The most useful thing to understand about overcurrent protection is what it is actually for. A fuse or breaker does not protect your inverter, your controller or your battery. It protects the wire — it opens before the conductor can carry enough current to overheat and start a fire. Once you hold that idea, where everything goes becomes obvious: the fuse is sized to the cable it is guarding.
DC fuses are not AC fuses
This substitution is dangerous and it is common. An AC current crosses zero volts 120 times a second, so an arc drawn when contacts separate self-extinguishes almost immediately. DC never crosses zero, so an arc can sustain itself and keep burning.
DC-rated fuses and breakers are built to interrupt that sustained arc. An AC breaker used on a DC circuit may fail to break the arc at all, which turns the protective device into the ignition source. Always use devices rated for DC at or above your system voltage — a device rated 32V DC is not adequate on a 150V PV string.
Where protection goes
| Location | Device | Sized to | Why |
|---|---|---|---|
| Each PV string (3+ strings) | String fuse in combiner | Panel series fuse rating | Stops other strings backfeeding a faulted one |
| Array to controller | DC breaker or fuse | The conductor | Protects the run and provides a disconnect |
| Controller to battery | DC fuse or breaker | The conductor | High current, short run |
| Battery to inverter | Class T fuse or equivalent | The conductor | Highest fault current in the system |
| Each DC load branch | Fuse or breaker | The branch conductor | Standard branch protection |
| Inverter AC output | AC breaker | The AC circuit | Normal AC rules |
The battery fuse is the one that matters most
A lithium battery bank can deliver enormous fault current — thousands of amps into a dead short, far more than an ordinary fuse can safely interrupt. That is why the battery-to-inverter connection typically needs a Class T fuse or equivalent, which is specified by its high interrupt rating rather than only by its amperage.
It should sit as close to the battery terminal as practical, because any unprotected cable between the battery and the fuse is cable that nothing is guarding. This is the single most important overcurrent device in a battery system and the one most often omitted from DIY builds.
When string fuses are needed
With one or two strings in parallel, a fault generally cannot draw enough current from the remaining string to exceed the panel's series fuse rating, and fuses are typically not required. With three or more parallel strings, the healthy strings together can push dangerous current backwards into a faulted one, and each string needs its own fuse — usually housed in a combiner box. That threshold is the main reason combiner boxes exist; see solar combiner boxes explained.
Sizing, and the DC derating people miss
Size the device to protect the conductor: it must open below the wire's ampacity, and sit above the circuit's normal operating current. For PV circuits the convention is 1.56 × the module's short-circuit current, which combines a 1.25 factor for sustained irradiance above test conditions with another 1.25 for continuous duty.
One more detail: fuses and breakers are commonly derated for DC use. A device rated 30A on AC may be rated substantially lower on DC — read the DC rating specifically, not the headline number.
What not to do
- Never oversize a fuse to stop nuisance trips. A tripping fuse is reporting a real problem; a bigger fuse hides it while the wire keeps heating
- Never use an automotive fuse on a high-voltage DC string — the voltage rating is what matters, not just the amperage
- Never leave the battery cable unfused, whatever else you skip
- Never break a live DC connection under load. Open the disconnect first — see disconnects and what code requires
If any of this is unfamiliar territory, this is the part of a build worth having a licensed electrician review before you close anything up — an hour of their time is the cheapest insurance in the project.