Level 1 · Starter build
Budget AGM Starter wiring diagram
The lowest-cost way into van electrics: an AGM battery, a modest solar panel and alternator charging running a small fridge, lights and USB sockets. Everything is sized conservatively so the budget bank lasts.
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This example starts with 710 Wh of daily appliance demand. Trace each charging source to the battery, then follow each protected supply to its load and back through the return path. The table makes the drawing searchable and gives you a record to compare with your actual cable routes.
Cable sizes and protective devices below are calculated planning outputs for the stated assumptions. They need checking against equipment manuals, installation conditions, available fault current and the applicable standards. Changing route length, battery voltage or appliance demand can change the result. An AWG conversion alone is not an ampacity check.
- Replace example appliance hours with your measured or intended use. Check both daily energy and simultaneous power.
- Place equipment to preserve ventilation, access and manufacturer clearances. Measure each real cable route before ordering.
- Review the supply, protective device, cable and terminal ratings as one circuit. Check battery and charger limits together.
- Have mains switching, earthing and protective-device operation assessed for every supply mode before use.
- Keep the final drawing, settings, photographs and commissioning results with the van. Recheck them after an upgrade.
Cable schedule and assumed routes
| Connection | Current | One-way length | Cable | Fuse | Voltage drop |
|---|---|---|---|---|---|
| Leisure Battery Bank → Positive & Negative Busbars positive | 12.0 A | 0.5 m | 1 mm² | 15 A MEGA | 1.80% |
| Solar Array → MPPT Charge Controller positive | 17.0 A | 5 m | 4 mm² | See protection schedule | 2.00% |
| MPPT Charge Controller → Positive & Negative Busbars positive | 45.0 A | 1.5 m | 10 mm² | 60 A MIDI / MEGA at busbar | 2.00% |
| Starter Battery / Alternator → DC-DC Charger positive | 48.0 A | 5 m | 25 mm² | 60 A MIDI at starter battery | 2.80% |
| DC-DC Charger → Positive & Negative Busbars positive | 40.0 A | 1.5 m | 10 mm² | 50 A MIDI at busbar | 1.80% |
| Positive & Negative Busbars → 12 V Fuse Box positive | 12.0 A | 2 m | 4 mm² | 15 A MIDI at busbar | 1.80% |
| 12 V Fuse Box → Compressor Fridge (Small) positive | 3.8 A | 4 m | 1.5 mm² | 5 A Blade, way #1 | 3.00% |
| 12 V Fuse Box → LED Spot Lights positive | 1.7 A | 6 m | 1 mm² | 3 A Blade, way #2 | 3.00% |
| 12 V Fuse Box → Water Pump positive | 5.0 A | 5 m | 1.5 mm² | 7.5 A Blade, way #3 | 4.90% |
| 12 V Fuse Box → USB Sockets positive | 0.8 A | 4 m | 1 mm² | 2 A Blade, way #4 | 0.90% |
| Leisure Battery Bank → Positive & Negative Busbars negative | 12.0 A | 0.5 m | 1 mm² | See protection schedule | 1.80% |
| Solar Array → MPPT Charge Controller negative | 17.0 A | 5 m | 4 mm² | See protection schedule | 2.00% |
| MPPT Charge Controller → Positive & Negative Busbars negative | 45.0 A | 1.5 m | 10 mm² | See protection schedule | 2.00% |
| Starter Battery / Alternator → DC-DC Charger negative | 48.0 A | 5 m | 25 mm² | See protection schedule | 2.80% |
| DC-DC Charger → Positive & Negative Busbars negative | 40.0 A | 1.5 m | 10 mm² | See protection schedule | 1.80% |
| Positive & Negative Busbars → 12 V Fuse Box negative | 12.0 A | 2 m | 4 mm² | See protection schedule | 1.80% |
| 12 V Fuse Box → Compressor Fridge (Small) negative | 3.8 A | 4 m | 1.5 mm² | See protection schedule | 3.00% |
| 12 V Fuse Box → LED Spot Lights negative | 1.7 A | 6 m | 1 mm² | See protection schedule | 3.00% |
| 12 V Fuse Box → Water Pump negative | 5.0 A | 5 m | 1.5 mm² | See protection schedule | 4.90% |
| 12 V Fuse Box → USB Sockets negative | 0.8 A | 4 m | 1 mm² | See protection schedule | 0.90% |
