DC Wire Size Calculator: Prevent Fire in Your Van Build
DC Wire Size Calculator: Prevent Fire in Your Van Build
In a house, a 1500 Watt heater runs on a thin 14-gauge wire. In a 12V van build, that same 1500 Watt load requires a cable as thick as your thumb. Why? Because as voltage drops, amperage skyrockets. High amperage creates heat, and heat creates fire. If you guess your wire size in a van conversion, you are driving a potential incendiary device. Use this ABYC-compliant tool by ShockTrail to size your cables safely.
The 12V Marine/Auto Estimator
Calculate the critical gauge for Inverters, Fridges, and Solar Charge Controllers.
12V/24V Wire Estimator
Van Build • Marine • SolarPART 1: The “Amperage Avalanche”
The formula for Power is $Watts = Volts \times Amps$.
If you have a 1000 Watt Microwave:
- At 120V (Grid): It pulls 8.3 Amps. (Safe for thin wire).
- At 12V (Battery): It pulls 83.3 Amps. (Requires massive cable).
This is why you cannot use household “Romex” wire in a van. It is simply too thin to carry the massive current required by low-voltage appliances.
Stranded vs. Solid Core
Never use solid core wire (like Romex) in a moving vehicle. The constant vibration of the road will work-harden the copper, causing it to crack and arc. You must use Stranded Wire (like Marine Grade or Welding Cable), which is flexible and vibration-resistant.
PART 2: Real-World Case Studies
Where do DIY builders fail most often? Let’s analyze the three most dangerous circuits in a van.
Case Study 1: The 3000W Inverter (The Fire Hazard)
The Project: Installing a 3000W Victron MultiPlus to run an induction cooktop.
The Distance: 6 feet from battery to inverter.
The Math:
- 3000 Watts / 12 Volts = 250 Amps.
- Efficiency Loss (10%): Total Draw ~275 Amps.
- Requirement: You need 4/0 AWG (0000 Gauge) cable.
The Failure: Many beginners buy “2 Gauge” amplifier kits from AutoZone. Those wires will melt insulation at 275 Amps, potentially igniting the plywood bed frame above them.
Case Study 2: The 12V Fridge (The Compressor Killer)
The Project: A Dometic 12V fridge located at the back of the van (20 feet from the battery).
The Load: Only 5 Amps. Sounds small, right?
The Voltage Drop: Over 40 feet (round trip), a thin 16 AWG wire causes a 10% voltage drop.
If your battery is at 12.6V, the fridge sees 11.3V.
Result: The fridge’s “Low Voltage Protection” trips immediately. The fridge runs for 10 seconds and shuts off. Your food spoils.
The Fix: Upsize to 10 AWG wire to keep the voltage drop under 3%.
Case Study 3: Solar Panels to Controller
The Project: 400 Watts of solar on the roof. 20 feet down to the MPPT controller.
Configuration Matters:
- Parallel (12V): 400W / 18V (Panel Vmp) = 22 Amps. Needs 8 AWG wire. Hard to route.
- Series (48V): 400W / 72V = 5.5 Amps. Needs only 14 AWG wire.
Lesson: Wiring panels in Series increases voltage and lowers amperage, allowing you to use thinner, cheaper wire (see our Solar Wiring Guide).
PART 3: Fusing (Fuse the Wire, Not the Device)
A fuse has one job: prevent the wire from glowing red hot. It is not there to protect your expensive TV; it is there to save your life.
- Placement: The fuse must be as close to the battery Positive terminal as possible (ABYC recommends within 7 inches).
- Sizing: The fuse rating must be lower than the wire’s max amperage rating.
Example: If your wire can handle 100A, use an 80A or 100A fuse. Never put a 150A fuse on a 100A wire.
PART 4: Marine Grade vs. Welding Cable
What should you buy?
| Type | Pros | Cons | Best Use |
|---|---|---|---|
| Marine Grade (Ancor) | Tinned (Silver color). Corrosion proof. | Expensive ($$$). | Boats, humid vans, coastal areas. |
| Welding Cable (OFC) | Oxygen-Free Pure Copper. Very flexible. | Insulation isn’t oil/gas resistant. | Inverter Cables (High current connections). |
| CCA (Copper Clad Aluminum) | Cheap. | GARBAGE. High resistance, corrosion risk. | Never. Avoid cheap amp kits. |
PART 5: Chassis Ground vs. Negative Run
Should you run a black wire all the way back to the battery, or just bolt it to the van metal (Chassis)?
- High Current (Inverters): Always run a dedicated black Negative cable back to the battery. The chassis resistance is unpredictable for 200A loads.
- Low Current (Lights/Fans): Chassis grounding is acceptable if you grind the paint off to bare metal, but dedicated wires are always more reliable (prevent “Ground Loops”).
Frequently Asked Questions (FAQ)
Does voltage drop hurt LED lights?
It causes dimming or flickering. If you run thin wire to LEDs, the ones closest to the battery will be bright, and the ones at the end will be dim. Upsize the wire to ensure uniform brightness.
What is the “Round Trip” distance?
Electricity travels out on Positive and back on Negative. Total circuit length is 2x the distance. Our calculator asks for “One Way Distance” and doubles it automatically for you.
Why did my fuse holder melt?
Loose connections create heat. If the fuse didn’t blow but the plastic melted, the screw/bolt holding the fuse was likely loose or dirty. Always torque your connections.
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