Design your 12V system with confidence.
A practical, regulation-aware toolkit for sizing the power system behind your next adventure. Adjust your load below and get a clear architecture in seconds.
Live build preview — updates with your loads
Official VanSpark 12V Pro Blueprint
The live calculation grids are intentionally protected from direct printing. Purchase the official £7.99 PDF Blueprint for the professional, build-ready documentation.
Includes the Master Wiring Schematic, Full Circuit Fuse/Cable Schedule, and BS 7671 Compliance Checklist.
01 / Daily load breakdown
What are you powering?
Add a custom load
Your estimated daily use
Tap any appliance to include or exclude it from your system sizing. Add your own loads below the grid — watts or amps both work. Estimates use 12V nominal voltage.
02 / Architecture grid
Your system, sized instantly.
LiFePO4 battery bank
130 Ah
2 days autonomy · 80% usable DoD
Solar array output
250 W
Spring / Autumn · 3.2 peak sun hrs
Includes a 30% real-world derate: flat mounting, heat, cloud and dirt all cut brochure yield.
Inverter rating
Not needed
No 240V loads selected
Main supply cable
25 mm²
2.1% drop at 60A over 3m
03 / Triple-charging pillar
Balance every source of energy
Build the system around three complementary charging paths: harvest daylight, recover energy while driving, and top up safely when parked.
Solar
250W array on a 30A MPPT, sized against the spring / autumn yield preset.
DC-DC alternator
Victron Orion-Tr Smart, fused at both batteries for smart alternator vehicles.
Shore power
A mains charger and RCD-protected hookup provide a reliable parked-up top-up.
Daily energy balance — can your solar keep up?
614 Wh/day demandWinter solar covers 47% of your daily use — the bank buffers ~3.8 days before you need alternator driving or shore hookup. Summer is self-sufficient.
Assumes real-world derated yield. As a rule of thumb, 1 hour of driving adds ~360Wh via the 30A DC-DC, and shore hookup refills the bank overnight.
04 / Cable check
Voltage drop calculator
Set your run length and continuous current to validate the main cable. Drop is calculated on the full round trip — out on the positive and back on the negative.
Take the guesswork out of your build.
Get the complete printable blueprint with wiring diagrams, fuse schedules, component links, and an installation checklist.
05 / Quick reference
Cable sizing at a glance
Typical copper cable guidance for 12V campervan runs. Keep positive and negative runs together.
| Cross-section | Typical load | Max amps | Safe run |
|---|---|---|---|
| 0.5mm² | LED lighting | 3A | 3m |
| 1.0mm² | LED lighting | 6A | 5m |
| 1.5mm² | Water pump | 10A | 6m |
| 2.5mm² | USB sockets | 16A | 7m |
| 4.0mm² | 12V sockets / USB | 25A | 8m |
| 16mm² | Main inverter | 60A | 3m |
| 25mm² | Main inverter | 90A | 4m |
| 35mm² | Large inverter | 120A | 5m |
| 50mm² | Large inverter | 150A | 6m |
12V DC only. Cable ratings are indicative; confirm installation method, ambient temperature, grouping and voltage drop before selecting final protection.
06 / Circuit guide
Individual circuit sizing
Select a branch circuit and run length for an instant recommendation.
Recommended wire
2.5 mm²
Fuse size
10 A
07 / Wiring schematic
Your system, drawn live
Single-line diagram generated from your configuration — cable sizes, fuse ratings and branch circuits update as you change the load list.
Click any component marked ⓘ — MPPT, DC-DC, fuse block, inverter, shunt — for an exploded view with terminal-by-terminal wiring steps.
08 / Live bill of materials
Everything you need to buy
Quantities update with your load list. Branch circuits assume a typical one-way run of 4m — adjust it below to match your van.
