Fuse size calculator
Find the correct fuse size and type for your marine cable. The fuse protects the cable — not the device.
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Worked example
You're wiring a VHF radio (6A peak transmit) on 2.5 mm² cable:
- 2.5 mm² cable carries a maximum of 20A
- Maximum fuse size: 20A blade fuse (ATO/ATC)
- The 6A device load is well within the cable rating — 70% headroom
- Fuse location: within 300 mm of the positive bus bar or battery terminal
Always fuse as close to the power source as possible.
Key principle: fuse the cable, not the device
The fuse must blow before the cable overheats. This means the fuse rating should never exceed the cable's current-carrying capacity. If your device needs less current than the cable can carry, that's fine — the fuse protects the cable, and the device has its own internal protection or a smaller inline fuse.
Fuse rating ≤ Cable current capacity
BS EN ISO 13297 requires fuses within 300 mm of the positive distribution point, or at the battery terminal for direct connections.
Fuse type guide
| Fuse type | Typical range | Typical use |
|---|---|---|
| Mini blade (ATM) | 2–30A | Small instruments, accessories |
| Standard blade (ATO/ATC) | 1–40A | Most 12V circuits — VHF, lights, pumps |
| Maxi blade | 20–80A | High-current circuits, sub-panels |
| MIDI / MEGA fuse | 30–200A | Battery feeds, large inverters |
| ANL fuse | 80–500A | Main battery cable, bow thrusters, windlass |
| Class T | 110–400A | High-current lithium systems |
Frequently asked questions
What size fuse for a VHF radio?
A VHF radio draws about 1.5A on receive and 5–6A briefly on transmit. Wire it on 2.5 mm² cable with a 20A fuse (protects the cable). Some installations use a 10A fuse, which is also fine — it still protects the cable and gives some protection to the radio circuit.
Do I need an ANL fuse?
ANL fuses are recommended for any high-current cable run connected directly to the battery — typically anything over 60–80A, or any cable going to a windlass, bow thruster, or large inverter. They should be mounted within 300 mm of the battery positive terminal.
Can I use a higher-rated fuse if the one I need isn't in stock?
No — never upsize a fuse. A 30A fuse in a 20A cable means the cable can overheat and potentially start a fire before the fuse blows. If you don't have the right fuse, don't complete the circuit until you do.
Do circuit breakers work the same way as fuses?
Yes — a circuit breaker rated to the cable's capacity provides equivalent protection and has the advantage of being resettable. Use the same current ratings as you would for fuses. Marine-rated circuit breakers are preferred over automotive types in damp environments.