Design a complete 12V auxiliary electrical system for your overlanding vehicle — wire gauge, fuse sizing, LiFePO4 battery bank, DC-DC charger, and solar. Free Power System Builder tool with NEC/ABYC safety standards.
Most overlanders get their 12V electrical system wrong. Not a little wrong — fire-hazard wrong.
They watch a YouTube video, buy a battery and some wire, and start crimping. Nobody calculates voltage drop. Nobody checks if their fuse actually protects the wire behind it. Nobody thinks about what happens when you try to charge a LiFePO4 battery at 15 degrees in the Rockies.
The result? Melted wire, blown fuses, dead batteries, and in worst cases, a vehicle fire 40 miles from the nearest paved road.
We built the Power System Builder to fix that. It’s a free, step-by-step calculator that designs your entire 12V auxiliary electrical system — wire gauge, fuse sizing, battery bank, charging sources, and a wiring diagram — all based on NEC and ABYC E-11 standards. No account, no email gate, no cost.
Try the Power System Builder here →
Your house runs on 120V or 240V AC. At those voltages, you can push serious power through relatively small wire. Drop down to 12V DC and the math changes completely.
At 12 volts, a 600-watt load pulls 50 amps. That same load at 120V pulls 5 amps. Ten times the current means ten times the heat in the wire, ten times the voltage drop over distance, and ten times the consequences if you get the wire gauge wrong.
This is why undersized wire is the number one cause of 12V electrical fires in vehicles.
The formula is straightforward:
Voltage Drop = (2 x Distance x Current x Resistance) / 1000
For charging circuits and anything sensitive, you want 3% drop or less (0.36V at 12V). For lighting-only circuits, up to 5% is acceptable. Go beyond that and you’re wasting power as heat in the wire — and risking a fire.
The Power System Builder runs this calculation for every circuit in your system automatically.
It’s a 5-step wizard that walks you through the entire design:
Select your vehicle type and alternator rating. If you’ve already built a Vehicle Profile in the Ops Deck, it auto-loads your alternator amps — no re-entering data.
The tool also asks about smart alternators. Most 2017+ vehicles have ECU-controlled alternators that don’t maintain a constant 14.4V output. If you have one, you need a DC-DC charger with smart alternator compatibility. Direct battery-to-battery charging won’t work and can trigger check-engine codes.
Pick your devices from 30+ pre-loaded options across 8 categories:
Each device has realistic wattage, duty cycle, and default run times pre-filled. Adjust hours and duty cycle to match how you actually use it. The calculator handles inverter overhead automatically — devices that need an inverter get 15% added to their wattage.
You can also add custom devices for anything not in the database.
Real talk: Most people underestimate their daily amp-hour consumption by 30-50%. The duty cycle matters. A fridge doesn’t run 24 hours straight — it cycles on and off. A compressor runs for 5 minutes, not 8 hours. The defaults in this tool reflect real-world usage, not worst-case spec-sheet numbers.
Choose your chemistry, capacity, and count.
LiFePO4 vs AGM vs Lead-Acid:
| LiFePO4 | AGM | Lead-Acid | |
|---|---|---|---|
| Usable capacity | 80% DOD | 50% DOD | 50% DOD |
| Weight per Ah | 0.14 lb | 0.30 lb | 0.35 lb |
| Cycle life | 3,000-5,000 | 400-800 | 200-500 |
| Cold charge limit | 32F (HARD STOP) | -4F | -4F |
LiFePO4 is the recommendation for overlanding. A 200Ah LiFePO4 bank gives you 160Ah of usable capacity. To get the same usable capacity from AGM, you’d need 320Ah — at more than double the weight and a fraction of the lifespan.
The one critical gotcha: LiFePO4 cannot be charged below 32F / 0C. Charging below freezing causes permanent lithium plating damage. The tool flags this automatically based on your ZIP code climate zone.
Set your days-of-autonomy target (how long you want to run without any charging), and the calculator tells you the recommended bank size. It also cold-derates your capacity if you’re in a cold climate zone.
