Glossary · DC and AC electrics
Maximum short-circuit current ISC MAX
The design current of a PV array, I_SC MAX = K_I·I_SC STC with K_I of at least 1.25; cables and protection are sized by it.
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Definition
The datasheet short-circuit current ISC STC is measured at 1000 W/m². Real irradiance can be higher: cloud edges, reflection from snow, a light roof. So DSTU HD 60364-7-712:2022 (Annex B.2) adds a margin:
ISC MAX = KI · ISC STC, KI ≥ 1,25The minimum KI is 1.25. The standard says plainly that under certain conditions, such as increased reflection, KI must be increased, but it gives no number for that. Clause 712.512.1.2: ISC MAX is taken as the design current when selecting devices for a PV array.
Example: a panel with ISC STC = 13.9 A gives ISC MAX = 1.25·13.9 = 17.4 A per string. Two strings in parallel on one MPPT input, joined before a common cable, put 34.8 A on that cable. The DC cable section and the decision on string fuses depend on this number.
Bifacial panels get extra current from the rear side. For them 1.25 is the lower bound, and above a reflective surface (snow, gravel, a light membrane) it makes sense to take ISC with the rear-side gain from the datasheet.
Three things are compared with ISC MAX, and they should not be mixed up. First, the cable rating Iz: it must be at least ISC MAX of the string, or of the sum of strings on a common cable. Second, the fuse rating where one is needed: from 1.1·ISC MAX up to the limit on the panel datasheet. Third, the inverter input. Here makers rate the maximum input current in different ways, often against the array's ISC STC. So 1.25·ISC above the input rating is a reason to check the inverter datasheet, not an automatic ban. Cables and protective devices have no such leeway: for them ISC MAX is the design current.
How Solispec applies it
Solispec applies KI = 1.25 to each string and to the sum of strings joined in parallel on one MPPT input. If an input’s ISC MAX exceeds the inverter’s input rating, the engine warns and suggests checking how the inverter datasheet rates that input. For a bifacial panel it notes that KI may be higher and lets you enter the current with the rear-side gain in the "panel Isc, A (own)" field.
Norms and clauses
- DSTU HD 60364-7-712:2022, Annex B.2; cl. 712.512.1.2
Related terms
- Maximum open-circuit voltage UOC MAXThe open-circuit voltage of a panel or string at the site minimum temperature; inverters, fuses and isolators are selected by it.
- PV DC cableThe cable between the panels and the inverter; its section is chosen by I_SC MAX at 70 °C and checked for voltage drop.
- String fuse (gPV)Protects a string from reverse current fed by parallel strings; under DSTU HD 60364-7-712 it is needed with three or more strings in parallel.
- PV stringA chain of panels connected in series: voltages add up, the current is shared; frost and the inverter MPPT window limit its length.
- MPPT and the inverter voltage windowThe inverter’s maximum power point tracker; its voltage range sets the minimum and maximum number of panels in a string.
- DSTU HD 60364-7-712:2022: the electrics of PV systemsNorms explained
Solispec’s calculation is preliminary: for a design it has to be checked by an engineer. The values and clauses follow the current editions of the norms on the date of the page.