Electrical, AC and DC

The DC system distributes battery power—normally 12V—through main switches, busbars, fuses/breakers, panels and tinned copper wiring to pumps, lights, navigation gear and electronics. A fuse protects the wire, so its rating follows conductor ampacity, not merely the appliance. Blue Sea's ABYC summaries place main overcurrent protection close to the power source, commonly within 7in of a battery connection unless specified sheath/enclosure exceptions apply.

Charging sources include engine alternator, solar controller, shore-power charger, wind or hydro generation. The AC system begins at a shore inlet and includes a main double-pole breaker, reverse-polarity protection, equipment-leakage protection, branch breakers and GFCI receptacles. An inverter makes AC from DC; an inverter/charger also charges batteries. Bonding, AC safety ground, DC negative and lightning systems have distinct purposes and must not be casually merged.

Electrical faults are a major boat-fire source. A high-current lithium bank can turn an unfused cable short into an intense fire. Corroded connections disable bilge pumps, navigation lights, autopilot and communications. Poor charging design can overheat an alternator or leave the boat energy-deficient. Two-handed cruising also needs an energy budget that works on cloudy passage days, not only at a sunny dock.

Start with a diagram and energy audit. Service by cleaning/torquing accessible connections to manufacturer specifications, protecting terminals, labeling, securing wiring and testing shore polarity/GFCI/ELCI. Repair individual faults and replace damaged conductors end-to-end rather than hiding splices in wet bilges.

A partial refit can add main fuses, busbars, battery switches, shunt, solar controller or a correctly protected inverter while retaining good branch wiring. A wholesale rewire is justified by pervasive untinned/undersized wire, widespread corrosion, heat damage, inaccessible unsafe splices or an unusable distribution architecture. Lithium conversion is a system redesign covering BMS-controlled charge/load disconnects and alternator protection, not just batteries.

• Covered by a standard survey — visible wire type/routing, batteries and switches, panels, shore inlet, obvious missing protection, corrosion, polarity/GFCI tests and operation of accessible equipment. Scope varies, and panels may not be removed.
• Not covered — conductor-by-conductor ampacity/voltage-drop calculations, high-current load testing, insulation-resistance testing, thermal imaging under full load, inverter programming or a lithium charge/disconnect analysis. An ABYC electrical audit ($1,000–$3,000) is worth doing before purchase when there is lithium, a large inverter, obvious DIY work, heat damage or no diagram; otherwise immediately after purchase before new loads are added.

• The broker or owner — “Send the current AC/DC single-line diagram and invoices for batteries, solar, inverter/charger, alternator, shore power and panels.” “Who designed and installed each change?” “Provide model numbers, fuse/breaker ratings and cable sizes for every charge source and inverter.” “Have there been hot connections, nuisance trips, electrical fires or unexplained battery depletion?”
• The surveyor — “Open accessible panel backs; trace and report protection at batteries, alternator, charger, solar and inverter; test shore polarity, GFCI/ELCI and charging voltages; and identify which findings require an ABYC electrician rather than pricing a blanket rewire.”

With loads off, look behind panels and near batteries. Household wire nuts, solid Romex, automotive clamp terminals, unsupported cable, multiple wires stacked on battery posts, green corrosion, blackened insulation and fuses far from the source are red flags. Turn on large loads and feel—not touch bare metal—for warming cable/terminals; an infrared thermometer is safer. Run shore charger, solar and engine charging and record current/voltage. Trip-test every GFCI. Labels should correspond to reality.