Voltage Drop in Campervans: How to Calculate and Prevent It
Voltage drop is one of the most misunderstood and underestimated problems in campervan electrical systems. At 12V, even small losses in the cable become significant — a drop that would be trivial in a 230V household circuit can make the difference between a fridge running properly and a fridge shutting down on a low-voltage alarm. Understanding voltage drop, calculating it correctly, and designing your wiring to minimise it is essential for a reliable system.
This guide explains the physics in practical terms, gives you the formula, walks through real-world campervan examples, and shows you how to fix excessive voltage drop. For cable sizing guidance that incorporates voltage drop, see our wire gauge calculator and cable sizes guide. For the full wiring safety picture, start with the wiring and safety guide.
Get Cable Sizes Right First Time
Our free calculator sizes every cable in your system, accounting for both current capacity and voltage drop. No formulas needed — just enter your devices and cable lengths.
What Is Voltage Drop?
Every cable has resistance. When current flows through a cable, some of the electrical energy is lost as heat due to that resistance. The voltage at the far end of the cable is therefore lower than the voltage at the battery — this difference is the voltage drop.
The basic relationship is Ohm's Law:
Voltage Drop (V) = Current (A) x Resistance (ohms)
The resistance of a cable depends on three things:
- Length — longer cables have more resistance
- Cross-sectional area — thinner cables have more resistance
- Material — copper has low resistance; aluminium has higher resistance
In a campervan, you are working at 12V (nominally). A fully charged lithium battery sits at about 13.4V, and a battery at 50% state of charge is around 13.0V. Your devices expect to receive somewhere in the range of 12.0-13.6V. Every volt lost in cable resistance is a volt your devices do not receive.
Why Voltage Drop Matters More at 12V
This is the critical point that many builders miss. A 1V drop at 230V represents only 0.4% of the supply voltage. A 1V drop at 12V represents 8.3% — more than twenty times the proportional loss.
| System Voltage | 1V Drop as % | 2V Drop as % |
|---|---|---|
| 230V AC | 0.4% | 0.9% |
| 24V DC | 4.2% | 8.3% |
| 12V DC | 8.3% | 16.7% |
At 12V, voltage drop is not a minor efficiency concern — it is a functional problem. Excessive voltage drop can cause:
- LED lights flickering or dimming — visible and annoying
- Fridge compressor failing to start — the compressor needs a burst of current at startup, and if the voltage sags below about 11.5V, the compressor controller shuts down
- Inverter low-voltage cutoff — most inverters shut down at 11.0-11.5V. If your battery is at 12.8V but the cable drops 1.5V, the inverter sees 11.3V and shuts off
- USB chargers delivering less current — voltage regulators in USB sockets compensate for some drop, but below about 11V they cannot maintain 5V output
- Charge controllers underperforming — voltage drop in solar cables means less energy reaches the battery
Voltage Drop Is a Round Trip
A common mistake is calculating voltage drop for only one cable (positive). Current must travel from the battery through the positive cable, through the device, and back through the negative cable. The total cable length for voltage drop calculations is always twice the one-way distance — the positive run plus the negative return.
The 3% Rule
The widely accepted guideline for 12V systems is to keep voltage drop below 3% of the system voltage. At 12V nominal:
Maximum acceptable voltage drop = 12V x 0.03 = 0.36V
Some designers use a stricter 2% target (0.24V) for critical circuits like fridge and inverter feeds. Others allow up to 5% (0.6V) for non-critical circuits like interior lights. The 3% rule is a sensible default for most circuits.
| Drop Limit | Voltage Loss | Application |
|---|---|---|
| 2% (strict) | 0.24V | Inverter feed, fridge, sensitive electronics |
| 3% (standard) | 0.36V | General circuits — water pump, fans, USB, lighting |
| 5% (relaxed) | 0.60V | Non-critical loads — LED strips, secondary lighting |
The Voltage Drop Formula
The formula for voltage drop in a DC circuit using copper cable is:
Vdrop = (2 x L x I x 0.0175) / A
Where:
- Vdrop = voltage drop in volts
- L = one-way cable length in metres (not the total round-trip — the formula accounts for both directions with the "2 x" multiplier)
- I = current in amps
- 0.0175 = resistivity of copper at 20 degrees C (ohm-mm2/m)
- A = cable cross-sectional area in mm2
To rearrange for the minimum cable size needed:
A = (2 x L x I x 0.0175) / Vdrop(max)
Temperature Affects Resistance
The resistivity figure of 0.0175 is for copper at 20 degrees C. At higher temperatures, resistance increases. In a hot engine bay or behind a sun-baked panel, cables can reach 50-60 degrees C, where the resistivity is closer to 0.020. If your cable runs through hot areas, add a 15-20% safety margin to your cable size calculation.
