DC cabling
String design current and DC cable rating check (1.25 × Isc)
- where
- design current (A)
- module short-circuit current at STC (A)
- strings in parallel on this cable
- corrected cable rating (A)
- cable current rating for the installation method (A)
- combined correction factor
- cable utilisation (%)
- maximum reverse current into one string (A)
Explain this equation
Checks that a PV DC cable is rated for the current the strings on it can deliver, and how much reverse current could flow into a faulty string.
A PV module cannot deliver much more than its short-circuit current, but on bright days with reflections it can exceed the STC figure, so DC design uses 1.25 × I_sc per string. Parallel strings add. The cable's tabulated rating is reduced by correction factors for heat and grouping. Utilisation is design current as a percentage of that. If one string faults, all the others can feed into it, hence N_p − 1.
Currents in parallel add; the 1.25 factor is the standard design margin for PV.
Sense check: one string of 13.9 A modules gives 17.4 A, about a third of a 52 A cable rating. With three strings, a fault could push about 35 A back into one string.
Watch out: when reverse current can exceed the module's reverse current rating (often with three or more strings in parallel) string fuses are needed.
PV modules can deliver more than their STC current in strong sun. This applies the 1.25 × Isc rule and checks it against the cable rating after correction factors.
Enter your values to see the result.
Good to know
The 1.25 multiplier covers irradiance above 1000 W/m² and module tolerance. Take Iz from the cable maker's table for the installation method; for example, one UK maker rates 4 mm² PV cable at 52 A clipped in 60 °C ambient, with 0.91 at 70 °C and 0.82 at 80 °C. Cables behind modules on a roof run hot, so apply the ambient factor. Where three or more strings are paralleled, compare the reverse current with the module's maximum series fuse rating; string fuses may be needed.
Common mistake
Sizing on Imp or plain Isc without the 1.25 factor, or using a free-air rating for cable run under modules in a hot loft or behind an array.
Limits
Current check only; also check voltage drop and the cable's d.c. voltage rating. Cable must be to BS EN 50618 or equivalent. Full rules in BS 7671 Section 712 and the IET Code of Practice.
Sources
- IET Wiring Matters, April 2026: Solar PV d.c. switch-disconnector selection (UOC MAX = modules × UOC STC × 1.2; ISC MAX = strings × ISC STC × 1.25; alternative method using site minimum temperature and module coefficient)
- myCableEngineering: PV design – voltage and current (worked examples: design current = Isc × strings × 1.25)
- BS 7671:2018+A4:2026 Requirements for Electrical Installations, Section 712 Solar photovoltaic (PV) power supply systems
- IET Code of Practice for Grid-connected Solar Photovoltaic Systems, 2nd edition (2022)
- British Cable Company (Doncaster Cables) PV ULTRA datasheet: 4 mm² 52 A, 6 mm² 67 A clipped at 60 °C ambient; derating 0.91 at 70 °C, 0.82 at 80 °C, 0.71 at 90 °C; 90 °C conductor
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow MCS standards, manufacturer data, BS 7671, the ENA connection rules and Building Regulations. System design and DNO applications need a competent, certified installer.