Question:
Is it permissible to combine PV strings on the roof with a single output to a PV inverter when using TS4-F or -2F with the RSS Transmitter?
Here is an illustration of this question.
Answer:
If the rest of the RSS Transmitter design criteria are met (references below), combining strings into fewer outputs from a combiner box is acceptable. The powerline communication signal from the RSS Transmitter will travel through the circuit.
Additional Considerations
System Efficiency:
Generally speaking, the system will be more efficient for residential systems and produce more energy in the long run if each string is connected to its own MPPT terminal in the inverter.
PV inverters have multiple DC terminals that accept individual array strings. Each terminal has its own maximum PowerPoint tracker (MPPT) that can compensate for various module mismatches throughout the system's lifetime. This is not as effective as optimizers on each module, but that is a different discussion. Here is a residential rooftop with complex roof geometries.
If using a combiner box on a rooftop with various pitches and angles, the inverter will not see three different inputs and compensate accordingly. It only sees one PV string input and could have difficulty finding the true maximum power point of the array.
Here are two images representing multiple modules at different orientations (left) and tilts (right). Imagine the PV inverter deciding where to go on the IV curve if all the strings were combined!
While there are good reasons to combine, they mainly deal with the cost of copper (wiring). Generally, your PV system will get more bang for its buck by running separate strings to the inverter.
Resources
Design Essentials for Rapid Shutdown using TS4-A-F and TS4-A-2F