12V Electrical Wiring Pack

Van Electrical Wiring Pack

**A companion bonus for *Van Life for Beginners* (Open Road Publishing)**

This is the part every "just watch YouTube" answer skips: the actual wiring. Below are three complete, labeled 12V systems you can copy, the exact wire and fuse sizes so nothing melts, and a real parts list with 2026 prices. You do not need to be an electrician to follow it. You do need to follow it carefully, because 12V looks harmless and isn't — see the safety block at the end before you touch a wire.

Everything here is 12-volt LiFePO4 (lithium), because that's what a modern van build uses.

1. How a 12V system actually connects (the chain)

Electricity flows in a loop: out of the battery's positive (+) terminal, through a fuse, through your device, and back to the battery's negative (−) terminal. Break the loop and nothing works. Short the loop (+ touches − with no load between them) and you get a fire. That's the whole game: keep the loop intact, and put a fuse in every path so a fault blows a $2 fuse instead of your van.

Power OUT (battery to your stuff):

BATTERY (+)  →  MAIN FUSE  →  POSITIVE BUS BAR  →  small fuse per circuit  →  each LOAD
LOAD  →  NEGATIVE BUS BAR  →  BATTERY (−)      (the return path — every load comes home here)

• Battery bank — your energy tank (LiFePO4, with a built-in BMS that protects the cells).
• Main fuse / breaker — one big fuse right at the battery + terminal. Protects the thick main cable. Use a Class-T fuse with lithium (explained in Safety).
• Positive bus bar — a metal bar that splits the one main + cable into many. The negative bus bar does the same for all the return wires.
• Per-circuit fuses — a small fuse for each device, sized to protect *that device's wire*.
• Loads — fridge, fan, lights, pump, USB, inverter, etc.

Power IN (three ways to recharge — each has its own charger):

SOLAR PANELS   →  MPPT CHARGE CONTROLLER  →  BATTERY      (sun)
ALTERNATOR     →  DC-DC CHARGER           →  BATTERY      (driving)
SHORE POWER    →  CONVERTER / CHARGER     →  BATTERY      (plugging into 120V)

You never wire a solar panel, the alternator, or a wall plug straight to a lithium battery. Each source runs through a charger that converts its raw output into the exact voltage lithium wants. Skip the charger and you cook the battery.

• MPPT controller — turns high panel voltage into correct charging voltage; "MPPT" squeezes ~20-30% more out of the panels than the cheap "PWM" type. Worth it.
• DC-DC charger (a.k.a. B2B) — lets your engine's alternator safely charge lithium while you drive. Required for lithium; a direct alternator connection can destroy the alternator or the battery.
• Converter/charger — for when you plug into a campground or house outlet (shore power).

2. Three complete systems (copy one of these)

Pick by how much power you use, not by your van size. Rough guide: 100Ah = weekender / lights-fan-phone-fridge. 200Ah = full-timer with a laptop, induction burner now and then. 400Ah = heavy use, off-grid for days, runs most things like a small apartment.

All three use the identical chain from Section 1. Only the sizes change.

System A — 100Ah "Weekender" (~1.3 kWh usable)

 200W SOLAR ─┬─► [MPPT 20A] ──►┐
             │                 │
 ALTERNATOR ─┴─► [DC-DC 20A] ──┤ (each source fused 30A)
                               ▼
   ┌─────────────────────────────────────────────────────┐
   │  BATTERY  1 x 100Ah LiFePO4  (use a 120A+ BMS)       │
   │    (+)──[ MAIN FUSE 150A Class-T ]──► (+) BUS BAR     │
   │                                          │           │
   │        ┌──────────┬──────────┬──────────┼─────────┐  │
   │     [15A]      [7.5A]      [10A]      [5A]     [125A]│
   │    FRIDGE       FAN        PUMP      LIGHTS   INVERTER│
   │                                                1000W ─┼─► 120V AC
   │        └──────────┴──────────┴──────────┴─────────┘  │
   │    (−) BUS BAR ◄── all returns ──────────────────────┤
   │    (−)◄──────────────────────────────────────────────┘
   └─────────────────────────────────────────────────────┘

• Panels: 200W (2 × 100W) → MPPT 20A
• Alternator charging: DC-DC 20A (8 AWG, fused 30A each end)
• Inverter: 1000W (fine for a blender/laptop charger; run *one* big thing at a time on this battery)
• Main fuse: 150A Class-T at the battery; main cable 2 AWG
• Note: a 100Ah with a 100A BMS will trip if the 1000W inverter runs flat-out. Buy a 100Ah rated for 120A+ continuous, or keep inverter loads under ~700W.

