In a PV combiner box, each solar string's positive wire connects to a fuse or circuit breaker and then to the positive busbar, while negative wires connect directly to the negative busbar, with the combined output routed to the inverter.

Positive and Negative Terminal Connections

• Positive Terminals: Each string's positive conductor is connected to the line side of a fuse or miniature circuit breaker (MCB) inside the combiner box. The load side of the fuse or breaker then connects to the positive busbar, which combines all strings into a single output to the inverter .
• Negative Terminals: Negative conductors from each string are connected directly to the negative busbar without fusing, following NEC 690.9(B) guidelines .
• Output to Inverter: The combined positive and negative busbars are connected to the inverter using appropriately sized cables, ensuring secure and weatherproof connections .

Overcurrent Protection

• Each string should have a dedicated fuse or breaker to protect against overcurrent and short circuits. This isolates any faulty string without affecting the rest of the system .
• Fuses are typically replaced after a fault, while circuit breakers can be reset .

Grounding and Bonding

• The combiner box must be properly grounded to provide a fault current path and protect against lightning or electrical faults .
• Bonding jumpers may be installed to maintain an equipotential plane across metallic components, separate from grounding conductors .

Wiring Best Practices

• Use strain relief connectors at entry points to prevent wire damage .
• Strip insulation carefully (~0.5 inch) and ensure firm connections to terminals .
• Verify wire gauge according to system current: small systems may use 12–10 AWG, larger systems 8–6 AWG .
• Check all connections with a multimeter for continuity and correct polarity before energizing the system .

Summary

Proper wiring of a PV combiner box involves connecting each string's positive wire through a fuse or breaker to the positive busbar, connecting negative wires directly to the negative busbar, grounding the box, and routing the combined output to the inverter. Following these steps ensures safety, system reliability, and compliance with electrical codes .