- SVB / Oceanflex — single-core tinned cable
tinned conductor, 1.5–6 mm² sections, price per metre€0.80–3 (≈$0.93–3) / m - SVB / Oceanflex — 2-/3-core tinned cable
round sheathed cable, 2×1.5 to 3×2.5 mm², price per metre€3.80–8 (≈$4–9) / m - SVB / Oceanflex — tinned battery cable 16–50 mm²
heavy sections for the starting / main circuit, price per metre€6–18 (≈$7–21) / m - Ancor — Marine Grade Duplex / Primary Wire
tinned UL 1426 cable, available from EU chandlers€1–4 (≈$1–5) / m approx.
Fuses, circuit breakers and a fuse panel
- Blue Sea Systems — Fuse Box D6 (5035)
compact 6-circuit panel, up to 30 A per position, 100 A max€52–62 (≈$60–72) - Blue Sea Systems — Fuse Box C12 (5026)
panel for 12 blade fuses, for 12 consumers€64–75 (≈$74–87) - Blue Sea Systems — MAXI Fuse Block
MAXI fuse block for high-power circuits€37–45 (≈$43–52) - Victron Energy — MEGA fuses
MEGA elements 40–500 A for the main circuit€19–35 (≈$22–41)
Main battery switch and main fuse
- Blue Sea Systems — Battery Switch 300 A (6006)
m-Series ON/OFF, 6–48 V, ABYC compliant€38–48 (≈$44–56) - Blue Sea Systems — Battery Switch 600 A
high-power main switch, 600 A continuous€104–120 (≈$121–139) - Blue Sea Systems — MRBF Terminal Fuse Block (5191)
main fuse on the battery terminal, 10 mm (0.39 in) / M8 hole€32–40 (≈$37–46) - Victron Energy — MEGA fuse holder (surface mount)
holder for the main fuse, the element chosen separately€12–15 (≈$14–17)
Crimp terminals and adhesive-lined heat-shrink
- SVB — ring terminals for crimping / soldering
copper terminals, 6–95 mm² sections, price each€0,95–6 (≈$1–7) - SVB — set of adhesive-lined heat-shrink sleeves, 110 pcs
various diameters, inner adhesive layer, case€24–30 (≈$28–35) - SEATEC — solder sleeve connectors, 60 pcs
connectors with solder and adhesive-lined sleeve, sealed joint€12–16 (≈$14–19) - Ancor — Heat Shrink Ring Terminals (adhesive)
tinned ring terminals with adhesive heat shrink€10–25 (≈$12–29) / set approx.
Crimping tool, wire stripper, multimeter
- SVB — professional crimping tool 0.5–6 mm²
crimper for insulated terminals, ratchet mechanism€76–90 (≈$88–105) - SEATEC — digital multimeter
31 ranges, voltage / current / resistance, rubber sleeve and probes€14–20 (≈$16–23) - SVB — AC/DC clamp meter
measures current without breaking the circuit€35–60 (≈$41–70) approx. - KNIPEX — self-adjusting crimping tool for ferrules 0.08–16 mm²
together with a wire stripper as a workshop addition€40–70 (≈$46–81) approx.
Grommets, ties / clips, labels
- SVB — cable grommets, 5 pcs
rubber grommets for holes, cables up to 9/11 mm (0.43 in)€6,70–12 (≈$8–14) - QUICK — cable glands
resistant to corrosion and salt water, O-ring seal€11–20 (≈$13–23) - SVB — cable ties (100 pcs, black)
all-purpose nylon ties, various lengths€4,95–9 (≈$6–10) - SVB — set of coloured markers, 800 pcs
for coding circuits and labelling cables, boxed€10–16 (≈$12–19)
Moisture-repelling protective product for contacts
- Boeshield T-9 — protective anti-corrosion spray
displaces moisture and leaves a waxy-oily protective film on contacts€24–28 (≈$28–33) - CRC — Contact Cleaner & Protectant
cleans contacts and leaves a thin anti-corrosion film€10–16 (≈$12–19) approx. - WD-40 Specialist — Corrosion Inhibitor
long-lasting protection for metal and electrical connections€12–18 (≈$14–21) approx.
Insulated tools, gloves, goggles
- SEATEC / VDE — insulated 1000 V screwdrivers (set)
set of insulated tools for working on the installation€20–45 (≈$23–52) approx. - Work / protective gloves
assembly gloves for electrical and mechanical work€8–20 (≈$9–23) approx. - Safety goggles
eye protection when cutting and crimping cables€5–15 (≈$6–17) approx.