Component tier
| Item | Spec | Guide | Buy |
|---|---|---|---|
| Leisure battery | LiFePO4 · Fogstar Drift · 2× 100Ah | £250–450 | Fogstar |
| Solar panels | Victron BlueSolar rigid · 2× 215W | £110–190 | Victron dealer |
| MPPT controller | Victron SmartSolar · 30A | £60–140 | Victron dealer |
| DC-DC charger | Victron Orion-Tr Smart · 30A | £130–230 | Victron dealer |
| Mains charger | Victron Blue Smart IP65 · 12/25 | £110–190 | Victron dealer |
| Battery monitor | Victron SmartShunt · 500A | £90–130 | Victron dealer |
| Isolator + busbars | Master cut-off · pos/neg pair | £30–70 | 12 Volt Planet |
| Blade fuse block | 4 circuits + spares · 6-way | £15–35 | 12 Volt Planet |
| Item | Detail | Guide | Buy |
|---|---|---|---|
| 25mm² tri-rated cable | Main + branches · red + black · 7m | £62–100 | 12 Volt Planet |
| 2.5mm² tri-rated cable | Branches · red + black · 24m | £29–47 | 12 Volt Planet |
| 1.5mm² tri-rated cable | Branches · red + black · 8m | £6–10 | 12 Volt Planet |
| MEGA fuse + holder | ≤300mm from battery + · 80A | £15–30 | 12 Volt Planet |
| Tinned copper lugs | 25mm² main run · ×4 | £2–6 | 12 Volt Planet |
| Blade fuses | 1.25× circuit load · 1× 5A · 2× 10A · 1× 15A | £5–12 | Amazon UK |
| Circuit | Load | Wire | Fuse | Cable needed |
|---|---|---|---|---|
| 12V Compressor Fridge | 3.8A | 2.5mm² | 10A blade | 8m |
| Diesel Heater | 2.9A | 2.5mm² | 15A blade | 8m |
| LED Lighting | 1.0A | 1.5mm² | 5A blade | 8m |
| USB Device Charging | 1.5A | 2.5mm² | 10A blade | 8m |
Cable lengths include positive and negative runs (7m on the main feed). Add your own margin for routing around the van, grommets, heat-shrink and a few spare fuses. Guide prices are estimates — compare suppliers before buying. Some links are affiliate links: we may earn a commission at no extra cost to you, which keeps the tool free.
09 / 240V AC mains sizing
Inverter output to 3-pin sockets
This is a separate AC circuit and must not be sized like the 12V DC wiring above. Use 2.5mm² 3-core flexible cable from the inverter output to a proper consumer unit with RCD protection, installed and tested by a qualified person.
Cable
2.5mm² 3-core flex
Protection
Consumer unit + RCD
Scope
240V AC only
10 / Smart alternator advisor
Euro 6 and BlueMotion charging
Modern Euro 6 / BlueMotion vehicles use smart alternators that reduce output once the starter battery is charged. A conventional split-charge relay may undercharge the leisure bank; use a smart DC-DC charger such as Victron Orion-Tr Smart or Renogy with ignition/D+ sensing, correctly fused at both batteries.
11 / Fuse & protection schedule
Protect every cable and connection
The fuse protects the cable, not the appliance. Place high-current protection within 300mm of the battery positive terminal, use correctly rated interrupt capacity, and have AC work installed and tested by a qualified person.
| Inverter | Fuse type | Rating | Cable |
|---|---|---|---|
| 300W | MEGA slow-blow | 40A | 10–16mm² |
| 600W | MEGA slow-blow | 80A | 16–25mm² |
| 1000W | MEGA / ANL slow-blow | 125–150A | 25–35mm² |
| 1500W | MEGA / Class T | 200A | 35–50mm² |
| 2000W | Class T preferred | 250A | 50mm² |
| 2500W | Class T / ANL | 300A | 70mm² |
| 3000W | ANL / Class T | 350–400A | 70–95mm² |
| Circuit | Fuse | Wire |
|---|---|---|
| LED lighting | 3–5A | 1.0–1.5mm² |
| Water pump | 5–10A | 1.5–2.5mm² |
| Diesel heater | 15A | 2.5–4.0mm² |
| USB / 12V sockets | 10–15A | 2.5mm² |
| Compressor fridge | 10–15A | 2.5–4.0mm² |
Solar protection
Panel side: 1.25 × Isc. Battery side: match MPPT output and cable; a 100/30 typically uses 40A.
DC-DC chargers
Fuse both starter and leisure battery ends: 30A/40A/70–80A for 20A/30A/50A chargers.
System management
SmartShunt stays in the full negative return with a 1A supply fuse. BatteryProtect guards loads; isolate the main positive with a heavy-duty cut-off.
