2011 NEC 690.8(B)(2)(c) When is OCPD required?

[SolarAlternativesDesign]

One of the sizing requirements conductors is that the conductor shall be protected by the OCPD, "where required." [2011 NEC 690.8(B)(2)(c)] Our common practice has been to size the inverter - service panel conductor to be protected by the backfed breaker in the panel. The only time the current would be higher than the inverter output is in the case of a fault causing current to flow from the panel to the inverter.
What about the PV output circuit between the combiner box and inverter? Our common practice has been to install fuses only in the combiner, between the combiner output bus and the modules, but not in the inverter.
Similarly, if there are only 2 modules we do not protect the circuit with fuses. We have assumed that current can only flow from modules to inverter and not the other direction.
Is this valid? Could there be some kind of fault current flowing from the inverter to the combiner box?
:?

 

[jaggedben]

...
What about the PV output circuit between the combiner box and inverter? Our common practice has been to install fuses only in the combiner, between the combiner output bus and the modules, but not in the inverter.

When a combiner is used, fuses are installed to protect source circuits (i.e. between modules and combiner) from the combined current of the other strings. If you have the current from, say, 3 strings flowing back through the combiner to a fault on one string, the fuses will blow so that the fault only carries the current from the string where the fault is. If the wiring for each string is rated to carry the current for all strings then the fuses are not required. That is often the case with 2 strings, but usually not with more.

If there is only one combiner box, the output circuit (between combiner and inverter), then the same logic applies as if there was only one string; no OCPD is required. However if there are multiple combiners feeding a 're-combiner' as in some larger systems, then the same logic applies to fuses in the re-combiner.

Whether fuses should or should not be installed in the inverter depends on the inverter instructions.

We have assumed that current can only flow from modules to inverter and not the other direction.
Is this valid? Could there be some kind of fault current flowing from the inverter to the combiner box?
:?

With transformerless inverters there could be fault current flowing from the AC source to the combiner box, but the ground-fault protection in a listed inverter (and possibly an internal fuse) will shut this down very quickly. You are not required to consider the DC side when choosing an AC OCPD (unless, perhaps, the inverter instructions say you must, but I've never seen that). Inverters instructions should include a maximum OCPD rating for the AC side.

 

[SolarAlternativesDesign]

Thanks

Thanks

Thanks, jaggedben.