Configure how your bank gets charged:
The tool shows your daily charge budget in real time: alternator Ah + solar Ah + shore Ah. You can see immediately whether you’re running a surplus or a deficit — and how long your bank lasts parked vs. driving.
This is where it gets safety-critical.
The tool auto-generates every circuit in your system:
Enter the one-way distance in feet for each run. The calculator sizes the wire gauge and fuse for every circuit, live.
Wire gauge is selected by iterating from 16 AWG up until both conditions pass: voltage drop is within target AND current is within the wire’s ampacity rating.
Fuse sizing follows the 125% rule: load amps x 1.25, rounded up to the next standard fuse size. The fuse must never exceed the wire’s ampacity — if it does, the tool flags it as a failure and tells you to upsize the wire.
Fuse type is selected automatically:
Hit “Generate Report” and you get:
Power Budget — A donut chart showing your charge sources vs. consumption, plus surplus/deficit, days of autonomy, and parked vs. driving balance.
Battery Assessment — Your selected bank vs. the recommended bank size for your loads and autonomy target.
Wire Gauge Schedule — Full table with every circuit: distance, amps, recommended AWG, voltage drop in volts and percent, and pass/warn/fail status. Color-coded green (under 3%), amber (3-5%), red (over 5%).
Fuse Schedule — Every fuse: rating, type, and placement notes.
Wiring Diagram — An SVG block diagram showing your entire system: starter battery, DC-DC charger, aux bank, charge controller, fuse panel, and branch circuits. Each connection labeled with wire gauge and fuse rating. Color-coded by voltage drop.
Shopping List — Every component you need, matched to your specific system design, with links to buy them. Batteries, chargers, panels, fuse blocks, disconnect switches, wire kits, bus bars, and lugs.
Safety Checklist — 10 items you must verify before connecting anything. Every item sourced from NEC, ABYC E-11, or established best practices.
These are non-negotiable:
I am a union lineman who has done storm restoration. I understand 12V systems from both a professional and a camping perspective. My current 2025 GMC Sierra build includes an EcoFlow DELTA 3 Ultra with dual 220W solar panels and a WAVE 3 portable AC, and I have wired and troubleshot 12V setups across three different overland rigs. This guide and the Power System Builder tool come from that hands-on experience.
Who it’s NOT for: This tool designs 12V DC auxiliary systems. It does not design 120V AC house circuits, solar grid-tie systems, or high-voltage EV systems.
The Power System Builder is part of the Ops Deck — our suite of vehicle-aware tools that share data. If you’ve built a Vehicle Profile, the Power System Builder already knows your alternator amps. Future updates will tie into the Fuel & Range Planner and the Overland Load Balancer so your electrical system weight automatically factors into payload calculations.
Planned additions:
Most overlanding setups run well on a 100-200Ah lithium battery. A 100Ah LiFePO4 battery provides about 1,280Wh of usable power. That runs a fridge, lights, and fans for 2-3 days without charging. Add solar to extend that indefinitely.
Yes, with a DC-DC charger (like a Renogy or Redarc) wired from your alternator to the auxiliary battery. Most chargers output 20-40A, which means you can add 25-50Ah per hour of driving. A 2-3 hour drive can significantly recharge a depleted battery.
It depends on the current draw and wire length. For a 30A circuit running 10 feet, you need at minimum 10 AWG wire. For higher draws or longer runs, go thicker. Undersized wiring causes voltage drop, heat, and fire risk. The Power System Builder calculates this for you.
Yes. Every circuit needs a fuse sized to the wire gauge, not the device. An unfused circuit is a fire waiting to happen. Use a blade fuse distribution block and fuse every positive wire within 12 inches of the battery connection.
Not recommended without a battery isolator or DC-DC charger. A fridge running overnight can drain your starting battery to the point where your vehicle will not start. An auxiliary battery with proper isolation protects your starting battery while keeping your fridge running.
Build it right the first time. Bad wiring doesn’t give you a second chance at 2 AM on a forest road.
Stay prepared,
Mike + The Prepper Evolution Team