A friend of mine called with questions about overcurrent protection for direct current (DC) output of photovoltaic (PV) modules, the DC output of combiner boxes, the location of disconnects for both DC side and the alternating current (AC) side of an inverter, and sizing conductors on both sides. My friend is very knowledgeable about the National Electrical Code (NEC), was an electrical inspector for many years, and now teaches the NEC and PV installations. As I explained the requirements and provided the answers with references within the NEC, I realized our discussion and the information was important for anyone working on PV systems.
PV modules are listed as an assembly of solar cells, optics, a connection box and DC output leads, which are typically assembled within an aluminum frame with specific mounting and grounding means, such as bolts, mounting clamps or grounding clamps. The solar cells within the module are connected in series or are parallel combinations of cells connected in series.
Based on Ohm’s Law, the current is the same in a series circuit, but the voltage from one cell or parallel combination of cells to the next is additive. Modules often are connected in series to the next module by connecting the positive lead from the first module to the negative lead of the next module and so forth for the entire string of modules. The manufacturer sizes each set of leads on the modules to be large enough for the total current of the module string. However, oversizing string conductors to reduce losses is a very common installation technique. Since current in the series-connected string of modules is the same, the conductors connecting each module to other modules in the string are sized for the same amount of current. The maximum output current of one string of series-connected modules is defined in Section 690.8 as 125 percent of short-circuit current rating for the modules. Strings of series-connected modules usually are connected at combiner boxes with the parallel voltage of the strings being the same with the current from each set of strings in parallel as an additive current value. Under normal conditions, the voltage across all the parallel strings will be the same or equal.
On the nameplate of each module is the rated maximum power current, the short-circuit current, the rated maximum power voltage, the open-circuit voltage, and the maximum series fuse size permitted for each module. This series fuse rating is specified as part of the listing of the module and is used for protecting the module or string of modules to react under any abnormal single-fault condition, such as wiring faults or module failures.
PV modules are different from traditional power source circuits where a series fuse is used to protect the power source output wiring and load devices from source overcurrent conditions during fault conditions. In a PV circuit, the series fuse protects a string of modules and associated wiring from reverse current of other parallel PV strings or other power sources, such as batteries or from backfeed from inverters.
In 690.9(E), the text states: “A single overcurrent protective device [the fuse size shown on the nameplate] shall be permitted to protect the PV modules [in series in that string] and the interconnecting conductors [connecting each module in the series string].” An exception to 690.9(A) states, “an overcurrent device shall not be required for PV modules or PV source circuit conductors sized in accordance with 690.8(B) [since the manufacturer has already sized these interconnecting conductors to be able to handle the current of the series string], if there are no external sources such as parallel-connected source circuits [other strings connected in parallel can add to the current in a fault situation], batteries, or backfeed from inverters [current from the DC side of the inverter could feed back into the fault under some circumstances], or the short-circuit currents from all sources do not exceed the ampacity of the conductors or the maximum overcurrent protective device size specified on the PV module nameplate.”
The installation of the series fuse for each series string becomes necessary when the series strings are connected in parallel within the combiner box or directly to an inverter that could potentially provide reverse current under fault conditions. These series fuses are often located in a combiner box. While a PV combiner box appears very similar to a panel board, load center or breaker enclosure, the safety evaluation program of a PV combiner box is significantly different to address the unique electrical and environmental conditions within the PV array.
Due to this topic’s complexity, I’ll continue the discussion next month.
ODE is a staff engineering associate at Underwriters Laboratories Inc., based in Peoria, Ariz. He can be reached at 919.949.2576 and firstname.lastname@example.org.