PV DC isolator equations
Cold module voltage and combined string current establish the minimum electrical rating screen.
Voc,cold = N × Voc,STC × [1 + βVoc(Tmin − 25°C)]Idesign = Isc × Nparallel × FcurrentUe,DC >= Voc,coldIe,DC >= Idesignwhere:
- Voc,cold
- Temperature-corrected array open-circuit voltage[V]
- βVoc
- Module Voc temperature coefficient[/°C]
- Isc
- Module short-circuit current[A]
- Ie,DC
- Isolator rated operational current for the selected DC-PV duty[A]
Select the utilization category and pole-series wiring from the exact switch-disconnector documentation.
How to size a PV DC isolator
Enter module Voc, its temperature coefficient, the lowest design cell temperature, modules per string, module Isc, and parallel-string count. The calculator derives cold array voltage and corrected current before checking the candidate isolator.
- Calculate cold-condition array Voc.
- Combine the current of every string passing through the isolator.
- Apply the required current factor.
- Choose isolation-only or operational load-switching duty.
- Verify voltage, current, utilization category, and pole wiring together.
Cold Voc and DC-PV switching duty
PV open-circuit voltage normally rises as cell temperature falls. The isolator voltage screen therefore uses temperature-corrected Voc rather than nominal operating voltage. Operational switching also requires the correct IEC 60947-3 DC-PV utilization category for the application.
Voc,cold = N × Voc,STC × [1 + βVoc(Tmin − 25°C)]
Idesign = Isc × parallel strings × current factor
Pole configuration and safe isolation
Many PV isolators obtain their declared DC voltage through a specific series-pole connection. The installation must disconnect every required live conductor and follow the manufacturer's polarity, wiring, mounting, and enclosure instructions.
Check PV string lengthSize a PV combiner boxSelect a DC breaker
PV isolator sizing example
Eighteen modules with 49.5 V Voc and a -0.28%/°C coefficient reach about 978 V at -10°C. Two 13.7 A strings with a 125% factor require 34.25 A, pointing to a 40 A current class while the voltage and pole configuration require exact product verification.
Assumptions
- Linear module Voc temperature coefficient around STC
- Entered minimum cell temperature is suitable for the site
- All parallel strings can contribute current through the isolator
Important Warnings
- Preliminary engineering estimate only. Verify the final design against the current applicable standard, local rules, manufacturer data, and project conditions.
- Use the exact IEC 60947-3 DC-PV utilization category and manufacturer wiring diagram for the intended switching duty.
- PV switching voltage can depend on poles connected in series; do not infer voltage capacity from pole count alone.