How does the fuse calculator work?
The fuse calculator determines the correct fuse rating for every DC (direct current) circuit in your camper van. A properly sized fuse protects the cable against overload and prevents fire. The tool also calculates voltage drop, recommends the right fuse type and accounts for temperature and cable bundling.
Determining the recommended fuse
The recommended fuse sits roughly 25% above the current you entered, because a fuse carrying its rated current continuously will eventually blow. Standard values are 1, 2, 3, 5, 7.5, 10, 15, 20, 25, 30, 40, 50 and 60 A. That margin is only applied when it stays comfortably below your cable's capacity; if it does not fit, the tool falls back to the smallest rating that still covers your current and the status flags that it is tight.
Cable capacity and derating factors
Each cable cross-section (measured in mm² or AWG) has a maximum current capacity. Under certain conditions, this capacity is reduced — this is called derating. Three factors are considered: cable length (longer cables build up more heat), ambient temperature (cables in the engine bay get warmer than in the living area) and bundling (multiple cables in a single duct cool less effectively). These derating factors are multiplied together to give the effective maximum current.
Voltage drop
Every cable has electrical resistance, which causes a voltage drop (measured in volts). The longer the cable and the thinner the cross-section, the more voltage is lost. The formula is: 2 × resistivity × length × current / cross-section. The factor 2 accounts for both the positive and negative conductor. For sensitive electronics, keep voltage drop below 3%. For lighting and motors, up to 5% is acceptable.
Motor circuits and slow-blow fuses
Appliances with a motor (water pump, roof fan, compressor fridge) have an inrush current that is 3 to 5 times higher than the rated current. This brief spike can blow a standard fuse. For these circuits a slow-blow fuse (also called time-delay) is recommended. A slow-blow fuse tolerates short current peaks without tripping, while still protecting against sustained overload.
Fuse types explained
Different current ranges require different fuse form factors. Blade fuses (ATO/ATC) are the most common for circuits up to 30 A — they fit in standard fuse boxes. MIDI fuses are used for mid-range circuits (30–40 A). For heavy circuits like inverters and battery cables (40–200 A), use ANL or Mega fuses. Above 200 A, an automatic circuit breaker is safer and easier to reset.
Sources and standards
Cable current-carrying capacities are based on IEC 60364-5-52 (low-voltage installation requirements) and the associated capacity tables. The temperature correction follows table B.52.14 of that standard, assuming XLPE or EPR insulation. The correction factors for bundling and for cable length are our own practical factors, not standard values: they keep a margin below what a single cable in free air can carry. Fuse sizes (ATO/MIDI/ANL) follow the common automotive sizing per SAE J1284. The voltage drop calculation uses the resistivity of copper (κ = 56). These standards were written for fixed low-voltage installations; for a 12V installation in a vehicle we use them as a guideline, with a safety margin.
Frequently asked questions
Which type of fuse should I use in my camper?
It depends on the current. For circuits up to 30 A, use blade fuses (ATO/ATC) or mini blade fuses in a fuse box — these are the most common in campervans. For heavier circuits (30–40 A), MIDI fuses are suitable. For the main battery cable or inverter (40–200 A), use ANL or Mega fuses. For very heavy loads above 200 A, an automatic circuit breaker is recommended because it can be reset without replacement.
How do I size a fuse correctly?
The fuse sits above the current you enter and stays below the cable's maximum capacity. Within that band the tool aims for roughly 25% margin, so the fuse does not blow during normal use. If that margin does not fit under your cable, the tool picks the smallest rating that still covers your current and the status shows that it is tight; choose a thicker cable in that case. For motor loads (pumps, fans) also pick a slow-blow fuse to absorb the inrush current.
What are derating factors?
Derating reduces the safe current capacity of a cable under unfavourable conditions. Three factors play a role: length (cables over 3 m build up more heat), ambient temperature (a cable in a 50 °C engine bay can carry less current than one at 25 °C) and bundling (multiple cables in a duct cool less effectively). The tool automatically calculates the combined derating factor.
Should I place the fuse near the battery or the appliance?
Always place the fuse as close to the battery (or power source) as possible — ideally within 30 cm. The fuse protects the cable, not the appliance. If the cable short-circuits anywhere along its length, the fuse near the battery will blow and cut power before the cable overheats.
What is the difference between a fuse and a circuit breaker?
A fuse is a one-time protection device: when it blows, you must replace it. A circuit breaker (also called automatic fuse) can be reset with a switch after it trips. Circuit breakers are more expensive but practical for main battery cables and circuits that may trip occasionally. For most camper circuits, blade fuses are simpler and cheaper.
How many fuses do I need in my camper?
Every individual circuit needs its own fuse. A typical campervan has 8–15 circuits: lighting (1–2), fridge, water pump, USB chargers, diesel heater, roof fan, radio, and possibly an inverter and solar charge controller. Use the Electrical Planner to map all circuits with the correct fuse sizes.