How to choose a fuse
- Multiply the normal continuous current by 1.25, so the fuse doesn't blow under normal use or brief start-up surges.
- Round up to the next standard fuse size.
- Check the cable is rated for at least the fuse size. If not, use thicker cable.
Where to fit the fuse
As close to the battery positive as practical, because any cable between the battery and the fuse is unprotected. US marine practice (ABYC) asks for the fuse within 7 inches (18 cm) of the battery for most circuits. Every positive cable from a battery needs its own fuse, including cables to other batteries in a parallel bank.
Fuse types
| Type | Typical range | Used for |
|---|---|---|
| Blade (ATO/ATC, mini, maxi) | 1–40 A (maxi to 120 A) | Lights, pumps, fridges, sockets via fuse box |
| MIDI / strip | 30–200 A | DC-DC chargers, small inverters |
| MEGA / ANL | 60–500 A | Inverters, battery main feeds |
| Class T | 100–400 A+ | Lithium battery banks (very high fault current) |
| MRBF (terminal fuse) | 30–300 A | Mounted directly on the battery terminal |
Lithium batteries can supply enormous short-circuit currents. Make sure your main fuse has a high enough interrupt (breaking) rating, which is why Class T fuses are often recommended.
Current ratings are guide figures for single-core copper cable. 'Standard PVC' uses conservative values for 70 °C insulation, bundled or enclosed; '105 °C' uses typical marine-table values for high-temperature cable run on its own. Always check the cable maker's rating, and derate for hot spaces such as engine bays.
Frequently asked questions
What size fuse for a 12V fridge?
A fridge drawing 5 A needs at least 6.25 A, so a 7.5 A or 10 A fuse, with cable rated above that.
Do I need a fuse on the negative cable?
Normally no. Fuse the positive side. Some systems use a shunt or isolator on the negative, but the fuse goes on the positive.
Can I use a bigger fuse to stop it blowing?
No. A fuse that's bigger than the cable rating can let the cable overheat and start a fire. Find out why it's blowing instead.