12V Van and Camper Wiring Guide
A step-by-step sizing walkthrough for a 12V DC leisure electrical system in a van, camper or motorhome. Covers battery capacity, wire gauge for each circuit, fuse selection and voltage drop — with a worked example for a typical weekend build.
Last updated: May 2026
The scenario
You are fitting out a panel van or motorhome with a 12V leisure system. The load list for a typical weekend build: four LED lights (10 W each), a 12V water pump (40 W), USB charging for phones and tablets (15 W combined), and a 12V compressor fridge (averaging 45 W). Total simultaneous draw is around 130 W, and the system runs roughly 10 hours per day, giving a daily energy budget of about 550 Wh.
The five steps below take you from a blank sheet to a working wiring plan, with the specific numbers to enter in each calculator.
Step 1: Load inventory and daily energy budget
List every load with its wattage and the estimated hours per day it runs. Multiply to get the daily Wh for each load, then sum.
| Load | Power (W) | Hours/day | Daily energy (Wh) |
|---|---|---|---|
| LED lights (4x) | 40 | 4 | 160 |
| 12V water pump | 40 | 0.5 | 20 |
| USB charging | 15 | 3 | 45 |
| 12V compressor fridge | 45 | 8 | 360 |
| Total | — | — | 585 Wh/day |
Step 2: Size the leisure battery
A lead-acid battery (AGM or gel) should not be discharged below 50% — deeper cycling shortens its life significantly. Lithium (LiFePO4) can safely go to 20% state of charge. For the calculation, divide your daily Ah draw by the allowable depth of discharge (DoD), then add enough capacity for the number of days between charges.
For this build: 48.75 Ah/day, 2 days of autonomy, AGM at 50% DoD:
Required Ah = (48.75 × 2) ÷ 0.50 = 195 Ah → choose a 200 Ah AGM
With LiFePO4 at 80% DoD: (48.75 × 2) ÷ 0.80 = 122 Ah → a 120 or 130 Ah lithium battery is sufficient.
Use the calculator: Enter 585 Wh/day, 2 days autonomy, 12V, 50% DoD into the Battery Capacity Calculator. Adjust DoD to 80% and compare the lithium result.
Step 3: Size the wire gauge for each circuit
12V DC systems run higher currents than 230V AC for the same power, so wire sizing matters more. Use the power formula to get current, then pick wire gauge for the run length and acceptable voltage drop. A 3% drop at 12V is 0.36V — keep drops small on 12V to avoid lights dimming and motors running slow.
| Circuit | Power (W) | Current (A) | Run (m) | Recommended wire |
|---|---|---|---|---|
| Main battery feed | 140 W peak | 11.7 A | 1 | 6 mm² (AWG 9) |
| LED lighting sub-circuit | 40 W | 3.3 A | 4 | 1.5 mm² (AWG 15) |
| 12V fridge | 45 W | 3.75 A | 3 | 2.5 mm² (AWG 13) |
| Water pump | 40 W | 3.3 A | 3 | 2.5 mm² (AWG 13) |
| USB hub | 15 W | 1.25 A | 2 | 1 mm² (AWG 17) |
The main feed wire runs from battery to the fuse block or distribution bar. Size it for the maximum total current the system can draw, not just the normal load — here 140 W / 12 V = 11.7 A, so 6 mm² gives safe headroom and keeps the main voltage drop under 0.1 V for a 1 m run.
Use the calculator: Enter current, run length (one-way), 12V, copper, 3% drop limit into the Wire Gauge Calculator for each circuit. The result gives both mm² and AWG and shows the actual voltage drop.
Step 4: Select a fuse for each circuit
Every positive wire leaving the battery or the fuse block needs a fuse rated just above the circuit's expected current. The fuse protects the wire, not the device. Size the fuse at 125% of the circuit's normal current, then round up to the next standard blade fuse size (5, 7.5, 10, 15, 20, 25, 30 A for blade fuses).
| Circuit | Normal current | 125% value | Fuse to fit |
|---|---|---|---|
| Main battery feed | 11.7 A | 14.6 A | 20 A (close to battery, within 30 cm) |
| LED lighting | 3.3 A | 4.1 A | 5 A |
| 12V fridge | 3.75 A | 4.7 A | 7.5 A (allows startup surge) |
| Water pump | 3.3 A | 4.1 A | 7.5 A (allows pump startup) |
| USB hub | 1.25 A | 1.6 A | 5 A |
The most important fuse is the main one directly at the battery terminal — within 30 cm of the positive terminal. If a fault melts the main wire with no battery fuse, the battery will deliver hundreds of amps and could start a fire.
Use the calculator: Enter the circuit current and select "non-continuous" into the Fuse / Breaker Sizing Calculator to verify ratings and see derating tables.
