Most failed RV electrical installs are not wiring mistakes. They are sizing mistakes: wire that is a gauge too thin, a fuse that protects the appliance but not the cable, a charger mounted in a sealed locker, or a lug torqued by feel. This page collects the four sets of numbers you need before drilling a hole, and points to the step-by-step install guides (C1–C8 below) for each job. Where a figure depends on the specific unit, it says so; the manual for the product you bought is the final word.
1. Wire gauge
Two things size a DC cable: how much current it carries (ampacity) and how far it goes (voltage drop). At 12 V, voltage drop is the constraint almost every time.
The table gives a starting-point gauge for a 3% drop on a 12 V circuit. Length is the round trip (positive and negative added together). Copper stranded wire, marine or automotive grade.
| Continuous current | 10 ft round trip | 20 ft round trip | 30 ft round trip |
|---|---|---|---|
| 10 A (lights, fans, pump) | 14 AWG | 12 AWG | 10 AWG |
| 20 A (fridge, small charger) | 12 AWG | 8 AWG | 6 AWG |
| 30 A (MPPT 100/30, Orion-Tr 30 A) | 10 AWG | 6 AWG | 6 AWG |
| 50 A (MPPT 100/50, Orion XS 50 A) | 8 AWG | 4 AWG | 2 AWG |
| 100 A (small inverter, DC distribution) | 4 AWG | 2 AWG | 1/0 AWG |
| 150 A (MultiPlus 12/2000 class) | 2 AWG | 1/0 AWG | 2/0 AWG |
| 250 A (MultiPlus 12/3000 class) | 1/0 AWG | 4/0 AWG | Relocate or parallel runs |
Notes:
- Inverter and charger battery cables should be as short as possible; move the unit, not the wire size. Victron specifies battery cable size and maximum length in each manual (for example, larger MultiPlus units call for very heavy cable or two parallel runs) and those figures override this table.
- For 24 V systems, the same table works at roughly half the current for the same power; for 48 V, a quarter.
- Confirm the wire's ampacity rating for its insulation temperature class, especially in engine bays or hot lockers.
- The 12 V voltage drop guide explains the math; the Victron cable, fuse, and busbar chart has the Victron-specific figures.
Solar panel wiring is separate: size for the array's short-circuit current with margin, and note that series strings raise voltage, not current, which is why longer roof runs favor series wiring into a higher-voltage MPPT like the 150/35.
2. Fusing
A fuse protects the wire. Its rating must be below the cable's ampacity and above the circuit's continuous load with margin (commonly 125% of continuous current). Mount it within a few inches of the source (battery positive or busbar). Every positive cable that leaves the battery needs one, including the cable to the DC-DC charger, the MPPT, the shunt's sense lead (a small fuse), and the inverter.
Typical values, to be confirmed against the unit's manual:
| Device | Fuse (starting point) | Fuse type |
|---|---|---|
| SmartSolar MPPT 100/30 | 40 A | MIDI or blade in a proper holder |
| SmartSolar MPPT 100/50 | 60 A | MIDI |
| Orion XS 12/12-50 | 60 A each side | MIDI |
| Orion-Tr Smart 12/12-30 | 60 A | MIDI |
| MultiPlus 12/1200–12/2000 | 200–300 A by model | MEGA |
| MultiPlus 12/3000 | 400 A class | MEGA 400 A or Class T |
| Lynx Distributor positions | Sized per branch | MEGA, in the Lynx |
| 12 V accessory circuit (lights, fan, pump) | 5–15 A per branch | Blade fuse at the 12 V panel |
| Diesel heater | Per manual; glow-plug start current is the sizing case | Blade or MIDI |
| Compressor fridge | 10–15 A | Blade |
Every figure in this table is a starting point for the common configuration. The number that governs your install is the one in your model's manual, and where they differ the manual wins.
