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For the first example, I’m going to start with panels with THESE stats:
Here is how it would look wired in a 3-panel series with 10 AWG (30 Amps Max Capacity) wire and a 15 amp fuse between the charge controller and the panels:
In the case of a short circuit or short to ground, the 15 amp fuse would not trip because the max amps that could flow to the short would be 9.78A; which would look something like this:
It is not reasonably possible to protect this wire with an fuse/breaker because under normal operation, there is 9.33A flowing through the wire. To ‘protect’ the wire, the OCPD would, technically, have to be between 9.33A and 9.78A. In other words, higher than 9.33A to allow for normal operation of the panel and lower than 9.78A to protect from a short.
This margin between 9.33A and 9.78A is too small to reasonably place an OCPD to protect the circuit as you would technically be asking for something like a 9.5 amp fuse, which doesn’t exist (or at least, isn’t readily available). Also, as the temperature of the panel cools off and therefore gets more efficient, there is a potential for the panels to produce MORE power than what’s listed above, making nuisance trips possible. This is why a fuse is not required, nor useful in this location.
Fusing Solar Panel Arrays wired in Parallel
Now, in the case of 3 panels in series and 4 strings in parallel, if one string shorts, the max amperage to short is 39.12, which exceeds the 30 amp max current rating of the 10 AWG wire.
This means each series needs a fuse/breaker placed in the combiner box as shown here:
Now: on the OTHER side of the combiner box. Between the combiner box and the charge controller, the current flowing through the wire is:
- Normal Operating Current: 37.32
- Short Circuit Current: 39.12
This brings us back to the same point as earlier meaning that the fuse would have to be above 37.32A to allow for normal usage, but below 39.12A to protect from short, which is not reasonable. This reason is why no OCPD is necessary between the combiner box and the solar controller given proper wire size.
The code seeming to refer to this particular scenario is NEC 690.9 as an array of parallel-connected modules are capable of producing amperages higher than the short circuit voltage of a single panel. The code I’m referring to states:
NEC 690.0 Overcurrent Protection.
(A) Circuits and Equipment. PV system DC circuit and inverter output conductors and equipment shall be protected against overcurrent. Overcurrent protective devices shall not be required for circuits with sufficient ampacity for the highest available current. Circuits connected to current limited supplies (e.g., PV modules, dc to dc converters, interactive inverter output circuits) and also connected to sourcces having higher current availability (e.g., parallel strings of PV modules, utility power) shall be protected at the higher current source connection.
Exception: An overcurrent device shall not be required for PV modules or PV source circuit or dc-to-dc converters source circuit conductors sized in accordance with 690.8(B) where one of the following applies:
- There are no external sources such as parallel connected source circuits, batteries, or backfeed from inverters.
- The short-circuit currents from all sources do not exceed the ampacity of the conductors and the maximum overcurrent protective device size rating specified for the PV module or dc-to-dc converter