Blueprint preview
Your professional build blueprint
A concise, build-ready PDF with the decisions behind this estimate, prepared for a qualified installer to review.
Secure checkout
Download the full plan
Stripe-secured purchase for the complete PDF blueprint.
12 / Tested hardware
A reliable parts shortlist.
Premium picks — Victron / Fogstar / Blue Sea
Fogstar Drift Heated LiFePO4
105Ah · built-in heater
Aluminium Z-Brackets
Universal · 4-piece set
Flexible Solar Panel
100W · low-profile option
IP68 Roof Entry Gland
Twin cable · UV stable
Victron SmartSolar MPPT
75/15 · Bluetooth
Victron Orion-Tr Smart DC-DC
30A · isolated
Victron Phoenix Inverter
Pure sine · heavy-load ready
Blue Sea Systems Fuse Block
MIDI / MEGA compatible
MIDI / MEGA Heavy-Duty Fuses
High-current circuit protection
Battery Isolator Switch
Continuous-duty master cut-off
Tri-Rated Copper Cables
16 · 25 · 35 · 50mm²
Tinned Marine Copper Lugs
Crimp terminals · corrosion resistant
12 Volt Planet
Best for wiring, terminals & marine accessories
Fogstar / Fogstar Drift
UK heated LiFePO4 specialists
Van Junkies / Wired Campers
Victron gear & complete off-grid kits
Mid-tier picks — Renogy
Renogy Core LiFePO4 Battery
100Ah · 12.8V · Bluetooth option
Renogy 200W Solar Kit
Rigid mono · MC4 leads + brackets
Renogy Rover MPPT
40A · LCD + Bluetooth
Renogy DCC30S DC-DC
30A · MPPT dual input
Renogy Pure Sine Inverter
2000W · 12V · induction-ready
Renogy 500A Battery Monitor
Shunt-based · 2.4in display
Budget-friendly picks — Eco-Worthy / generic
Eco-Worthy LiFePO4 Battery
100Ah · 12.8V · budget pick
Eco-Worthy Pure Sine Inverter
2000W · 12V · budget pick
Eco-Worthy Solar Panel Kit
195W mono · mounts included
Generic MPPT Controller
30–40A · budget class
Budget DC-DC Charger
20–40A · battery-to-battery
Generic Battery Monitor
Shunt + LCD display
Welding Cable 35–70mm²
Heavy inverter feed · cut to length
MEGA / MIDI Fuse + Holder Kit
80–400A · high-current
Heavy-Duty Isolator Switch
275A continuous rating
13 / Common questions
Campervan electrics, answered.
What size leisure battery do I need for a campervan?
Add up your daily watt-hours and divide by 12 to get daily amp-hours, then multiply by the days of autonomy you want and divide by usable depth of discharge — 80% for LiFePO4, 50% for AGM. A typical weekend build with a fridge, heater and lights lands around 150–250Ah of lithium.
What cable size should I use for 12V campervan wiring?
Size cable on the full round trip — positive out and negative back — keeping voltage drop under 3%. As a guide: 1.5mm² for lighting, 2.5mm² for pumps and USB sockets, 16–25mm² for inverter feeds up to 1000W, and 50–95mm² for 2000–3000W inverters on short runs. Always check both drop and ampacity.
Do I need a DC-DC charger with a smart alternator?
Almost always, yes. Euro 6 and BlueMotion vehicles reduce alternator output once the starter battery is charged, so a basic split-charge relay undercharges the leisure bank. A DC-DC charger such as the Victron Orion-Tr Smart with ignition sensing maintains proper multi-stage charging.
Is a 2000W inverter enough for an induction hob?
Just about — a single-zone induction hob draws 1500–1800W, so a 2000W pure sine inverter covers it with a thin margin. It will pull 150–170A from a 12V battery, so you need at least a 200Ah LiFePO4 bank with a 200A+ BMS, Class T protection, and 50mm²+ cable kept as short as possible.
Where should the main fuse go in a campervan electrical system?
Within 300mm of the battery positive terminal. The fuse protects the cable, not the appliance, so it must sit before any cable run that could short to the chassis. High-current inverter feeds should use MEGA, ANL, or Class T fuses rated above the inverter draw but below the cable ampacity.