Voltage drop in a camper van: formula and examples

Voltage drop is the voltage lost along the length of a cable because copper has a (small) resistance. The longer the cable and the higher the current, the bigger the loss.

In a household 230V installation you barely notice it: 230V can easily absorb a loss of a few volts. In a 12V camper system the same absolute loss is far more critical. A loss of 0.5V is almost 5% at 12V, while at 230V it would be only 0.2%.

Consequences of too much voltage drop:

• Compressor fridges cut out on undervoltage (around 11.3V)
• Water pumps run slower and deliver less pressure
• LED lighting becomes visibly dimmer
• Inverters shut down when the input voltage is too low
• Cables get warm, and at high currents even hot (fire risk)

Voltage drop is therefore not an academic detail but a design parameter that directly determines whether your system works.

The standard formula for voltage drop in volts:

Udrop = (2 × L × I) / (κ × A)

Where:

• L is the one-way cable length in metres (the factor 2 accounts for the return path)
• I is the current in amps
• κ (kappa) is the conductivity of copper, 56 m/(Ω·mm²) at 20 °C
• A is the cable cross-section in mm²

The other way round, if you have a maximum acceptable voltage drop and want to know which cross-section you need:

A = (2 × L × I) / (κ × Umax)

For aluminium cable use κ = 34 instead of 56. Aluminium is rarely used in campers because of its higher resistance and because it is harder to terminate.

Note: this formula applies to direct current (DC). For alternating current (AC, for example 230V through an inverter) a power factor and frequency effects come into play as well. For camper runs up to roughly 3 metres after the inverter you can use the same DC formula; the difference is negligible.

Rules of thumb per circuit type, expressed as a percentage of the system voltage:

Maximum 1% loss:

• Main cable from battery to busbar/fuse box
• Cable from solar panel to MPPT (maximum power point tracking, the technique that gets the most out of a panel) charge controller
• Interconnections between batteries

Maximum 3% loss:

• Branches to most consumers (LED, USB, fans)
• Water pump and fridge (provided they work properly at 11.6V)

Maximum 5% loss (only when there is no alternative):

• Short runs to non-critical devices
• Lighting in far corners

In absolute volts: on a 12V system 1% equals 0.12V, 3% equals 0.36V and 5% equals 0.60V. At 24V you can use the same percentages. The absolute values are then twice as high, which means 24V tolerates long cables and high currents better.

These percentages are common practice in the camper and marine world, not a legal standard. Our cable calculator and fuse calculator use the same assumptions.

Example 1, fridge at 3 metres (12V, 5A):Udrop = (2 × 3 × 5) / (56 × 2.5) = 30 / 140 = 0.214V2.5 mm² cable gives 1.8% loss here, comfortably within the 3% guideline.

Example 2, 1500W inverter at 1.5 metres (12V, 125A):Udrop = (2 × 1.5 × 125) / (56 × 35) = 375 / 1960 = 0.191V35 mm² gives 1.6% loss at full power, acceptable, but 50 mm² (1.1%) is safer.

Example 3, 200W solar panel at 4 metres (12V system, open circuit voltage approx. 22V, 9A):Udrop = (2 × 4 × 9) / (56 × 4) = 72 / 224 = 0.321VAgainst 22V open circuit voltage that is 1.5% loss, fine for an MPPT input.

Example 4, LED strip at 6 metres (12V, 2A):Udrop = (2 × 6 × 2) / (56 × 1.5) = 24 / 84 = 0.286V1.5 mm² gives 2.4% loss, just within the guideline. When dimmed this is no problem; at full brightness you may see a slight shift in the LED colour.

The vanalyze cable calculator uses this formula and gives you the minimum required cross-section plus a check against your preferred percentage.

Voltage drop is not a theoretical number. It has consequences you notice, and different ones per appliance.

With lighting you see it immediately: LEDs go dimmer or start flickering, and on dimmable lighting the lower part of the range stops working.

With an inverter it works differently. At a lower input voltage it draws more current to deliver the same power, which increases the loss further. At some point voltage falls below its cut-out threshold and it shuts down while the battery is still half full.

With a pump or compressor the start becomes the problem. It asks for a lot of current briefly, and that is exactly when voltage sags deepest. A fridge that occasionally refuses to start while the battery looks full is a classic case of undersized cable.

When charging it works the other way and is more treacherous: the charger sees the right voltage at its own terminals, but less arrives at the battery, which therefore never gets completely full. On long charging cables that is the most common reason a battery stays consistently below its capacity.

You calculate beforehand; you measure when something does not work as expected. Measuring is simple and needs only a multimeter.

Switch on the load in question, because without current there is no drop and every point reads the same. Then measure voltage in two places: at the battery terminals and at the appliance's connection. The difference is your voltage drop over the whole cable, there and back.

If that difference is larger than expected, split the measurement. Measure across the positive alone by holding the meter between the battery positive and the appliance positive, then do the same across the negative. If most of the loss sits in the negative, the earth connection is the problem, not the cable size.

What you often find: a connection running warm. A poorly crimped lug, a corroded earth bolt or a loose terminal produces a local loss larger than ten metres of cable. Feel the connections after a minute under load; anything warm is wrong.

When calculation or measurement shows too much loss, there are four ways out, in order of how often they work.

Upsizing the cable is the direct answer. Doubling the cross section halves the loss. That is usually simplest, because the rest of the installation stays as it is.

Shortening the cable works better than people expect. Loss is directly proportional to length, so moving the inverter next to the battery instead of next to the sockets can take a 70 mm² cable down to 25 mm².

Checking the connections costs nothing. A poor lug or earth point can produce more loss than the cable itself, and thicker copper does not fix that.

Raising the system voltage is the last answer and the most far-reaching. At 24 volts the current is halved and the percentage loss becomes a quarter of what it was at 12 volts. That is rarely a retrofit solution, but it is worth weighing while the installation is still on paper.

1. Forgetting that current has to travel out and back. A device 3 metres away has 6 metres of copper. The factor 2 in the formula accounts for this, but when buying cable you still need twice the one-way distance.
2. Calculating with nominal current instead of peak current. A 1500W inverter draws up to 2-3 times nominal at peak (motor start-up, microwave switching on). Size the cable for the expected peak.
3. Confusing "cross-section" with "thickness". Cross-section is in mm², the area of the conductor. "Thickness" is often used for the diameter, which is something else. AWG tables use yet another numbering system (a lower AWG number means a thicker cable).
4. Underestimating the conductivity of tinned cable. Tinned copper (used in marine and automotive cable) has practically the same conductivity as pure copper. You can simply use 56 as the κ value.
5. Forgetting that warm cables lose more. At 60 °C the resistance of copper is about 16% higher than at 20 °C. In a hot engine bay or with cables bundled tightly together you therefore need a safety margin. Our cable calculator applies this automatically.