Electricity basics for your campervan build
Before you start pulling cables in your campervan, it helps to understand the basic principles of electricity. You don't need to become an electrical engineer, but with this knowledge you'll understand why certain choices are made and you can work safely. In this guide, we explain the key concepts without jargon.
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Voltage, current and resistance
The three fundamental quantities of electricity are voltage (volts), current (amps) and resistance (ohms). The easiest way to understand them is the water pipe analogy:
Voltage (V, volts) = water pressure. The higher the voltage, the more 'pressure' on the electrons. Your camper battery provides 12V, a home outlet provides 230V (or 120V in the US).
Current (A, amps) = the amount of water flowing through the pipe per second. The more devices you turn on, the more current flows. An LED light draws 0.5A, an inverter can draw 80A.
Resistance (ohms, symbol Omega) = how narrow the pipe is. A thin cable has more resistance than a thick one. Resistance causes heat, which is why the correct cable thickness is so important.
Ohm's law: V = I x R
Ohm's law is the most important formula in electrical engineering. It describes the relationship between voltage (V), current (I) and resistance (R):
V = I x R
Or: voltage = current times resistance. From this also follows:
I = V / R (current = voltage divided by resistance)
R = V / I (resistance = voltage divided by current)
Practical example: if you have a 12V battery and a device with 6 ohms resistance, then 12 / 6 = 2 amps of current will flow.
Why this matters: Ohm's law determines how much current flows through your cables. Too much current through a too-thin cable = overheating = fire risk.
Power: watts, watt-hours and amp-hours
Power (W, watts) indicates how much energy a device uses per second. The formula is:
P = V x I
Or: power = voltage times current.
Practical example: a fridge drawing 5A at 12V uses 12 x 5 = 60 watts.
Watt-hours (Wh) is power times time. If that fridge runs for 10 hours: 60W x 10h = 600 Wh per day.
Amp-hours (Ah) is current times time. The same fridge: 5A x 10h = 50 Ah per day. This is how battery capacity is expressed. A 200 Ah battery can theoretically deliver 1A for 200 hours, or 20A for 10 hours.
Conversion: Wh = Ah x V. So 200 Ah at 12V = 2400 Wh.
Direct current (DC) vs alternating current (AC)
There are two types of electrical current:
Direct current (DC): current always flows in the same direction. Batteries, solar panels and USB chargers work on DC. Your camper system is a DC system.
Alternating current (AC): the current reverses direction 50 times per second (50 Hz in Europe, 60 Hz in the US). Home outlets provide AC. Appliances like coffee makers, hair dryers, or laptop chargers run on 230V AC (120V in the US).
In a campervan, you have both: the 12V system is DC, and if you want to use AC appliances, an inverter converts 12V DC to 230V AC (or 120V AC).
Important: never mix DC and AC wiring. Use separate cables, separate fuse boxes, and label everything clearly.
Series vs parallel wiring
You can connect batteries and solar panels in two ways:
In series: you connect the positive of one to the negative of the next. Voltages add up, capacity (Ah) stays the same.
Example: 2x 12V 100Ah batteries in series = 24V, 100Ah.
In parallel: you connect all positives together and all negatives together. Voltage stays the same, capacity adds up.
Example: 2x 12V 100Ah batteries in parallel = 12V, 200Ah.
The same applies to solar panels:
In series: higher voltage, same current. Advantage: thinner cables needed. Disadvantage: if one panel is shaded, the whole string drops.
In parallel: same voltage, higher current. Advantage: shade on one panel doesn't affect the others. Disadvantage: thicker cables needed.
For batteries: never connect more than 4 batteries in parallel. Preferably use the same brand, type and age.
Voltage drop: why cable thickness matters
Every cable has resistance. That resistance causes voltage drop: the voltage at the end of the cable is lower than at the beginning. This is the biggest practical issue with 12V systems.
At 230V, a 0.5V loss is negligible (0.2%). But at 12V, 0.5V loss is already 4%. Many devices won't work properly below 11V.
The formula: voltage drop = current x resistance x cable length x 2 (out and back).
How to minimise voltage drop:
1. Use thick enough cables (larger cross-section = less resistance)
2. Keep cables as short as possible
3. Consider a 24V system for high-current applications
Rule of thumb: keep voltage drop under 3% for lighting, and under 5% for other circuits. Our cable calculator works this out precisely.
Fuses: the insurance for your wiring
A fuse protects the cable, not the device. During a short circuit or overload, the fuse blows before the cable overheats.
Choose the fuse based on the cable, not the device:
1.5 mm2 cable: max 10A fuse
2.5 mm2 cable: max 16A fuse
4 mm2 cable: max 21A fuse
6 mm2 cable: max 28A fuse
10 mm2 cable: max 40A fuse
Types of fuses:
Blade fuse (ATO/ATC): the standard automotive fuse, up to 40A.
ANL/MEGA fuse: for thick cables (100-500A), like the battery cable.
Circuit breaker: can be reset, useful for circuits you switch frequently.
Golden rule: every cable needs a fuse, placed as close to the power source (battery) as possible.
The 5 essential formulas at a glance
These five formulas are all you need when designing your campervan electrics:
1. Ohm's law: V = I x R
Voltage = current times resistance
2. Power: P = V x I
Watts = volts times amps
3. Calculate current: I = P / V
Amps = watts divided by volts. Example: 60W light on 12V = 5A.
4. Energy consumption: E = P x t
Watt-hours = watts times hours. Example: 60W x 8 hours = 480 Wh per day.
5. Battery capacity: Ah = Wh / V
Amp-hours = watt-hours divided by volts. Example: 480 Wh / 12V = 40 Ah per day.
With these formulas and our tools, you can design your complete electrical installation.
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Use our free tools to put the theory into practice.
Frequently asked questions
- What is the difference between volts, amps and watts?
- Volts (V) is voltage (pressure), amps (A) is current (flow rate), and watts (W) is power (energy per second). They are related by the formula: watts = volts times amps. A 12V system delivering 5 amps provides 60 watts of power.
- Why do campervans use 12V instead of 230V?
- Batteries provide direct current (DC) at low voltage. 12V is safe to work with and is the standard in vehicles. 230V alternating current (AC) is dangerous and is only used through an inverter for specific appliances.
- How much current does a device draw?
- Divide the wattage by the voltage: amps = watts / volts. A 60W fridge on 12V draws 5A. A 2000W kettle on 230V draws 8.7A. On 12V, that same kettle would draw 167A — that's why you use an inverter.
- What happens if my cable is too thin?
- A cable that is too thin has too much resistance. That resistance converts current into heat. With mild overload, you lose voltage (devices perform poorly). With severe overload, the cable can melt and cause a fire. That's why every cable needs a fuse.
- Should I choose 12V or 24V for my campervan?
- Most campervans use 12V. It's the standard with the widest range of 12V appliances available. Choose 24V if you have very high energy consumption (over 300Ah), because cables at 12V become impractically thick. Current at 24V is halved, so cables can be thinner.
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