RV and boat DC power

Choose the system voltage at the top (12, 24 or 48 V); every tab uses it.

Wire & fuse — the load in amps or watts, the one-way length (the return is counted for you), whether the circuit is critical (3 % drop: electronics, pumps, navigation lights, panel feeders) or not (10 %: lighting), the cable’s insulation rating (105 °C is usual for boat cable), whether it runs through the engine room and how many current-carrying conductors are bundled with it. You get the size by drop and by ampacity, which one wins, the fuse range (at least 125 % of the load, at most 150 % of what the wire can carry — the wire goes up a size if needed) and fuse types that fit. Below it, the placement check: where the conductor starts, whether it is sheathed all the way, and how far away the fuse is.

Battery bank — chemistry, the battery or cell, and how many strings in parallel. It draws the series and parallel connections with the main leads on opposite corners, lists the wiring practices that keep the strings even, and compares a rough short-circuit current with the interrupting rating of Class T, MRBF and ANL fuses.

Charging — the alternator through a DC-DC charger (limited by what the alternator can give for hours, about half its rating, and by what the bank should take), with the input current and its cable; and a shore-power or generator charger sized to put back the energy in the hours you have, with the mains current it draws.

Busbar & fuse blocks — list the circuits (or fill them from the budget’s DC loads). Small ones go in blade fuse blocks within their position count and total rating, big ones bolt to the busbar with their own MIDI, ANL or Class T fuse; the feeder, main fuse and busbar ratings follow from what is on at once.

Daily budget — the load list (shared with the off-grid solar sizer: From off-grid solar sizer copies it), the inverter’s efficiency and idle draw, and what comes in on a typical day from solar, driving and shore power: the net amp-hours and how many days until the bank reaches its usable limit.

Related: off-grid solar sizing for panels, controllers and inverters by month and place, battery runtime, battery pack builder, battery life, LiPo care, energy usage, generator sizing, and wire size for 120/240 V AC circuits under the NEC.

Voltage drop — circular mils needed = 10.75 × amps × round-trip feet ÷ allowed volts (10.75 is copper’s resistance in ohm-circular-mils per foot, the figure ABYC uses). The conductor areas are the minimum circular mils listed for stranded boat cable (16 AWG 2,426; 10 AWG 9,880; 4 AWG 40,930; 1/0 104,500 …), a little under nominal AWG; SAE automotive cable of the same number is smaller.

Ampacity — the ABYC E-11 table for conductors under 50 V, by insulation rating (60–200 °C), outside engine spaces; inside them multiplied by 0.58 (60 °C), 0.75, 0.79, 0.82, 0.86 (105 °C), 0.89 and 1.0 (200 °C); bundles of 3, 4–6, 7–24 and 25+ current-carrying conductors by 0.7, 0.6, 0.5 and 0.4.

Fuse placement — within 7 inches of the source; 72 inches if the conductor comes straight off a battery terminal and is sheathed or enclosed along its whole length; 40 inches if it comes off another source (a switch or busbar) and is sheathed; starter cables exempt.

Fault current — a rough rule of 40 × Ah per string for LiFePO4, 25 × for AGM and 15 × for flooded and gel, against interrupting ratings from makers' data sheets (blade 1 kA, MIDI 2 kA, ANL 6 kA, MRBF 10 kA at 14 V, Class T 20 kA).

Batteries — usable depth of discharge 80 % for LiFePO4 and 50 % for lead-acid; charge limits 0.5 C (LiFePO4), 0.3 C (AGM) and 0.2 C (flooded, gel), with at least C/10 for lead-acid; round-trip efficiency 95 % and 80–85 %.

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