Worked Examples
Example 1: Fridge Circuit
A 12V compressor fridge draws 5A and is located 4 metres from the battery.
Given:
- L = 4m (one-way)
- I = 5A
- Cable: 2.5mm2
Calculation: Vdrop = (2 x 4 x 5 x 0.0175) / 2.5 Vdrop = 0.70 / 2.5 Vdrop = 0.28V (2.3% at 12V) -- within the 3% limit
With 2.5mm2 cable, this circuit meets the 3% rule. However, remember that the fridge compressor has a startup surge of 10-15A. At 15A:
Vdrop = (2 x 4 x 15 x 0.0175) / 2.5 = 0.84V (7%)
This startup surge causes a momentary 7% voltage drop. For a fridge, upgrading to 4mm2 cable would be wise:
Vdrop at 15A = (2 x 4 x 15 x 0.0175) / 4.0 = 0.53V (4.4%) -- much better
Example 2: Rear Lighting Circuit
A set of LED lights draws 2A, located 6 metres from the fuse box.
Given:
- L = 6m
- I = 2A
- Cable: 1.5mm2
Calculation: Vdrop = (2 x 6 x 2 x 0.0175) / 1.5 Vdrop = 0.42 / 1.5 Vdrop = 0.28V (2.3%) -- within the 3% limit
1.5mm2 cable is adequate for this circuit.
Example 3: Inverter Feed (High Current)
A 1000W inverter at full load draws approximately 90A from a 12V battery. The inverter is mounted 1.5 metres from the battery.
Given:
- L = 1.5m
- I = 90A
- Cable: 25mm2
Calculation: Vdrop = (2 x 1.5 x 90 x 0.0175) / 25 Vdrop = 4.725 / 25 Vdrop = 0.19V (1.6%) -- within the 2% target for critical circuits
25mm2 cable is appropriate here. Dropping to 16mm2 would give:
Vdrop = (2 x 1.5 x 90 x 0.0175) / 16 = 0.30V (2.5%) -- still acceptable at the 3% level but tight for a critical circuit
Example 4: Solar Panel Cable
A 200W solar panel producing 11A at maximum power is mounted on the roof, 5 metres from the charge controller.
Given:
- L = 5m
- I = 11A
- Cable: 4mm2
Calculation: Vdrop = (2 x 5 x 11 x 0.0175) / 4 Vdrop = 1.925 / 4 Vdrop = 0.48V (2.6% at 18V panel voltage)
Note that for solar cables, the relevant voltage is the panel's operating voltage (typically 18-20V for a 12V panel), not the battery voltage. 2.6% of 18V is within the 3% target. However, using 6mm2 cable would reduce the drop to 0.32V (1.8%) and deliver noticeably more energy to the battery over a full day.
Common Cable Sizes and Their Voltage Drop
This table shows the voltage drop per metre (one-way) per amp for common cable sizes. Multiply by 2 for the round-trip, then multiply by your one-way length and current.
| Cable Size (mm2) | Resistance (ohm/m) | Drop per Amp per Metre (one-way) |
|---|---|---|
| 1.0 | 0.0175 | 0.0175V |
| 1.5 | 0.0117 | 0.0117V |
| 2.5 | 0.0070 | 0.0070V |
| 4.0 | 0.0044 | 0.0044V |
| 6.0 | 0.0029 | 0.0029V |
| 10.0 | 0.0018 | 0.0018V |
| 16.0 | 0.0011 | 0.0011V |
| 25.0 | 0.0007 | 0.0007V |
| 35.0 | 0.0005 | 0.0005V |
How to Fix Excessive Voltage Drop
If your calculations show too much voltage drop — or if you are experiencing symptoms like flickering lights, fridge shutdowns, or inverter cutoffs — there are five approaches, listed in order of effectiveness.
