Quick answer
Put the fuse in the positive cable as close to the battery (or other power source) as practical, so almost none of the cable is left unprotected. Rate the fuse at or below what the cable can safely carry, and above what the accessory normally draws. The fuse’s job is to protect the cable from overheating if it shorts to the body, which is why it belongs at the source end, not at the accessory.
Is this a DIY job?
- Difficulty: Easy
- Time: 15 to 45 minutes, depending on access to the battery
- Tools: spanners for the battery terminals, crimp tool, wire stripper, multimeter, cable ties
- Skills: basic crimping and reading a current rating
Fitting a fuse holder is straightforward. Get help if you’re feeding high-current gear (inverters, winches, DC-DC chargers), working on a vehicle with a battery monitoring sensor on the negative terminal, or wiring anything in a hybrid or electric vehicle.
Step by step
- Work out the accessory’s current. Check its label or manual for amps. If it only lists watts, divide by 12 for a rough figure (for example, 120 W ÷ 12 V = 10 A).
- Size the cable first. Choose cable by copper area in mm² for the current and run length. The fuse is chosen to suit the cable, so this comes first. (see What size cable do I need for my 12V accessory?)
- Pick the fuse rating. It must be above the accessory’s normal draw (including any start-up surge the maker mentions) and at or below the cable’s current rating. If the accessory maker specifies a fuse, use that, as long as it’s no higher than the cable can carry.
- Pick the fuse type. Getting the protection right matters more than anything else in this job, because it’s what stops cable damage and fires. Blade fuses suit most small accessories. Higher currents need MIDI or ANL fuses, or a circuit breaker (see the table below). Under the bonnet, under the vehicle or anywhere it can get wet, use a waterproof (sealed) fuse holder or circuit breaker. A circuit breaker can be reset instead of replaced, which suits accessory circuits that may trip occasionally.
- Disconnect the battery. Negative terminal off first, then you can work near the positive safely.
- Mount the fuse holder close to the battery. Fix it firmly, away from sharp edges, exhaust heat and moving parts, and where you can reach it to check or replace the fuse. The short cable between the battery and the fuse is the only unprotected part, so keep it as short as you practically can and protect it with loom tube.
- Connect the battery side. Use a properly crimped lug on the battery positive terminal or an approved power stud. Don’t stack lots of lugs on one terminal.
- Run the cable to the accessory. Use loom tube, grommets through any metal panel, and secure it every so often so it can’t rub through.
- Run a proper earth. Use a negative cable of the same size back to the battery or a clean, solid chassis earth point.
- Fit the fuse last. With everything connected and checked, fit the fuse, then reconnect the battery: negative on last.
- Test. Switch the accessory on and confirm it works. Check that nothing, including the fuse holder, gets warm after a few minutes of running.
Two power sources, two fuses. If a cable can be fed from both ends, such as the link cable between a main and an auxiliary battery, or the feed to a DC-DC charger, fit a fuse or circuit breaker near each battery. A short anywhere along the cable is then cut off from both sides. (see How does a dual battery system work and do I need one?)
Specs table
Common fuse and breaker types for 12V accessories
| Type | Typical ratings | Notes |
|---|---|---|
| Standard or mini blade fuse | Up to 40 A | Colour-coded by rating; plugs into an inline holder or fuse block |
| MIDI (bolt-down) fuse | 30 to 200 A (32 V DC) | Time-delay type that tolerates short surges; the maker states 150 to 200 A sizes are for short-circuit protection only |
| ANL fuse | 35 to 750 A | Bolt-on, for heavy cables such as inverter and winch feeds |
| Circuit breaker, Type I (auto reset) | Various | Resets itself and keeps cycling until the fault clears |
| Circuit breaker, Type II (modified reset) | Various | Stays open while power is applied; resets when power is removed |
| Circuit breaker, Type III (manual reset) | Various | Stays off until you press the reset button or lever |
For accessory feeds, a manual-reset (Type III) breaker is usually the safer choice, because it stays off until someone looks at why it tripped.
Blade fuse colours
| Rating | Colour |
|---|---|
| 5 A | Tan |
| 7.5 A | Brown |
| 10 A | Red |
| 15 A | Blue |
| 20 A | Yellow |
| 25 A | Clear (natural) |
| 30 A | Green |
| 40 A | Orange |
Maxi fuses use a different colour code. Always go by the number printed on the fuse.