| Starter Battery / Alternator → DC-DC Charger signal | 0.5 A | 5 m | 0.75 mm² | 5 A Blade at ignition feed | 0.00% |
| MPPT Charge Controller → Leisure Battery Bank signal | 0.0 A | 1.5 m | 0.5 mm² | See protection schedule | 0.00% |
| Positive & Negative Busbars → Chassis Bond protective-earth | 0.0 A | 1 m | 16 mm² | See protection schedule | 0.00% |
Appliance branch circuits
| Load | Current | Route | Cable | Fuse |
|---|---|---|---|---|
| Compressor Fridge (Small) | 3.8 A | 4 m | 1.5 mm² | 5 A |
| LED Spot Lights | 1.7 A | 6 m | 1 mm² | 3 A |
| Water Pump | 5.0 A | 5 m | 1.5 mm² | 7.5 A |
| USB Sockets | 0.8 A | 4 m | 1 mm² | 2 A |
Protection and installation parts
- MEGA fuse 15 A + holder — 1; Main battery protection
- Battery isolator switch — 1; 100 A+ continuous rating
- Battery interconnect cables — 2; 1 mm² — identical length & cross-section per Victron guidance
- Busbar pair (+/−) — 1 pair; ≥ 150 A, insulated covers
- PV isolator switch — 1; DC-rated, ≥ 56 V
- Solar roof gland + MC4 connectors — 1 set; Waterproof cable entry
- MC4 extension cable pair — ≈ 2; Panel-to-panel and string-to-gland runs, UV-rated
- MIDI fuse holders + fuses — 2; 60 A and 50 A for DC-DC run
- 6-way 12 V blade fuse box — 1; With negative bus
- 12 V blade fuse assortment — 1 set; 5 A, 3 A, 7.5 A, 2 A
- D+ signal wire 0.75 mm² — ≈ 5 m; Ignition feed → DC-DC charger D+ terminal
- 25 mm² cable — ≈ 11 m total; Red + black pair (lengths include 10 % spare)
- 16 mm² cable — ≈ 3 m total; Red + black pair (lengths include 10 % spare)
- 10 mm² cable — ≈ 7 m total; Red + black pair (lengths include 10 % spare)
- 4 mm² cable — ≈ 16 m total; Red + black pair (lengths include 10 % spare)
- 1.5 mm² cable — ≈ 40 m total; Red + black pair (lengths include 10 % spare)
- 1 mm² cable — ≈ 46 m total; Red + black pair (lengths include 10 % spare)
- Crimp lugs & ferrules — assorted; Sized to each cable; use a proper crimper
- Heat shrink, grommets, cable clips — assorted; Abrasion protection everywhere a cable touches metal
Assumptions to review
- System voltage 12 V (12 V nominal AGM). Voltage drop calculated round-trip (2 × run length) with copper resistivity 0.0175 Ω·mm²/m.
- Voltage drop targets: ≤ 2.5 % main battery/charge runs (Victron), ≤ 3 % critical circuits (fridge, electronics), ≤ 5 % non-critical circuits (Blue Sea 3 %/10 % tiers, drawn conservatively).
- Cable ampacities from UK thin-wall (≤ 6 mm²) and Hi-Flex battery cable (≥ 10 mm²) published ratings, free-air.
- Fuses sized ≥ 1.25 × continuous load and ≤ cable ampacity, so the fuse always protects the cable.
- Solar panel Voc ≈ 23 V and Isc ≈ W ÷ 19 estimated for typical "12 V" panels; cold-weather Voc factor 1.2. Check your actual panel datasheet.
- Inverter DC current = W ÷ (11.5 V × 0.87 efficiency). Where a product manual states cable/fuse sizes, the manual takes precedence.
- Circuit run lengths are typical-van defaults (fridge 4 m, lighting 6 m, etc.) — measure yours and resize if longer.
- UK solar yield assumed 2.8 peak sun hours at 72 % system efficiency (annual average).
References and next steps
- Victron Energy — Wiring Unlimited — Cable sizing, fuse-protects-cable principle, <2.5 % drop target, lithium interrupt ratings, parallel bank interconnects, busbar sizing
- Blue Sea Systems — Choosing the Correct Wire Size for a DC Circuit — 3 % vs 10 % voltage-drop tiers by circuit criticality (ABYC E-11 method)
- BS 7671:2018 Section 721 (caravans & motor caravans) — 30 mA RCD per inlet disconnecting all live conductors, direct inlet-to-RCD connection, protective bonding
- 12 Volt Planet — Cable Sizing & Selection — UK cable ampacity ratings, voltage-drop methodology
- Victron Energy — solar series vs parallel guidance — String Voc cold-weather margin vs MPPT max input, series/parallel trade-offs, string fusing
Read the wiring safety guide or calculate your own appliance plan.
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