System B — 200Ah "Full-Timer" (~2.6 kWh usable)

 400W SOLAR ─┬─► [MPPT 40A] ──►┐
             │                 │
 ALTERNATOR ─┴─► [DC-DC 40A] ──┤ (solar fused 50A, DC-DC fused 50A)
                               ▼
   ┌─────────────────────────────────────────────────────┐
   │  BATTERY  200Ah LiFePO4  (2x100Ah or 1x200Ah, 150A+) │
   │    (+)──[ MAIN FUSE 250A Class-T ]──► (+) BUS BAR     │
   │                                          │           │
   │        ┌──────────┬──────────┬──────────┼─────────┐  │
   │     [15A]      [7.5A]      [15A]      [5A]     [250A]│
   │    FRIDGE       FAN        PUMP      LIGHTS   INVERTER│
   │                                                2000W ─┼─► 120V AC
   │        └──────────┴──────────┴──────────┴─────────┘  │
   │    (−) BUS BAR ◄── all returns ──────────────────────┤
   │    (−)◄──────────────────────────────────────────────┘
   └─────────────────────────────────────────────────────┘

• Panels: 400W (2 × 200W) → MPPT 40A
• Alternator charging: DC-DC 40A (6 AWG, fused 50A each end)
• Inverter: 2000W (microwave, induction burner, power tools — one at a time)
• Main fuse: 250A Class-T; main cable 2/0 AWG

System C — 400Ah "Off-Grid" (~5.1 kWh usable)

 800W SOLAR ─┬─► [MPPT 60A] ──►┐
             │                 │
 ALTERNATOR ─┴─► [DC-DC 60A] ──┤ (solar fused 80A, DC-DC fused 80A)
                               ▼
   ┌─────────────────────────────────────────────────────┐
   │  BATTERY  400Ah LiFePO4  (4x100Ah or 2x200Ah, 250A+) │
   │    (+)──[ MAIN FUSE 400A Class-T ]──► (+) BUS BAR     │
   │                                          │           │
   │        ┌──────────┬──────────┬──────────┼─────────┐  │
   │     [15A]      [7.5A]      [15A]      [5A]     [400A]│
   │    FRIDGE       FAN        PUMP      LIGHTS   INVERTER│
   │                                                3000W ─┼─► 120V AC
   │        └──────────┴──────────┴──────────┴─────────┘  │
   │    (−) BUS BAR ◄── all returns ──────────────────────┤
   │    (−)◄──────────────────────────────────────────────┘
   └─────────────────────────────────────────────────────┘

• Panels: 800W (4 × 200W) → MPPT 60A (or two smaller controllers)
• Alternator charging: DC-DC 60A (4 AWG, fused 80A each end)
• Inverter: 3000W (runs most household appliances)
• Main fuse: 400A Class-T; main cable 4/0 AWG

For a designer redrawing these: keep the left-to-right flow of *charge sources → chargers → battery → main fuse → positive bus → fanned-out fused loads → negative bus → back to battery*. Charge sources stack on the left feeding one junction into the battery +. The inverter is always the largest wire and largest fuse, drawn thickest. Fuses sit as squares directly on the wire they protect, right next to the bus bar.

3. Wire gauge (AWG) — the two things that size a wire

At 12V you size wire for two limits and use whichever demands the *thicker* wire:

1. Ampacity — can the wire carry the current without overheating?
2. Voltage drop — is the wire so long/thin that too much power is lost as heat before it arrives? At 12V this is the one that usually wins, because 12V systems push big amps and long runs bleed voltage fast.

Lower AWG number = thicker wire. (2 AWG is much fatter than 12 AWG.)

