How to Use This

Pick the right load type — this matters more than people think. ABYC defines a "continuous load" as anything running 3 hours or more. Continuous loads are sized at 125% of nameplate current. Intermittent loads are sized at 100%. Motors get 156% to handle inrush.

The wire size dropdown is optional but recommended. The calculator will warn you if your fuse rating exceeds the wire's ampacity — which is a fire hazard. The fuse exists to protect the wire, not the device. If your wire melts before your fuse blows, the fuse is wrong.

This is for branch circuits, not engine starting. Starter circuits have special rules — typically no fuse on the starter cable itself, with battery protection handled differently.

What ABYC E-11 Says About Fusing

ABYC Standard E-11 has specific rules for overcurrent protection on boats:

Every positive lead from a battery must be fused within 7 inches of the battery (175mm). The exceptions are limited: engine cranking circuits, and certain devices with their own internal protection.
The fuse rating must not exceed the wire's ampacity. This is the rule people miss most often. A 40A fuse on 14 AWG wire is a fire waiting to happen — the wire's ampacity is 20A.
Standard fuse sizes are accepted: 1, 2, 3, 5, 7.5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 100, 125, 150, 175, 200, 250, 300, 400, 500. Round up to the next standard size, never down.
Continuous loads sized at 125%. Anything running 3+ hours.
Motor / inductive loads sized at 156% to handle locked rotor / inrush current without nuisance trips.

Fuse Type Guide

The right fuse depends on current and battery chemistry:

Current	Recommended fuse	Notes
Up to 30A	ATO/ATC blade fuse	Easy to find, cheap, standard automotive
30–100A	MIDI / MEGA / ANL	Bolt-down, marine standard
100–300A	ANL or MEGA	Common for inverter feeds, alternator output
300–500A	ANL or Class T	Class T strongly recommended for lithium
500A+	Class T required	Only Class T has the interrupting capacity for lithium fault currents

Lithium note: Lithium batteries can deliver enormous fault currents (10,000+ amps in a dead short). Standard ANL fuses can fail to interrupt at these currents, melting closed and turning the fuse into a fire source. Class T fuses have a 20,000A interrupting rating and are the safe choice for any lithium bank.

Common Mistakes

Sizing fuse to load instead of wire. This is backwards. The wire dictates max fuse size. If your load needs 80A but you ran 8 AWG (55A ampacity), the maximum fuse you can use is 55A — and that won't be enough for the load. Upsize the wire.

Forgetting the 125% multiplier. A 30A continuous load needs at least a 40A fuse, not a 30A fuse. A 30A fuse will nuisance-trip on a load that's actually drawing 30A continuously.

Using the wrong fuse type for lithium. Standard ANL fuses are not safe on lithium banks above ~300A. Class T or DC-rated breakers only.

Mounting the fuse far from the battery. ABYC requires within 7 inches. Run the wire from the battery directly to the fuse, then from the fuse to wherever it needs to go. The unfused section between battery and fuse is the part that fails dangerously.