DC Isolator & Fuse Rating Calculator
String fuse, cable and d.c. isolator ratings for a PV array.
Modules
On the module datasheet, often "maximum series fuse rating" - the module's maximum overcurrent protection rating (IMOD MAX OCPR).
From the datasheet. Leave blank to use AS/NZS 5033 Table 4.1's correction for crystalline silicon instead.
Array and inverter
The strings this fuse, cable or isolator carries together - e.g. 2 strings sharing one MPPT input.
The current the inverter (or any other source) can feed back into the array, from its datasheet or installation information (AS/NZS 5033 Clause 3.3.2).
Check an isolator's datasheet ratings (optional)
For guidance only. These calculators are planning aids for licensed electrical workers and inspectors. All results must be independently verified against the current editions of AS/NZS 3000:2018, AS/NZS 3008.1.2, and other applicable standards before being relied on for any installation. CertMate accepts no responsibility or liability for any work carried out, decisions made, or certification issued in reliance on these results. The certifying electrical worker remains solely responsible for compliance with the Electricity (Safety) Regulations 2010 and all applicable New Zealand law.
One set of numbers, three components
PV modules are current-limited, but strings in parallel aren't: when one string faults, the others can push their current back into it. AS/NZS 5033 works everything from two figures - the maximum string current (1.25 x the module's short-circuit current) and the PV d.c. maximum voltage on the coldest morning - and this calculator follows those clauses through for the string fuses, the cable current ratings and the d.c. isolator.
It's for arrays without optimisers. Systems with d.c. conditioning units use the optimiser maker's maximum figures instead.
Worked examples
Three strings on one input
Modules with a 13.9 A short-circuit current and a 25 A maximum series fuse rating. The maximum string current is 17.38 A, so two strings can push 34.75 A into the third - more than the module's 25 A, so string fuses are needed. They must be rated above 20.85 A and no more than 25 A: a 25 A gPV fuse.
Isolator in the sun on two parallel strings
Two strings of 9 A modules: 1.25 x 9 A x 2 = 22.5 A. An isolator rated 19 A at 60°C (its "in the sun" rating) isn't enough for this circuit, even though its rating at 40°C would be.
Frequently asked questions
When are PV string fuses required?
Under AS/NZS 5033, strings connected in parallel need overcurrent protection when the fault current that can flow into one string - from the other parallel strings plus any backfeed - is more than the module's maximum series fuse rating. Whether that happens depends on the module: two strings are often fine, three or more often aren't - the calculator works it out.
How is a PV string fuse rated?
AS/NZS 5033 Table 3.1 sets the window: more than 1.2 times the maximum string current, and no more than the module's maximum series fuse rating. The maximum string current is 1.25 times the module's short-circuit current, adjusted for bifacial gain where it applies. The fuse must be a gPV type and rated for the full PV d.c. maximum voltage.
What current rating does a PV d.c. isolator need?
Its thermal rating at the installed temperature (40 degrees indoors or out of the sun, 60 degrees in the sun) must cover the circuit current, and it must be able to switch that current at the PV d.c. maximum voltage. For a single string that's 1.25 times the module short-circuit current; for parallel strings, the strings' total.
Why does the inverter type matter for the isolator?
With a non-separated (transformerless) inverter, a fault can put the full array voltage and current through either the positive or the negative side, so AS/NZS 5033 requires each side's switching to handle that on its own. With a separated inverter, the positive and negative poles in series share it.
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