I'm reading through 690.9 of 2020 NEC and I'm finding the language confusing related to combiner boxes and inverter DC inputs. If we have a combiner box with fusing on the input strings does there need to be overcurrent protection between the combiner box and the inverter DC input? The cables would have sufficient ampacity for the max circuit current. I've seen some combiner boxes with an option of positive fuse at the output but only on some models but then again I'm more on the rookie side of working through some of the language in 690.
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690.9 Overcurrent Protection Question
- Thread starter cyriousn
- Start date
I may have answered my own question and it comes down to it being a central vs. string inverter. The cables are already rated for the max circuit current so they should handle a fault so need for OCPD for string inverter since the individual PV circuits are fused at combiner box. For central inverters the output circuits between combiner boxes and inverter typically have fuses on the + cable for each set of cables going into the inverter. Without those fuses, the cables in parallel would not be able to handle the current seen in a fault.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
In a word, no. Your DC conductors should be sized so that they are able to withstand all the current that your modules can produce.I'm reading through 690.9 of 2020 NEC and I'm finding the language confusing related to combiner boxes and inverter DC inputs. If we have a combiner box with fusing on the input strings does there need to be overcurrent protection between the combiner box and the inverter DC input?
That's going to depend on the system design. If there is a recombiner between the combiner and the inverter with more than two inputs it will probably have fuses. If the inverter contains a recombiner with more than two inputs in parallel it will probably have fuses.In a word, no. Your DC conductors should be sized so that they are able to withstand all the current that your modules can produce.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
Well, yes, of course. Recombiners have input fuses for the same reason that combiners have them. The OP just mentioned a combiner feeding an inverter.That's going to depend on the system design. If there is a recombiner between the combiner and the inverter with more than two inputs it will probably have fuses. If the inverter contains a recombiner with more than two inputs in parallel it will probably have fuses.