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Campervan
Cable Size Calculator

Prevent electrical fire risks and optimise the performance of your 12V appliances.
Calculate the exact cable size (mm²) and fuse rating required for your build in real time.

Recommended Cable Cross-Section

-- mm²
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Estimated Current

-- A

Exact Calculated Area

-- mm²

Suggested Fuse Rating

-- A

Measurement Details

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Getting the Electrical System Right in Your Campervan Conversion

Taking on a DIY campervan conversion requires careful planning, especially when it comes to designing a safe 12V DC electrical system. Unlike a house powered by 230V AC mains, off-grid vanlife relies primarily on a 12V (or 24V) leisure battery bank powering low-voltage appliances.

The 12V Trap in Campervan Electrics

A common mistake when wiring a campervan is underestimating the required cable cross-section (mm²). At 12V, electrical current encounters much higher relative resistance than at 230V. The thinner and longer a wire is, the greater its internal resistance. Undersized cables lead to two major hazards:

  • Voltage Drop: Significant energy is lost as heat along the wire. Your leisure battery may output 12.8V, but your compressor fridge or diesel heater might only receive 11.2V. The appliance will trigger a low-voltage cut-off and shut down unexpectedly.
  • Thermal Overheating: An undersized cable carrying high current will heat up rapidly. Overheated wiring melts insulation and is a primary cause of electrical fires in self-built campervans.

The Official Cable Sizing Formula

Our calculator automates the standard electrical resistivity equation for multi-strand copper cable, fully aligned with UK road safety guidelines and leisure vehicle wiring standards (BS EN 1949 & BS 7671):

Cable Size (S in mm²) = (ρ × 2L × I) / ΔU

The calculation automatically factors in the complete round-trip distance (both the positive supply cable and the negative return to your Lithium or AGM leisure battery bank).

💡 Take Your Electrical System Further

Cable sizing is just one part of the installation. To learn how to design and protect your entire 12V/230V electrical system and minimise the risk of fire, check out our essential guide.

Read the Electrical & Energy Guide →

Frequently Asked Questions: Campervan Electrics

Can I use rigid domestic electrical cable in my campervan?

No, using rigid single-core domestic cable is extremely dangerous. A campervan is subject to continuous road vibration while driving. Solid copper core wires work-harden, crack, and break over time, creating a serious fire hazard from electrical arcing. You must strictly use flexible multi-strand automotive copper cable (such as Thin Wall ISO 6722 or Tri-Rated cable).

Why does the calculator recommend a 35mm² cable for my inverter?

This is completely normal and accurate. A 1000W inverter running at 12V DC draws up to 90–100 Amps under load. High currents require thick multi-strand copper cable (often 35mm² to 50mm²) to handle the load safely without overheating, and should be positioned as close to your leisure battery bank as possible (ideally under 1.5 metres).

Do I need to calculate the cable size for solar panels?

Yes, absolutely. Power continuously flows from your roof-mounted solar array to your MPPT charge controller. Even though solar panel operating voltage (Vmp) is often higher than battery voltage, using dedicated double-insulated, UV-resistant solar cable (typically 4mm² or 6mm²) is essential to prevent voltage drop and maximize solar yield.

How do I correctly protect the cable I have sized?

Sizing the cable cross-section (mm²) is only the first step. Every positive feed in your campervan must be protected by an appropriately rated fuse or circuit breaker placed as close to the power source (leisure battery or main busbar) as possible. Fuses are sized to protect the wire itself from melting, not the appliance.

The Electrical Workshop