Wire Your Rig

DC fuse size calculator

Pick a fuse that won't blow in normal use but will blow before the wire overheats.

Free printable: 12V Wire Gauge & Fuse Size Cheat Sheet (PDF, 2 pages), from the same formulas as this tool.

Enter values above.

The spreadsheet adds: fuse and wire sizing for up to 20 circuits at once, and a Fuse Guide tab with placement and fuse-type rules. Free vs. spreadsheet

Safety: this is a planning estimate, not an engineering sign-off. DC systems can start fires and lithium batteries can deliver very high fault current. Check every result against the manufacturer's instructions and current ABYC/NEC or local rules, put a correctly rated fuse or breaker close to the battery on every positive cable, and have a qualified installer check work you are not sure about. We are not responsible for how results are used.

How this is calculated

Minimum rating. A fuse should be at least 125% of the circuit's continuous current so it does not nuisance-trip, then rounded up to the next standard size (1, 2, 3, 5, 7.5, 10, 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 100, 125, 150 … 400 A).

Protect the wire. The fuse protects the wire, not the device, so its rating must not exceed the wire's ampacity (ABYC 105 °C table, derated for location and bundling). If it does, upsize the wire. We also show the largest standard fuse that still protects the wire you chose.

Placement. Blue Sea and ABYC place the fuse as close to the battery as practical (ABYC allows up to 7 inches for an unsheathed positive, longer when the cable is sheathed). Lithium banks can deliver very high fault currents: check the fuse's interrupt rating (AIC), and use a Class T fuse on large LiFePO4 banks when the battery maker calls for it.

Fuse type. As a rough guide, blade fuses cover 40 A and below. MIDI, MEGA, ANL or Class T cover larger feeds. Your device's manual wins if it names a fuse.

Planning a whole build?

The paid spreadsheets run these same tested formulas for a whole build in one saved file. Compare free vs. spreadsheet.

12V Load & Wire Gauge Calculator spreadsheet on Etsy, $9.00

Overland Rig Planning Bundle (12V + Payload + Trip spreadsheets) on Etsy, $18.99

Sources

  1. Blue Sea Systems: DC main circuit protection (fuse placement and sizing). www.arrid.com.au
  2. Blue Sea Systems: Allowable amperage in conductors / wire sizing chart (ABYC ampacity, bundling derates). www.bluesea.com
  3. ABYC: What is voltage drop? (3% / 10% guidance). abycinc.org
  4. Victron Energy: Wiring Unlimited (cable, fuse and inverter wiring guidance). www.victronenergy.com