Step 5: Verify voltage drop on the longest runs
Check that the voltage arriving at the end of each circuit is within 3% of 12V (i.e., above 11.64V). The fridge and pump are most sensitive to low voltage — compressors and pump motors run hotter and wear faster when supply voltage sags.
| Circuit | Wire | Run (one-way) | V-drop | V at load | Within 3%? |
|---|---|---|---|---|---|
| LED lights | 1.5 mm² | 4 m | 0.30 V | 11.70 V | Yes (2.5%) |
| 12V fridge | 2.5 mm² | 3 m | 0.16 V | 11.84 V | Yes (1.3%) |
| Water pump | 2.5 mm² | 3 m | 0.14 V | 11.86 V | Yes (1.2%) |
Use the calculator: Enter each circuit's current, run length, wire size and 12V into the Voltage Drop Calculator. If any circuit exceeds 3%, increase the wire size by one step and re-check.
Quick reference: example van build summary
| Item | Value | Notes |
|---|---|---|
| Daily energy | 585 Wh | LED + fridge + pump + USB |
| Battery (AGM, 50% DoD, 2 days) | 200 Ah | LiFePO4 option: 130 Ah at 80% DoD |
| Main feed wire | 6 mm² (AWG 9) | Battery to fuse block, 1 m run |
| Main battery fuse | 20 A | Within 30 cm of battery positive |
| Lighting circuit | 1.5 mm² + 5 A fuse | 4 m run, 3.3 A |
| Fridge circuit | 2.5 mm² + 7.5 A fuse | 3 m run, 3.75 A |
| Pump circuit | 2.5 mm² + 7.5 A fuse | 3 m run, 3.3 A, startup surge tolerance |
Common mistakes
- No main battery fuse: every van fire risk starts here. Fit a 20–30 A fuse or ANL fuse block within 30 cm of the battery positive terminal.
- Undersizing the main feed wire: running 2.5 mm² from battery to fuse block for a 10+ A system. Use 6 mm² for the main run — this wire carries the total current from all circuits simultaneously.
- Ignoring return wire: the negative return wire must be the same size as the positive. A thin earth return causes exactly the same voltage drop as a thin positive wire.
- Sharing a fuse between two circuits: each circuit needs its own fuse. A fuse block with one fuse per circuit is the safest layout.
- Connecting the fridge directly to the starter battery: the leisure battery must be isolated from the starter battery when the engine is off, typically via a battery-to-battery (B2B) charger or a voltage-sensitive relay (VSR).
Frequently Asked Questions
What wire size do I need for a 12V compressor fridge in a van?
For a typical 40–60 W fridge on a 3 m run: 2.5 mm² (AWG 13) keeps voltage drop under 2% and handles the startup current comfortably. Use a 7.5 A or 10 A fuse. If the run is longer than 5 m, increase to 4 mm² to keep the drop under 3%. Run both positive and negative wires at the same gauge — the return wire drops voltage too. Use the Wire Gauge Calculator with 12V and your actual current for exact results.
How close to the battery does the main fuse need to be?
Within 30 cm (12 inches) of the positive battery terminal. This short unfused section is the highest-risk segment: if it shorts against the van body, the battery will deliver hundreds of amps with no protection. An ANL or MIDI fuse holder bolted near the battery terminal is the standard approach for van builds. The fuse rating equals the maximum current capacity of the main feed wire — not the total of your loads.
AGM or lithium for a van build?
AGM is cheaper upfront and works in any climate, but should not go below 50% discharge — so a 200 Ah AGM gives 100 Ah usable. LiFePO4 (lithium iron phosphate) costs 2–3x more but is lighter, can discharge to 20% (80% usable), charges faster, and lasts 2–5x as many cycles. For a weekend build used 50+ days/year, lithium pays back within 3–4 years in battery replacement savings. For occasional use, AGM is simpler and cheaper. Use the Battery Capacity Calculator to compare required Ah at different DoD settings.
Can I connect solar panels directly to the leisure battery?
No — you need a solar charge controller (SCC) between the panels and the battery. Without it, a fully charged battery receiving panel current will overcharge and could be damaged or vent gas. A PWM controller is fine for small systems (under 100W); an MPPT controller extracts 10–30% more energy and is worth the extra cost for larger arrays. Size the controller for the panel's short-circuit current (Isc from the panel datasheet) plus 25% safety margin.
Do I need a separate earth (ground) wire or can I use the van body?
For a 12V leisure system, using the van chassis as the negative return is common and accepted practice. However, a dedicated return wire is better for sensitive electronics (fridges, inverters) because body earth connections corrode and add resistance over time. If you use chassis earthing, use large, clean bolts with star washers at both ends, clean the paint off contact points, and apply copper grease. For inverters over 300W, always run a dedicated negative cable back to the battery.