1. Use Thicker Cable
The most direct solution. Doubling the cable cross-sectional area halves the voltage drop. If 2.5mm2 gives too much drop, upgrade to 4mm2 or 6mm2. This is the correct approach for new installations.
2. Shorten the Cable Run
Halving the cable length halves the voltage drop. Can you move the fuse box closer to the battery? Can you relocate the inverter nearer to the battery bank? In a campervan, even reducing a run from 4 metres to 2 metres makes a significant difference at high currents.
3. Reduce the Current
If the device allows, run it at a lower power setting. Alternatively, consider whether you can use a more efficient device that draws less current for the same output.
4. Move to a Higher Voltage System
This is a design-level decision, not a retrofit fix. A 24V system carries the same power at half the current, which means one quarter the voltage drop (since drop depends on current, not power). If you are still in the design phase and have high-power requirements, a 24V system is worth considering. See our 12V vs 24V system comparison.
5. Use a DC-DC Converter at the Load End
For specific circuits with long runs (like a ceiling light at the far end of the van), a small DC-DC step-down converter at the load end can regulate the output voltage to a stable 12V regardless of the voltage drop in the feed cable. This is an advanced solution for specific problems, not a general approach.
Measuring Voltage Drop in an Existing System
If your system is already installed, you can measure actual voltage drop with a multimeter:
- Measure battery terminal voltage with the load switched on — note this as V1
- Measure voltage at the device terminals with the same load running — note this as V2
- Voltage drop = V1 - V2
For detailed multimeter techniques, see our guide on testing campervan wiring with a multimeter.
If the measured drop exceeds your acceptable limit, upgrade the cable, shorten the run, or both.
Measure Under Load, Not Open Circuit
Voltage drop only occurs when current is flowing. If you measure voltage at the device with the device switched off, you will read battery voltage regardless of cable size. Always measure with the device running at its normal operating current to get a meaningful reading.
Voltage Drop in Connections
Cable is not the only source of voltage drop. Every connection — crimp terminal, fuse, bus bar junction, switch — adds a small amount of resistance. In a well-made installation, each connection adds a negligible amount. But a poorly crimped terminal, a corroded bus bar connection, or a cheap switch can add 0.1-0.3V of drop each.
Over a circuit with six or eight connections, poor connection quality can contribute more voltage drop than the cable itself. This is why proper crimping technique, clean contact surfaces, and appropriate torque on terminal screws matter enormously. For cable routing and connection best practices, see our cable routing guide.
FAQ
What is an acceptable voltage drop for a campervan?
The standard guideline is a maximum of 3% of the system voltage, which equals 0.36V for a 12V system. For critical circuits like fridge and inverter feeds, aim for 2% (0.24V) or less. For non-critical circuits like LED lighting, up to 5% (0.60V) is usually acceptable without noticeable effects.
Does voltage drop waste electricity?
Yes. The energy lost to voltage drop is dissipated as heat in the cable. In a campervan, the amount of wasted energy is usually small in absolute terms (a few watts), but the impact on device performance is the bigger concern. A warm cable is also a sign that it is working harder than it should — and may be undersized for the current it carries.
Can I use an online voltage drop calculator instead of the formula?
Absolutely. Online calculators and our wire gauge calculator do the maths for you. Just make sure the calculator uses the correct resistivity for copper (0.0175 ohm-mm2/m at 20 degrees C) and that you input the one-way cable length, not the round-trip length (the calculator should account for the return path).
Why does my fridge work fine from the battery but not through the fuse box?
The fuse box adds cable length (from battery to fuse box) and connection resistance (fuse contacts, bus bar junctions). If the total cable run is significantly longer via the fuse box than a direct battery connection, and the cable gauge is borderline, the additional voltage drop through the fuse box path may push the fridge below its minimum operating voltage during compressor startup surges. The fix is usually thicker cable from the battery to the fuse box, or relocating the fuse box closer to the battery.