Short answer: a fuse protects the cable. Size it to the cable (or the one device on the end of it, whichever is lower), put one on every positive cable that leaves the battery, and use a fuse rated for DC at your system voltage. With lithium, at least one fuse must be able to safely break the battery's full short-circuit current. Then check the device's manual: makers print the fuse they want.
What is a fuse actually protecting?
The wire. A fuse stops too much current running through a cable and overheating it or starting a fire. It guards against two things:
- Overcurrent: more current in a wire than it's rated for.
- Short circuit: one conductor accidentally touching another.
That's why the fuse is sized to the cable, not to whatever you hope the circuit will carry.
What size fuse do I need?
Victron's rule, from Wiring Unlimited:
- One device on the circuit: the fuse matches the device's current rating or the cable's, whichever is lower.
- Several devices on one circuit: the fuse matches the cable's current rating.
For battery cables, Victron's Lithium Battery Smart manual says the same: match the lowest of the cable, battery and system current ratings.
Battery makers can add their own ceiling. Exotronic say to fuse the positive output of their Alpha lithium batteries at no more than 2C. 2C means twice the battery's Ah figure in amps, so for a 100 Ah Alpha that's no more than 200 A.
Cable size and fuse size go together, so if you haven't sized the cable yet, start with 12V cable sizing and voltage drop.
What do the manuals say for common gear?
Every device has its own numbers. Some Victron examples:
| Device | Fuse (maker's figure) | Cable (maker's figure) |
|---|---|---|
| Orion XS 12/12-50A DC-DC charger | External battery protection fuse 60-70 A, on both the input (starter battery) and output (auxiliary battery) cables | Minimum 16 mm² under 5 m; 4 AWG (21.2 mm²) for 5-10 m |
| SmartSolar MPPT 100/30 | Battery fuse 35 A minimum, 40 A maximum | (See manual) |
| SmartSolar MPPT 100/50 | Battery fuse 55 A minimum, 70 A maximum | (See manual) |
| MultiPlus-II 12/3000/120 inverter/charger | DC fuse 400 A | 2 x 50 mm² per terminal for 0-5 m; 2 x 70 mm² for 5-10 m |
The Orion XS manual says the installation must include a fuse as recommended. Its cable figures are minimums; for a long run, our cable guide explains why you might go thicker.
For the MultiPlus-II, DC cables should be copper with a 90 °C rating, battery cables shouldn't go in a closed conduit, and local installation rules apply. (The bigger 12/5000/220 wants a 600 A fuse and 2 x 95 mm² or 2 x 150 mm² cables.) That's the 12V side only. Fixed 230 V wiring in a van or caravan is a job for a licensed electrician; our 230V in a van or caravan guide covers why.
Where do the fuses go?
On the positive cable, one per device, plus a main fuse. Every device that connects to a battery needs its own fuse in the positive cable, no matter how big or small it is.
The usual layout is a main battery fuse, then branches off to each device. A DC system should always have a main fuse, and pulling it breaks the circuit, which is handy when you need to work on things.
Don't forget:
- Solar. There needs to be a fuse between the panels and the solar charger. Local rules vary, so check what applies.
- DC-DC chargers. The Orion XS wiring diagram shows a fuse at both the starter battery and the house battery. Both ends of that cable connect to a battery.
How close to the battery?
Two makers give a figure:
- Enerdrive, for their DC-DC charger: a fuse or breaker within 20 cm of both the start battery and the house battery.
- Blue Sea design their terminal fuse blocks to meet the US boating standard's (ABYC) 7-inch rule, about 178 mm, by mounting straight onto a battery post, battery switch or bus bar.
The idea is the same: the less unprotected cable between the battery and the fuse, the better. A terminal fuse block, like our MRBF terminal fuse block, sits right on the post.
Does any fuse do, as long as the amps are right?
No. Choose by current rating, voltage rating, interrupt rating, speed and type.
DC rated, at your system voltage
The fuse's voltage rating has to be at least the highest voltage in the system, and it has to be rated for DC. Most DC fuses suit 12 and 24 V but aren't necessarily suitable for 48 V and up.
Same with circuit breakers: not all of them work in both AC and DC circuits, and some DC breakers only work one way round, so it matters which way they're wired in.
Typical limits for the common types, from Victron's overview:
| Type | Current range | Voltage |
|---|---|---|
| Blade (automotive) | Up to 120 A | 32 V DC |
| MIDI | 23-200 A | 32 V DC |
| MEGA | 40-500 A | 32 V DC |
| ANL | 35-750 A | 32 V DC |
| MRBF | 30-300 A | 58 V DC |
Speed
Slow-blow fuses are the usual choice in automotive and marine DC circuits, because those circuits have things with a big start-up current, like motors or an inverter's capacitors.
Why does lithium need a special fuse?
Because of how much current a lithium battery can dump into a short: far more than a comparable lead-acid setup.
That's where interrupt rating (or breaking capacity) comes in. It's the biggest fault current a fuse can safely break. If it's lower than what the battery can deliver, Victron warn "the fuse may fail catastrophically."
Victron's rule for lithium is firm, and the wording matters: "In every lithium battery installation, at least one fuse in the DC line must have an interrupt rating equal to or greater than the maximum prospective short circuit current of the battery bank. This is a mandatory safety requirement."
Some interrupt ratings from Victron's table, for those makers' own fuses:
| Fuse (maker) | Voltage | Interrupt rating |
|---|---|---|
| Class T (Eaton Bussmann) | 160 V | 200 kA |
| ANL (Blue Sea) | 80 V | 6000 A |
| MRBF terminal fuse (Blue Sea) | 58 V | 2000 A |
| MEGA (Littelfuse) | 58 V | 1000 A |
The same table has a second Littelfuse MEGA row at 2500 A and 70 V, so check the actual fuse's datasheet. And these ratings belong to those makers' fuses; another brand of the same style has its own figure.
Victron rate Class T as well suited to lithium, with a very high interrupt rating and fast response, and it fits their Lynx system. That's what the Lynx Class-T Power In is for.
To use the rule, you need your battery bank's maximum short-circuit current. If it isn't in the battery's documents, ask the battery's maker.
Where do people go wrong?
- Saving money on fusing. A fuse is cheap next to a cable fire.
- Bypassing a fuse, a BMS or an isolator to "get it working". If a fuse keeps blowing, something is wrong with the load, the cable or the fuse size, and that's what needs fixing.
- Adding gear over the years. Each new device needs its own fuse, and the main fuse and cable need to suit the new total.
- Underestimating the inverter side. The DC side of an inverter carries big currents (see inverters, VA and watts). If you're not confident, an auto-electrician is money well spent.
Our fuses, fuse holders and circuit breakers are grouped by type.