Where to put the fuses
A fuse protects the wire that leaves a power source. A solar system has three: the panels, the batteries and the generator. Here's where each fuse and each switch goes, with a diagram.
The golden rule
Every wire that leaves a power source gets its fuse, on the source side. The most dangerous is the battery: in a short circuit, it can deliver thousands of amps at once.
The whole system
The diagram shows only the positive (+) wire, where the fuses go. Each number is explained below.
- 1
Between the panels and the controller
A disconnect to cut off the panels during work (even in the sun, they give current). With three or more strings in parallel, a fuse on each string.
Size: String fuse: the “maximum series fuse” value on the panel's label.
- 2
Between the controller and the battery
A fuse or a breaker rated for direct current (DC), on the battery or + bus bar side.
Size: The controller's output current × 1.25 (a 40 A controller: 50 A fuse).
- 3
The battery fuse
The most important one. Right on the battery's +, or within 18 cm. It must be able to cut a very big short circuit: a class T fuse or a terminal-mounted fuse (MRBF).
Size: Based on the biggest wire leaving the battery and the total of what's connected.
- 4
The main switch
A battery disconnect to cut everything at once: maintenance, work, or if something heats up.
Size: At least the battery fuse's current.
- 5
Between the + bus bar and the inverter
A fuse just for the inverter, near the + bus bar. It's often the biggest current in the system.
Size: Inverter watts ÷ voltage ÷ 0.9 × 1.25 (2,000 W at 12 V: about 250 A).
- 6
The 12 V fuse block
Each DC circuit (lights, pump, fan, 12 V fridge) gets its own small fuse, chosen for its wire.
Size: Based on each circuit's wire: see the wire tool.
- 7
The inverter's 120 V output
A real breaker panel, like in a house, with shock protection (GFCI) where the code requires it. That's an electrician's job.
Size: Per your country's electrical code.
- 8
The generator input
The generator's breaker protects the wire to the inverter-charger. Never plug a generator into a household outlet.
Size: Based on the generator and the wire to the inverter-charger.
Two details that matter
Words to know
- DC fuse
- Made for direct current. A fuse or breaker made for 120 V household power doesn't cut a DC arc well.
- Interrupt rating
- The biggest current a fuse can cut without blowing apart. A lithium battery can exceed 10,000 A in a short circuit.
- Class T
- A fuse with a very high interrupt rating, made for the output of big batteries.
- Bus bar
- A copper bar where all the + wires (or all the −) connect. Cleaner and safer than stacking lugs on the battery.
- Disconnect
- A switch made to cut a big direct current by hand.
Classic traps
- Putting the fuse at the far end of the wire, near the device: all the wire between the battery and the fuse stays unprotected.
- Using an AC (household) breaker on a battery circuit: it may not cut the arc.
- Stacking five lugs on the battery terminal: use a bus bar.
- Forgetting the negative wire: it must be as big as the positive. It has no fuse, but it often goes through a shunt (the battery monitor's sensor).
To calculate
Next step
You've covered every part. The calculator puts it all together for your project, with real weather for the spot.