PV combiner sizing equations
PV combiner voltage is governed by the coldest expected string open-circuit voltage, while string and output protection are governed by short-circuit current, parallel strings, and the adopted current factor.
Voc,cold = Voc,module × Ns × [1 + |βVoc| × (25 - Tmin)]Id,string = Isc × KpvId,out = Isc × Kpv × NpIreverse ≈ (Np - 1) × Iscwhere:
- Voc,cold
- Maximum corrected open-circuit voltage of one series string[V DC]
- Voc,module
- Module open-circuit voltage at STC[V]
- Ns
- Number of modules connected in series
- Np
- Number of parallel strings
- βVoc
- Magnitude of module Voc temperature coefficient[per °C]
- Tmin
- Minimum design temperature[°C]
- Isc
- Module short-circuit current[A]
Verify the result against the module and inverter instructions, adopted PV standard, DC component voltage ratings, conductor ampacity, enclosure temperature, and actual fault-current paths.
How to size a PV combiner box
Begin with module Voc, Isc, temperature coefficient, minimum temperature, modules per string, and parallel-string count. Cold voltage sets the DC voltage class while corrected short-circuit current sets string and output current requirements.
- Calculate cold string Voc.
- Check inverter and component voltage limits.
- Calculate string design current.
- Evaluate reverse-current contribution and gPV fusing.
- Size output conductors and switching devices.
- Select and coordinate the PV SPD and enclosure.
Cold string voltage example
Eighteen modules at 49.5 V Voc produce 891 V at STC. With a -0.28%/°C coefficient and -10°C minimum temperature, the correction factor is 1.098 and cold string Voc is about 978 V.
Voc,STC = 49.5 × 18 = 891 V
Voc,cold = 891 × [1 + 0.0028 × 35] ≈ 978 V
String fuse and output-current checks
The string fuse must carry corrected Isc without exceeding the module maximum series fuse rating. Combined output current grows with the number of parallel strings, while a faulted string may receive current contribution from the remaining strings.
Combiner components and environment
Verify gPV fuse and holder, DC isolator or breaker, Ucpv and SPD type, output terminals, busbar, cable glands, enclosure ingress rating, UV resistance, corrosion, condensation, temperature rise, labeling, and safe service access as one coordinated assembly.
Assumptions
- Module ratings are entered at STC
- Voc coefficient is entered as a negative percent per degree Celsius
- All parallel strings use matching modules and orientation
- Current and voltage factors must be confirmed against the adopted code and module instructions
Important Warnings
- Cold string Voc must remain below the inverter, isolator, breaker, fuse holder, SPD, terminal, and enclosure system voltage ratings.
- This tool does not replace IEC 62548, IEC 60364-7-712, NEC Article 690, project-specific protection studies, or manufacturer instructions.