Calculator
PV Overcurrent Protection Sizing Calculator
Photovoltaic overcurrent protection is unusual: the source is current-limited, so a fault does not produce the large prospective current that operates a normal breaker. Protection exists to stop reverse current from parallel strings damaging a faulted string, not to clear a short from the array itself.
This calculator applies the standard factors from IEC 62548 to size string fuses, combiner protection and the AC-side device.
Enter the array electrical data
1.25 covers edge-of-cloud and reflection effects.
String design current
17.4 A
Isc × 1.25
String fuse window
19.5 – 25 A
Choose a rating inside this window
Recommended string fuse
20 A
Next standard gPV rating above the minimum
Reverse current from parallel strings
52.1 A
Fusing required
Total array current at the combiner
69.5 A
Exceeds the cable rating — increase cross-section
AC breaker size
20 A
Design current 20 A
How the calculation works
String protection is sized between two bounds. The lower bound is 1.4 times the module short-circuit current, which guarantees the fuse will not nuisance-operate under enhanced irradiance. The upper bound is the maximum series fuse rating printed on the module datasheet, which is the module's own withstand limit.
Fuses are only needed when reverse current can reach a damaging level. With one or two strings in parallel there is no source of reverse current large enough to matter, so IEC 62548 permits omitting string fuses; from three strings upwards they are required.
Array and inverter circuits are sized on continuous current with a 1.25 factor, and the cable must carry that current after derating for grouping, ambient temperature and installation method. The AC breaker protects the cable, not the inverter, which limits its own output.
The formula
I_string = I_sc × 1.25 I_fuse,min= 1.4 × I_sc I_fuse,max= module maximum series fuse rating I_reverse = (S − 1) × I_sc × 1.25 I_array = S × I_sc × 1.25 I_breaker = I_ac,max × 1.25 → next standard size S number of parallel strings
Assumptions behind the numbers
- Fuses are gPV type to IEC 60269-6, rated for the full DC system voltage and for the ambient temperature inside the enclosure.
- The 1.25 irradiance factor covers cloud-edge enhancement. Sites with high albedo or bifacial rear gain may justify a higher factor.
- Cable ratings entered are already derated for grouping, ambient temperature and installation method to IEC 60364-5-52.
- This tool sizes protection only. Disconnection means, arc-fault detection, insulation monitoring and earthing arrangements are separate requirements.
- National deviations apply. Germany, France and the UK each add requirements on top of the IEC baseline.
How to read the result
If the fuse window closes — the minimum exceeds the module's maximum series rating — the array has too many parallel strings on one protected group. Split it into sub-arrays with separate combiners.
Fuse enclosures run hot. A fuse rated at 25 A at 25°C may be derated to 20 A at 60°C inside a sealed combiner in full sun, so check the manufacturer's temperature derating curve.
On the AC side, the breaker must be at or below the cable rating and at or above the design current. If no standard size fits both, the cable is too small.
Worked example
An array uses modules with Isc 13.9 A and a 25 A maximum series fuse rating, in four parallel strings per combiner. The inverter outputs 16 A AC maximum, and the AC cable is rated 41 A after derating.
String design current is 13.9 × 1.25 = 17.4 A. The fuse window runs from 1.4 × 13.9 = 19.5 A up to 25 A, so a 20 A gPV fuse fits, with 25 A as the alternative if enclosure temperature demands derating. Reverse current from three other strings is 52 A, well above the module withstand, so fusing is mandatory.
Total combiner current is 4 × 17.4 = 69.6 A — above the 41 A cable entered, so the array cable must be increased or the strings split across two combiners. On the AC side, 16 × 1.25 = 20 A, giving a 20 A breaker that sits comfortably under the 41 A cable rating.
Mistakes that skew the answer
- Fusing at the string design current rather than the 1.4 × Isc minimum, which causes nuisance operation on bright cloudy days.
- Exceeding the module's maximum series fuse rating, which leaves the module unprotected in a reverse-current fault.
- Using ordinary gG fuses or AC breakers on DC circuits — DC arcs do not self-extinguish and require DC-rated devices at the full system voltage.
- Ignoring fuse derating inside a hot combiner box.
- Sizing the AC breaker to protect the inverter instead of the cable, which is what the breaker actually protects.
Questions readers ask
- When do I need string fuses?
- When three or more strings are connected in parallel without individual protection, because the combined reverse current from the other strings can exceed a single string's withstand. With one or two strings there is no sufficient reverse source and fuses may be omitted.
- What fuse type is required?
- gPV fuses to IEC 60269-6, rated for the maximum system DC voltage — typically 1,000 or 1,500 V — and specifically designed to break DC arcs. Automotive or AC fuses are not acceptable at any rating.
- Can I fuse only the positive conductor?
- In an unearthed array both poles carry the same fault current, so protection is normally fitted in both, or an alternative arrangement is justified in the design documentation. Check the national annex, since practice varies.
- Does the AC breaker protect the inverter?
- No. Inverters are current-limited and protect themselves. The AC breaker protects the cable between the inverter and the distribution board, and provides a means of isolation. Size it to the cable, not the machine.
- Do microinverter systems need string fuses?
- The DC side has no parallel strings, so no. The AC trunk cable does need protection sized to the number of microinverters on the branch and the trunk cable's rating, which the manufacturer specifies as a maximum units-per-branch figure.