If you’re new to campervan electrics, the world of volts, amps and cable sizes can seem overwhelming.
Don’t worry – once you understand a few simple concepts, it all starts to make sense.
This guide explains the absolute basics of 12-volt systems in plain English so you can build safely and confidently.
Most campervans use 12 volts DC, the same as your vehicle’s starter battery.
A 12V system is:
Safe to work with (low voltage, low shock risk)
Compatible with most campervan appliances
Easy to charge from solar, alternator, or mains hook-up
Efficient for LED lights, USB devices, diesel heaters and fridges
Some larger vans use 24V, but 12V is by far the most common.
Think of voltage as pressure.
More volts = more electrical “push”.
Your leisure system will almost always be 12V.
Current is how much electricity a device draws.
LED light: ~0.2–1A
Diesel heater on startup: 8–10A
12V fridge: 3–6A (average while running)
Your cable size and fuse rating are based on this.
Watts is simply voltage × amps.
Example:
A 12V fridge running at 4A uses:
Cable thickness determines how much current it can safely handle and how much voltage is lost over distance.
Longer cables → more voltage drop → thicker cable needed.
This is why a diesel heater might need 4–6mm², even though it only draws 10A during startup.
As electricity travels along a cable, some energy is lost.
Too much voltage drop causes:
dim lights
fridges not starting
diesel heaters failing to ignite
pumps running weakly
A good rule for campervans:
👉 Keep voltage drop under 3%.
Our Cable Sizing Tool automatically calculates this for you.
Stores energy.
Common types:
AGM – reliable, budget-friendly
Gel – similar to AGM, slower charging
Lithium (LiFePO₄) – light, powerful, long life
You can charge your battery from:
Solar panels
DC-DC charger (from the alternator)
Mains hook-up charger
Portable power stations (indirectly)
Most vans use a combination.
Every circuit must be protected by a correctly sized fuse or breaker.
Fuses:
protect the cable, not the appliance
stop fires
isolate faults
keep your system safe
You’ll use automotive thin-wall cable for 90% of your wiring.
The key is choosing the correct size.
Use our Cable Sizing Tool to get it right every time.
Your 12V negative usually connects to the vehicle chassis.
For safety:
use clean metal
keep earth points secure
avoid multiple long earth routes
Plan circuits before buying any cable
Fuse every circuit
Use correctly sized ring crimps and heat-shrink
Label everything – future you will be grateful
Only join copper to copper (no aluminium)
Keep cable runs short and tidy
Never exceed a cable’s amp rating
Double-check polarity before powering up
❌ Using too thin a cable
→ Causes voltage drop, overheating, and appliance faults
✔ Always size cable properly using a calculator
❌ No fuse on the battery
→ Extremely dangerous
✔ Install a main fuse within 15–20cm of the battery
❌ Using domestic 240V cable for 12V systems
✔ Use automotive thin-wall cable only
❌ Mixing earth points randomly
✔ Choose one clean chassis point per zone
Once you understand these basics, you can move on to:
Fuse selection
How to wire a fridge
Solar wiring
Battery types
Inverter safety
Choosing the right cable size (use the tool above!)