Matching fuse to cable (examples)
Cable ratings from one Australian cable maker, for a single cable at 30°C. Ratings drop with heat and bundling, so in an engine bay choose a fuse well under the figure.
| Cable sold as | Copper area | Maker’s rating at 30°C | Fuse must be no higher than |
|---|---|---|---|
| 3mm auto cable | 1.13 mm² | 20 A | 20 A, lower in hot spots |
| 4mm auto cable | 1.84 mm² | 28 A | 28 A, lower in hot spots |
| 6mm auto cable | 4.59 mm² | 48 A | 48 A, lower in hot spots |
Fuses come in set ratings, so choose the nearest standard size at or below the limit. Twin-core and bundled cables have lower ratings than single-core; check the figure for the exact cable you buy.
Other brands rate the same sizes much lower. For example, some 3mm auto cable sold in Australia is rated at 10 A, and some 4mm twin-core at 15 A. Go by the rating printed on the cable or packet you actually buy, and if in doubt use the lower figure.
What you’ll need
- inline blade fuse holder (waterproof type under the bonnet)
- blade fuses
- MIDI fuse holder or ANL fuse holder (high-current feeds)
- manual reset circuit breaker
- fuse block (if you’re adding several accessories)
- cable lugs and crimp terminals
- split loom tube and rubber grommets
- auto cable (sized for the job)
When to call an auto electrician
- The accessory draws high current (inverter, winch, air compressor, DC-DC charger) or needs cable heavier than 6mm auto cable.
- The vehicle has a battery sensor on the negative terminal, a smart alternator, or a stop-start system. Adding loads in the wrong place can upset charging. (see What is a smart alternator and how does it affect accessories?)
- You’re adding several accessories and need a fused distribution block or auxiliary battery.
- Fuses keep blowing. That usually points to a short or a fault, not a fuse that’s too small.
- The vehicle is a hybrid or EV (high-voltage systems are professional-only), or the wiring runs near airbags or SRS looms.
Need an auto electrician? Find local auto electricians and mobile services in your city.
Related questions
Q: Should the fuse go near the battery or near the accessory? A: Near the battery. A short can happen anywhere along the cable, and only the part after the fuse is protected.
Q: Can I use a bigger fuse so it stops blowing? A: No. A fuse rated higher than the cable lets the cable overheat before the fuse blows. Find out why it’s blowing instead.
Q: Does the negative cable need a fuse? A: In a normal negative-earth vehicle, the fuse goes in the positive cable only. The negative is at earth potential, so a short to the body doesn’t draw current through it.
Q: Can I tap into the vehicle’s fuse box instead? A: For small loads, a fuse tap on a suitable circuit can work. Stay within that circuit’s rating and avoid safety circuits such as airbags.
Q: Fuse or circuit breaker: which is better? A: Both protect the cable. A breaker can be reset after a trip; a fuse is cheaper and simpler. For high currents, choose a type rated for the job.
Sources
- Tycab, automotive cable catalogue – low tension single-core cable by stranding, with copper areas and current ratings at 30°C (14/0.32 1.13 mm² 20 A; 23/0.32 1.84 mm² 28 A; 57/0.32 4.59 mm² 48 A) and temperature derating. https://www.tyree.nz/wp-content/uploads/2019/09/Automotive-Comp.pdf (checked October 2026)
- Littelfuse, ATO and MAXI blade fuse datasheet (via Farnell) – blade fuse colour codes by rating; maxi colour code differs. https://www.farnell.com/datasheets/82316.pdf (checked October 2026)
- Littelfuse, MIDI 498 series datasheet – 32 V DC, 30 to 200 A, time-delay; 150 to 200 A for short-circuit protection only. https://www.enika.eu/data/files/Littelfuse_MIDI_0498_auto32V.pdf (checked October 2026)
- Eaton Bussmann, ANL low-voltage limiter fuse datasheet – 35 to 750 A, 32 V AC / 80 V DC. https://static.chipdip.ru/lib/651/DOC044651693.pdf (checked October 2026)
- Littelfuse, aftermarket circuit breakers catalogue – Type I automatic reset, Type II modified reset, Type III manual reset definitions. https://www.littelfuse.com/~/media/automotive/datasheets/circuit-breakers/littelfuse_aftermarket_circuit_breakers.pdf (checked October 2026)
- Narva, 3mm single core cable – sold as 3mm and rated 10 A. https://narva.com.au/products/narva-10a-3mm-white-single-core-cable-100m (checked October 2026)
- Wiltronics, 4mm twin core auto cable – 23/0.32 stranding, 1.85 mm², rated 15 A. https://www.wiltronics.com.au/product/52830/4mm-twin-core-auto-cable (checked October 2026)