Lithium batteries can deliver very high short-circuit current. For a large bank feeding a 3,000 VA inverter, a Class T fuse (high interrupt rating) is the conservative choice; the Lynx Class-T Power In provides for it. Fuse holders and busbars are in the Fuses & Circuit Protection and Lynx & DC Distribution collections. Manual-reset DC circuit breakers ($14.99) are convenient for accessory circuits but check their interrupt rating before using one as a main.
3. Ventilation and mounting
Chargers and inverters turn a few percent of the power they handle into heat. In a sealed locker in summer that is enough to trigger a temperature derate or shutdown.
- MultiPlus / Quattro: mount vertically on a non-flammable surface with the manual's specified clearance on all sides (commonly around 4 in / 10 cm — use the figure in your model's manual); ambient above about 104°F (40°C) reduces output. Not in the same compartment as flooded lead-acid batteries. Keep the DC cables short; move the battery closer rather than lengthening them.
- MPPT controllers: vertical, clearance above and below, away from direct sun on the heatsink. Derate in high ambient.
- Orion XS / Orion-Tr Smart: vertical, heatsink fins vertical, clearance around. The Orion-Tr Smart derates noticeably in heat; the Orion XS less so.
- Lithium batteries: interior or a compartment that stays above freezing for charging and below the BMS high-temperature limit; not in the engine bay; secured against movement.
- Diesel heater: intake and exhaust outside, exhaust away from windows and vents, unit mounted on the floor with the specified gasket; never in the same sealed space as its combustion air intake.
- Compressor fridge: vent path for the condenser, and a condenser fan if the cabinet is tight.
A 12 V 120 mm equipment-bay fan ($17.99) on a thermal switch solves most locker heat problems; the equipment bay fan guide shows the layout.
4. Terminal torque
Loose lugs are the cause of a large share of "faulty inverter" and "faulty charger" calls: they heat, oxidize, and drop voltage under load. Every stud on a Victron device, busbar, or fuse holder has a torque figure in the manual. Use a torque wrench, not a feel.
Typical ranges for tinned copper lugs on brass or copper studs, to be confirmed against the specific product:
| Stud size | Typical torque |
|---|---|
| M5 | 2–3 Nm |
| M6 | 5–6 Nm |
| M8 | 10–14 Nm |
| M10 | 20–25 Nm |
Those are generic fastener-class figures for clean, dry threads. Any torque figure the manufacturer prints for a specific terminal or fastener overrides them.
Practices that matter as much as the number: use the correct lug size for the wire (a 2/0 lug on 4 AWG wire crimps badly), use a proper hex crimper, put the lug directly against the stud face with washers on top (not between lug and terminal), and re-check torque after the first trip because vibration settles lugs. On battery terminals, follow the battery maker's figure; overtightening a lithium battery's terminal can damage the internal connection.
The install guides, by job
- C1: How to Install a Victron MultiPlus
- C2: How to Install RV Roof Solar Panels
- C3: Victron Cerbo GX Wiring
- C4: Install a Victron SmartShunt Battery Monitor
- C5: How to Install a Victron Orion DC-DC Charger
- C6: How to Replace an RV Roof Vent Fan
- C7: How to Replace an RV Water Pump
- C8: How to Replace RV Awning Fabric
Also useful: installing a 280 Ah lithium battery, 200 A DC circuit breaker guide, and Anderson plugs explained.
Before you start: a one-page checklist
- Draw the system with every cable's current, length, gauge, and fuse written on it.
- Confirm each figure against the product manual.
- Buy cable, lugs, and fuses to that drawing; do not substitute at the store.
- Mount everything, then cable it, then torque it, then photograph it.
- Power up in order: battery, shunt, DC distribution, chargers, inverter last.
- Re-torque after the first trip.
If you would rather have this done, the Off-Grid Power & Heating Service ($349.00) covers an install check.
Shop this
- Victron MEGA-fuse 400A & 500A, 5-pack — $18.00
- Victron MIDI-fuse — $5.00–$18.00
- Victron Lynx Distributor — $215.00
- Victron Busbar, DC Distribution — $22.00–$238.00
- 120 mm 12V High-Power Cooling Fan — $17.99
- Fuses & Circuit Protection (collection)