05-06 2025
Commercial and Industrial Rooftop PV: How to Choose Combiner Box Branch Circuits Based on Installed Capacity
Commercial and Industrial Rooftop PV: How to Choose Combiner Box Branch Circuits Based on Installed Capacity
1. Key Factors Influencing Branch Circuit Selection
1.1 Solar Panel Electrical Parameters
Short-Circuit Current (Isc): The maximum current generated by a panel under standard test conditions (STC).
Rated Current (Irated): The typical operating current of a panel in real-world conditions (usually 80–90% of Isc).
Voltage Range: Ensure the combiner box’s voltage rating (e.g., 1000V or 1500V DC) matches the system’s string voltage.
1.2 String Design and Series/Parallel Configuration
String Voltage: Determined by the number of panels in series (e.g., 20 panels in series at 40V each = 800V DC).
String Current: Determined by the number of parallel strings feeding into the combiner box. For example, if each string has a rated current of 10A and 4 strings are paralleled, the total input current to the combiner box is 4 × 10A = 40A.
1.3 Combiner Box Ratings
Maximum Input Current per Branch: Typically 15–30A for standard fuses or circuit breakers.
Total Current Capacity: The combiner box’s bus bar and output cable must handle the aggregated current from all branches.
2. Step-by-Step Calculation Process
Step 1: Determine System Installed Capacity and Panel Count
Total panels = 500 kWp / 0.5 kWp per panel = 1000 panels.
Step 2: Define String Configuration
Series Panels per String: Assume 20 panels in series (common for 1000V systems: 20 × 40V = 800V DC).
Number of Strings: 1000 panels / 20 panels per string = 50 strings.
Step 3: Calculate String Current and Branch Circuit Requirements
Rated Current per String: Irated = 10A (from panel specs).
Max Current per Branch: Assume a safety margin of 1.25× (per NEC 690.8 for overcurrent protection), so 10A × 1.25 = 12.5A.
Branch Circuit Rating: Select fuses/circuit breakers rated for 15A (standard size above 12.5A).
Step 4: Determine Combiner Box Branch Count
Total Strings: 50 strings.
Branches per Combiner Box: Most combiner boxes have 8, 12, 16, or 24 branches. For 50 strings:
Using 16-branch combiner boxes: 50 strings / 16 branches ≈ 3.125 → Round up to 4 combiner boxes (4 × 16 = 64 branches, allowing for future expansion).
Step 5: Verify Total Current and Bus Bar Capacity
Total Current per Combiner Box: If each combiner box handles 16 strings × 10A = 160A.
Bus Bar Rating: Ensure the combiner box’s bus bar is rated for ≥160A (e.g., 200A bus bar).
3. Practical Considerations for C&I Rooftops
3.1 Safety and Code Compliance
Overcurrent Protection: Each branch must have a fuse or circuit breaker rated ≤1.56× Isc (NEC 2023, Article 690). For Isc = 12A, max fuse rating = 12A × 1.56 = 18.72A → Use 20A fuses.
Lightning Protection: Integrate surge protection devices (SPDs) compatible with the combiner box’s voltage and branch count.
3.2 System Expansion and Redundancy
Spare Branches: Reserve 20–30% of branches for future panel additions or string reconfiguration.
Redundancy: For critical systems, use parallel combiner boxes to avoid single points of failure (e.g., split strings across multiple boxes).
3.3 Cable Management and Cost Optimization
Wiring Length: Position combiner boxes near the center of the rooftop to minimize cable runs and voltage drop.
Cost Trade-off: Larger combiner boxes (e.g., 24 branches) reduce the number of boxes needed but may increase upfront costs. Smaller boxes offer flexibility for phased installations.
4. Example Scenarios for Different Installed Capacities
Installed Capacity | Panel Type | Strings (20 Panels/String) | Branches per Combiner Box | Number of Combiners Needed |
100 kWp | 500Wp | 200 panels / 20 = 10 strings | 8 | 2 (with 6 spare branches) |
300 kWp | 550Wp | 545 panels / 20 ≈ 28 strings | 12 | 3 (with 8 spare branches) |
1 MWp | 500Wp | 2000 panels / 20 = 100 strings | 16 | 7 (with 12 spare branches) |
