So you are wiring up a battery and asking: what fuse do I need? The short answer is a fuse just above the normal current, never bigger than the cable is rated for, and strong enough to stop a battery short.
Below you get the four-step sum, the fuse types you can pick from, and where each fuse goes. Our fuse calculator does the sum for you, so you can check your own numbers in a few seconds.
What A Fuse Actually Is
A fuse is a piece of thin metal wire inside a small tube.
When too much current tries to flow, the wire heats up and melts. The circuit breaks and the current stops.
It's the simplest electrical safety device ever invented, and one of the most important. Scientists and electricians call this job "overcurrent protection", which just means stopping too much current quickly, before it can damage the cable or your equipment.
Why Off-Grid Setups Need Them More Than Regular Houses
Your wall outlet at home? If something goes wrong, a breaker in the electrical panel flips. Annoying, but safe.
Your off-grid battery is a different beast. Batteries, and lithium batteries especially, can push a huge current into a short circuit. That is enough heat to melt copper wire and start a fire before you've even heard the bang.
So every part of an off-grid electrical system needs its own fuse, sized for the cable it protects.
The Three Fuses You'll Need
Battery To Inverter
This is the big one. The cable between your battery and your inverter handles a lot of current.
Look in the inverter's manual first. Many manufacturers list the correct fuse and cable for each model, and that always beats a rule of thumb.
Solar Panel To Charge Controller
The wire from your panels to the charge controller also needs a fuse. Otherwise a fault out on the roof could send a big current down that cable and damage it.
The charge controller's manual and the panel's datasheet say which fuse or breaker is specified, if one is needed at all. Solar fuses must be designed for the high DC voltage of your panels.
Branch Circuits
If you have separate runs for different things (lights, fridge, water pump), each one gets its own fuse. That way if your fridge has a problem, only the fridge shuts down and the lights stay on.
How Do I Size A Fuse?
Take the normal current, add 25 percent, round up to the next standard fuse size, then check that the cable is rated for at least that fuse. That's the whole rule, and it is the same sum our fuse size tool runs.
Here it is for a 1000W inverter on a 12V battery.
Step 1: the normal current. Divide the inverter's watts by the battery voltage. An inverter also loses some power as heat, so divide by its efficiency too. With 90% efficiency: 1000 ÷ (12 × 0.9) = 93 amps.
Step 2: add 25%. The fuse should never run right at its limit, so it gets a buffer: 93 × 1.25 = 116 amps.
Step 3: the next standard size. Fuses only come in fixed sizes, so you choose the next one up. The result: 125A.
Step 4: check the cable. A fuse is there to protect the cable, so the cable must carry at least 125 amps. With standard PVC cable that's 35 mm², or 25 mm² with cable rated for a higher temperature (90 °C). A 125A fuse on a cable made for 100 amps lets the cable overheat before the fuse blows. If the cable is too thin, use a thicker cable, not a smaller fuse that blows all the time.
Victron sizes fuses the same way: the fuse should match the rating of the cable or the device, whichever is lower. The cable calculator uses the same cable table.
Which Fuse Type Do You Need?
Picking the size is half the job. The other half is selecting a fuse type that suits the current and the voltage.
Here are the most common ones in off-grid systems, with the ranges Victron lists:
| Fuse type | Typical range | Used for |
|---|---|---|
| Blade | up to 120A, 32V DC | Small branch circuits, the same fuses as in vehicles |
| MIDI | 23 to 200A, 32V DC | Medium circuits |
| MEGA | 40 to 500A | Inverter cables on 12V and 24V |
| ANL | 35 to 750A | Inverter cables, fast-acting |
| MRBF | 30 to 300A, 58V DC | Mounted straight on a battery post or busbar |
| Class T | high current, very high interrupt rating | The main fuse on a big lithium battery |
What are blade fuses?
Blade fuses are the small plastic fuses with two flat metal legs, the same kind you find in cars and other vehicles. They're cheap, widely available and easy to swap, so they're great for lights, a pump or a USB socket, but they're far too small for an inverter.
What are resettable fuses?
A resettable fuse is a circuit breaker. Instead of melting, it trips, and you flip it back on once the problem is fixed.
Victron points out that breakers don't need to be replaced like fuses, which makes them handy for small loads. Read how they work in our article on the safety switch.
The Number Most People Forget
There is a third number on a fuse: how much current it can safely break. This is called the interrupt rating, or AIC.
A big lithium battery can push a huge current into a short circuit. If the interrupt rating is lower than the current the battery can push into a short, Victron warns the fuse may fail catastrophically: it can rupture or even explode instead of safely stopping the current. That makes the interrupt rating essential. Victron calls a main fuse with a high enough interrupt rating "a mandatory safety requirement" for lithium systems. That is why big battery banks typically use a Class T fuse as the main fuse.
