A DC-DC (B2B) charger pulls steady current from the alternator and feeds the house bank — both sides need the right cable and fusing. Wattonomy computes the gauge and fuse for any DC-DC charger at 12, 24 or 48 V, to ABYC E-11.
DC-DC charger output current over a 3 m run, sized to ABYC E-11 with a 3% voltage-drop limit.
Cable gauge and fuse for common DC-DC chargers by rated current, computed at a 3 m run (alternator runs are usually longer).
| Charger | 12 V | 24 V | 48 V |
|---|---|---|---|
| 18 A | 8 AWG18 A · 25 A fuse · 2% | 12 AWG18 A · 25 A fuse · 2.7% | 14 AWG18 A · 25 A fuse · 2% |
| 30 A | 6 AWG30 A · 40 A fuse · 2% | 10 AWG30 A · 40 A fuse · 2.9% | 10 AWG30 A · 40 A fuse · 1.4% |
| 50 A | 4 AWG50 A · 80 A fuse · 2.1% | 6 AWG50 A · 80 A fuse · 1.7% | 6 AWG50 A · 80 A fuse · 0.8% |
Figures are the charger’s rated current over a 3 m run, to ABYC E-11 with a 3% voltage-drop limit. Both the alternator feed and the battery output are fused. Alternator runs are often long — size your exact length in the designer, and check the alternator can supply the charger (the engine flags the 50% rule).
Alternator-to-house runs cross the vehicle, so a too-thin cable loses voltage and the charger underperforms or overheats the wire. Wattonomy sizes the conductor for the run length and fuses both ends, to ABYC E-11.
Charger current and a long run length drive the gauge; both ends get a fuse.
A DC-DC charger draws and delivers its rated current (30 A, 50 A) — that sizes both cables.
Alternator runs are often the longest in the build, so voltage drop dominates. The table assumes 3 m; the designer takes your real length.
A fuse near the starter battery and one at the house busbar — each conductor protected at its source, to ABYC E-11.
The charger can only deliver what the alternator supplies; the designer flags when the alternator is too small (50% rule).
This table sizes one run. The designer sizes your entire build — battery, inverter, solar, every cable and fuse — and draws the wiring diagram, free.
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At 12 V a 50 A charger needs about 6 AWG with an 80 A fuse over a short run — but alternator runs are long, so the gauge often goes up for voltage drop. Wattonomy sizes it to your exact length.
Yes — a fuse near the starter battery protects the feed, and one at the house busbar protects the output. The designer sizes both to ABYC E-11.
Roughly twice the charger’s current as a rule of thumb (the 50% rule) so the alternator isn’t overheated. The designer flags when your alternator is undersized.
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Plain version: these are the recognized rulebooks your cable and fuse are sized against, so the numbers hold up to a surveyor, an inspector or an insurer.
Wattonomy applies these standards in its calculations. It is not certified, sponsored or endorsed by ABYC, ISO, NFPA or Victron — it sizes your design to meet what they require, and shows the working.
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