Where to Place Fuses in an Off-Grid Solar System
- Every positive cable leaving a battery must be fused within 18 inches of the battery terminal — no exceptions.
- Fuses protect cables, not devices — size the fuse to the cable's ampacity, not the load.
- A solar system needs fuses in 4 places: battery to inverter, battery to controller, panel strings and DC loads.
- Use ANL fuses for high-current runs (inverter), MIDI or blade fuses for medium runs, string fuses for panels.
- An unfused cable can cause a fire within seconds of a short circuit — this is the most dangerous DIY mistake.
The 18-inch rule
The most important rule in solar system wiring: every positive cable leaving a battery terminal must have a fuse within 18 inches of that terminal.
Why 18 inches? A short circuit anywhere on an unfused cable sends the full current capacity of the battery through that cable. A battery bank can deliver thousands of amps — enough to instantly vaporise wire, ignite insulation and start a fire. The fuse limits the fault current to what the cable can safely handle.
The 18-inch rule comes from NEC 690 — the section of the National Electrical Code that governs solar installations. Even if you're in a jurisdiction that doesn't require permits, following this rule is essential for safety.
Where every fuse goes
1. Battery to inverter (highest priority)
This carries the highest current in the system. Use an ANL fuse holder mounted as close to the battery positive terminal as possible — ideally within 6 inches.
| Inverter size | 12V ANL fuse | 24V ANL fuse | 48V ANL fuse |
|---|---|---|---|
| 1,000W | 100A | 60A | 30A |
| 2,000W | 200A | 100A | 60A |
| 3,000W | 300A | 150A | 80A |
| 5,000W | 400A | 250A | 125A |
2. Battery to charge controller
Size to the controller's maximum output current × 1.25. A 40A controller needs a 50A fuse. Use a MIDI fuse or blade fuse in an inline holder.
| Controller amperage | Fuse size | Fuse type |
|---|---|---|
| 20A | 25A | Blade (ATC/ATO) |
| 30A | 40A | Blade or MIDI |
| 40A | 50A | MIDI |
| 60A | 80A | MIDI or ANL |
| 80–100A | 100–125A | ANL |
3. Solar panel strings
Each panel string needs a string fuse when 2 or more strings are wired in parallel. String fuses protect against reverse current from other strings flowing back through a faulted string. Place fuses at the positive lead of each string before the combiner box.
- Single string: no string fuse needed (nothing to flow back)
- 2+ strings in parallel: fuse each string at 1.5× the short-circuit current (Isc) of one string
- Typical 400W panel Isc ≈ 10A → string fuse = 15A
4. DC loads from controller load output
If you're running 12V DC loads directly from the charge controller's load terminals, fuse each load circuit appropriately. Size fuse to the cable ampacity — typically 15–20A for most DC load circuits.
5. Between battery banks (if using multiple batteries)
When connecting multiple batteries in parallel, each battery's positive cable to the busbar should have its own fuse. This protects against a faulty battery sending fault current through the rest of the bank.
Fuse types for solar systems
| Fuse type | Current range | Best for |
|---|---|---|
| ANL (bolt-down) | 80–500A | Battery to inverter, main system fuse |
| MIDI | 30–100A | Battery to charge controller, medium runs |
| ATC/ATO blade | 1–40A | DC loads, small controller connections |
| MRBF (terminal fuse) | 30–300A | Mounts directly on battery terminal — very compact |
| String fuse (cylindrical) | 10–20A | Individual panel strings in combiner box |
Sizing fuses correctly
The golden rule: fuse the cable, not the load.
- Find the ampacity of your cable (from the AWG ampacity table)
- The fuse should blow before the cable overheats — so fuse at or below cable ampacity
- Example: 10 AWG cable rated 55A → use a 50A fuse maximum
- Never upsize a fuse because it "keeps blowing" — a blowing fuse is telling you something is wrong
Complete fuse placement checklist
- ☐ ANL fuse within 18" of battery positive → inverter
- ☐ MIDI/blade fuse within 18" of battery positive → charge controller
- ☐ String fuses on each parallel panel string (if 2+ strings)
- ☐ Individual fuses on each DC load circuit from controller
- ☐ Fuse on each battery's positive cable to busbar (if parallel battery bank)
- ☐ All fuse ratings at or below cable ampacity
- ☐ All fuse holders rated for DC voltage (AC fuse holders are not suitable)
Frequently asked questions
Do I need a fuse on the negative cable?
No — fuses go on positive cables only. The negative cable connects directly from the battery to a common negative busbar or directly to components. Some systems use a shunt (for battery monitoring) in the negative line — this is not a fuse.
Can I use a circuit breaker instead of a fuse?
Yes — DC-rated circuit breakers are an excellent alternative for the main battery connections. They have the advantage of being resettable. Ensure any breaker is rated for DC voltage — standard AC breakers are not suitable for DC applications and may not interrupt DC fault current correctly.
What happens if I don't fuse the panel strings?
With a single string: no immediate danger, but good practice to fuse at the combiner. With multiple parallel strings: a fault in one string allows reverse current from other strings to flow through the faulted cable, potentially causing a fire at the fault location. Always fuse parallel strings.