Also look at the fuse's voltage rating. It must be higher than your battery's full charge voltage. On a 48V battery, a standard 32V fuse is not enough, and the calculator warns you about that.
Where To Put The Fuse
As close to the battery as possible. The boat wiring rules many DIY builders follow ask for about 18 cm (7 inches), with a bit more allowed only when the cable is in an extra protective sleeve.
The reason: the fuse only protects the wire that comes after it. If you put the fuse 3 meters away from the battery, those 3 meters of cable have no protection. A fault in that section is a fire waiting to happen.
So: battery, then fuse, then everything else.
Don't Cheap Out On The Holder
The fuse itself is a few euros. The holder it sits in might cost the same, so people often skip the good holder to save money.
Bad move. A cheap holder corrodes, gets loose connections, heats up, and can fail without anyone noticing. Spend a bit more.
How To Check A Fuse With A Tester
Some fuses, like glass tube fuses, show a broken wire you can see. Most don't, so you need a tester. A cheap multimeter does the job.
Switch the power off and take the fuse out. Set the meter to continuity (the mode that beeps) and touch one probe to each end. A beep means the fuse is good. No beep, or "OL" on the screen, means it has blown.
One Last Thing
If a fuse blows, don't just put a bigger one in. The fuse blew for a reason, so find the reason first.
A fuse is doing its job when it blows. Replacing it with a bigger one is like turning off your smoke alarm because it's beeping.
- Size it: normal current × 1.25, next standard size up.
- Check the cable is rated for that fuse.
- Check the interrupt rating and the voltage rating.
- Put it right next to the battery.
Sources
- Victron Energy, Wiring Unlimited: DC wiring and fuses
- Blue Sea Systems, Select a fuse and fuse holder for your DC installation
- Blue Sea Systems, DC main overcurrent protection requirements
- Portlandia Electric Supply, How to size circuit breakers: the 125% rule
- Fluke, How to check a fuse with a multimeter
Hard Words
| Term | What it means |
|---|---|
| Fuse | A thin piece of metal wire in a small tube. Melts and breaks the circuit when too much current flows. The simplest fire-safety part. |
| Overcurrent protection | Anything that stops too much current, like a fuse or a breaker. |
| Interrupt rating (AIC) | The biggest short-circuit current a fuse can safely break. Big lithium batteries need a fuse with a high one. |
| Amps (A) | How much current is flowing. Fuses are rated in amps. A 100A fuse blows when more than 100 amps tries to pass through it. |
| Watts to amps | Watts divided by voltage equals amps. For an inverter, also divide by its efficiency. |
| 25 percent margin | Extra buffer added when calculating fuse size, so the fuse never runs at its limit. The fuse must still be no bigger than the cable is rated for. |
| DC fuse | A fuse rated for direct current. DC arcs differently from AC, so the fuse needs a DC voltage rating. |
| Blade fuse | A small plastic fuse with two flat legs, as used in cars. Fine for small branch circuits. |
| Class T fuse | A fuse with a very high interrupt rating, used as the main fuse on big lithium batteries. |
| Circuit breaker | A resettable fuse: it trips instead of melting and you switch it back on. |
| Branch circuit | A separate run from the battery to one device or group of devices. Each branch gets its own fuse. |
| Short circuit | When current finds a path with almost no resistance, usually because two wires touch. Current spikes to dangerous levels almost instantly. |
| Fuse holder | The plastic and metal housing that holds the fuse. Quality matters; cheap holders corrode and heat up. |
| 7-inch rule | Boat wiring rules put the main fuse within about 18 cm (7 inches) of the battery terminal. The cable between battery and fuse is unprotected, so keep it short. |
| Standard fuse sizes | The fixed ratings fuses come in (100A, 110A, 125A, 150A and so on). Round up to the next one, as long as the cable is rated for it. |
Common Questions
What fuse do I need for a 1000W inverter on 12V?
A 125A fuse. 1000 watts ÷ (12 volts × 90% efficiency) is 93 amps, times 1.25 is 116 amps, and the next standard size is 125A. The cable must be rated for at least 125 amps, which is 35 mm² standard PVC cable. Check the inverter's manual too.
Why do off-grid setups need more fuses than houses?
Because a battery can push a huge current into a fault in a split second. Without a fuse, cables melt and fires start before anyone reacts.
Where does the fuse go?
As close to the battery as possible; boat wiring rules say about 18 cm (7 inches). The cable between the battery and the fuse is not protected by the fuse.
Are DC fuses the same as the ones in my house?
No. DC arcs differently from AC, so a DC fuse needs a DC voltage rating that is higher than your battery's full voltage.
Can I use a circuit breaker instead of a fuse?
On branch circuits, yes; a breaker is a resettable fuse. For the main fuse on a big lithium battery, use a fuse with a high enough interrupt rating, such as a Class T.
What do I do if a fuse blows?
Find out why. Fix the short or the bad device, then replace the fuse with one of the same rating, never a bigger one.