Budget AGM Starter wiring diagram

Read and adapt this wiring plan

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.

1. Replace example appliance hours with your measured or intended use. Check both daily energy and simultaneous power.
2. Place equipment to preserve ventilation, access and manufacturer clearances. Measure each real cable route before ordering.
3. Review the supply, protective device, cable and terminal ratings as one circuit. Check battery and charger limits together.
4. Have mains switching, earthing and protective-device operation assessed for every supply mode before use.
5. Keep the final drawing, settings, photographs and commissioning results with the van. Recheck them after an upgrade.

Cable schedule and assumed routes

Appliance branch circuits

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; 50 A and 40 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)
• 6 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 cross-sections are shown in mm² (SI). US builders: 6 mm² ≈ 9 AWG, 10 mm² ≈ 7 AWG, 16 mm² ≈ 5 AWG, 25 mm² ≈ 3 AWG, 35 mm² ≈ 2 AWG — confirm ampacity against NEC / ABYC tables for your install.
• 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.
• US solar yield assumed 4 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)
• NEC (NFPA 70) Article 551 & NFPA 1192 — Recreational Vehicles — AC shore-power inlets, GFCI protection, RV branch circuits and grounding/bonding
• ABYC E-11 — AC & DC Electrical Systems — DC conductor ampacity and overcurrent protection widely applied to RV/van builds
• 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.