Fuses, Breakers and Isolators for 12V to 48V Solar
What each protective part does, where it goes in a DC solar system, and the mistakes that cause fires.
Batteries can deliver an enormous current into a short circuit, far more than a cable can carry. Fuses, breakers and isolators are what stop a fault turning into a fire. This guide covers the extra-low voltage DC side of a solar system.
1. What each part does
- Fuse: melts and breaks the circuit when the current is too high. Cheap and reliable, but it has to be replaced after it blows.
- Circuit breaker: trips on overcurrent and can be reset. Many also work as a switch.
- Isolator: a switch that safely disconnects part of the system for maintenance or an emergency. A plain isolator doesn't protect against overcurrent.
2. The golden rule: protect the cable
A fuse or breaker is there to protect the cable it feeds. Its rating must be at or below what that cable can safely carry. A fuse that's too big for its cable lets the cable overheat before the fuse blows.
3. Where protection goes
- At the battery: a main fuse or breaker on the positive lead, as close to the battery terminal as practical. This protects the heavy cable that runs from it.
- Between the charge controller and the battery: protection sized to the controller's output.
- Between the panels and the controller: a DC-rated isolator, so you can disconnect the panels. Panels keep making power whenever there's light, so you can't just "turn them off".
- Each branch circuit: every circuit that leaves the main busbar, such as lights or a 12V outlet, needs its own fuse or breaker sized to its cable.
4. DC is not AC
- DC arcs are harder to break than AC arcs. Use fuses, breakers and isolators that are rated for DC at your system's voltage.
- Don't use an ordinary AC breaker or switch on the DC side, even if its voltage rating looks high enough.
- Some DC breakers are polarity sensitive. Wire them the way the markings show.
5. Lithium batteries need a high-rated main fuse
A lithium (LiFePO4) bank can deliver thousands of amps into a dead short. The main fuse needs an interrupt rating high enough to break that current safely. Check the battery maker's manual for the fuse type and rating they specify, and follow it.
6. Cable size and voltage drop
Cable has to be thick enough to carry the current without overheating, and thick enough to keep voltage drop low. Voltage drop matters most on 12V systems, because a small drop is a big share of 12V. Use a cable sizing chart or calculator, with the full length of the run there and back.
7. Common mistakes
- No fuse at the battery, or a fuse a long way from it.
- Fuses rated higher than the cable can carry.
- AC-only switches and breakers used on the DC side.
- Loose or poorly crimped lugs. A bad crimp gets hot under heavy load.
Safety: in Australia, any work on 240V AC wiring, including connecting an inverter, battery or solar system to a home's switchboard or fixed wiring, must be done by a licensed electrician. Grid-connected systems also need approval from your electricity network.