Table 3a — Ampacity (max continuous current, copper, 105°C insulation):

Table 3b — What gauge do I need? (holds voltage drop to 3%, the safe default)

Read down to your circuit's amps, across to your one-way run length (measure the wire's actual path, not straight-line). The cell is the minimum AWG. Round runs up.

*Why runs matter: a 10A fridge on a 20 ft run needs 8 AWG — not the 14 AWG the amps alone suggest — because the extra length would otherwise drop too much voltage and the fridge runs weak and hot. When in doubt, go one size thicker. It's never wrong, only pricier.*

4. Fuse sizing — the fuse protects the WIRE, not the device

The rule: size a fuse to about 125-150% of the circuit's continuous current, then make sure that fuse is at or below the wire's ampacity (Table 3a). The fuse exists to blow before the *wire* overheats. Never fit a fuse bigger than the wire can carry — that's how wires melt inside walls.

Motors and compressors (fridge, pump, inverter) surge when they start, so their fuses are set higher than 150% — follow the appliance's label if it specifies one.

Inverter DC amps = AC watts ÷ ~11 (that's 12.8V × ~85% efficiency). That's why a 2000W inverter pulls a shocking ~185A and needs cable as thick as your thumb.

5. Parts list + realistic 2026 US prices

Prices are budget-but-decent brands (LiTime / Renogy / ECO-WORTHY tier). Premium gear (Victron) runs roughly 1.5-2× on the controller, charger, and inverter. Wiring "kit" = the right-gauge cable, ring lugs, heat-shrink, and cable ties for that build.

System A — 100Ah (~$1,050)

System B — 200Ah (~$1,780)

System C — 400Ah (~$3,200)

Always buy pure-sine inverters (not "modified sine") — modified sine buzzes, runs motors hot, and can damage sensitive electronics. Buy all your lugs and cable in the same gauge system, and get a proper hydraulic crimper (~$35) or have a shop crimp the big lugs — a bad crimp on the main cable is a fire waiting to happen.

6. Read this before you touch a wire (safety)

This pack is educational. It teaches the standard sizing so you can plan and understand your build — it is not a substitute for a qualified installer.

• 12V DC is not "safe" just because it's low voltage. It won't shock you across dry skin, but a wrench dropped across a battery's terminals delivers hundreds of amps instantly — enough to flash-weld metal, cause severe burns, and start a fire. Remove watches and rings before working near the battery.
• The fuse protects the wire. Every wire that leaves the battery or a bus bar must have a fuse sized *at or below that wire's ampacity*, as close to the power source as practical. No exceptions. An unfused wire is the single most common cause of van fires.
• LiFePO4 must be charged correctly. Only charge through a lithium-rated MPPT, DC-DC, and converter set to lithium profiles. The battery's BMS is its last line of defense, not your first — never rely on it to do a fuse's job. Never charge lithium below freezing (~32°F / 0°C) unless the battery is a self-heating model.
• Use a Class-T fuse for the main lithium battery fuse. Lithium can dump enormous fault current in a short; a Class-T is built to interrupt it safely where a cheap fuse can arc and fail.
• Disconnect before you work. Turn off the battery disconnect switch and confirm circuits are dead before touching them. Connect the battery last, negative terminal final.
• Torque terminals properly. Loose connections heat up and start fires; over-tightened ones crack. Follow the battery and bus-bar torque specs.
• AC is a different animal. Anything on the 120V side of an inverter (outlets, shore power) can kill you. If your build includes shore power or hard-wired AC outlets, have that part done or inspected by a qualified electrician.
• When unsure, get help. For anything you're not confident about, consult a qualified marine or RV electrician and follow ABYC (American Boat & Yacht Council) standards — the accepted reference for DC systems in vans and boats. Local codes may also apply if you ever want insurance or resale certification.

Do it once, do it right, and a 12V system is quietly reliable for years. Rush it and it's the most dangerous thing in your van.

*This wiring pack pairs with your other two bonuses: the Build-Cost Calculator (plug these part prices straight in to budget your whole conversion) and the Conversion Checklist (where the electrical install slots into the full build order). Use all three together.*