The second case that eliminates the need for OCPDs is when there are external sources of potential backfeed current, typically from parallel strings of modules. To exercise the exception, the short-circuit current from those sources cannot exceed the ampacity of the conductors and the maximum OCPD rating specified on the PV modules—known as the maximum series fuse rating. The important part of this exception is to count only the external sources as possible overcurrent. We do not need to consider the current produced by a module or string as a source of overcurrent.
This second exception is what leads to the industry standard of one or two strings connected to a batteryless grid-tied inverter not requiring OCPD for the PV source circuits. In these cases, if a fault occurs within one of the strings, the only possible source of external current is the second string placed in parallel.
Consider a PV module with an Isc rating of 8.8 A and a series fuse rating of 15 A. If you consider the maximum short-circuit current the second string can impose on the first string (8.8 Isc × 1.25 = 11 A), the external source cannot provide more current than the modules’ and conductors’ ratings. This is because, in a normal condition, all the components are required to carry at least that same amount of current.
In the case of a third string in parallel, there is the potential of two 11 A sources, or 22 A that can be pushed back toward the fault. In that case, the conductors may be carrying too much current. In this example, the module’s maximum series fuse rating has been exceeded. There are cases where three strings of modules in parallel do not need OCPDs, but those are the exception. For example, many thin-film modules have low Isc ratings but have been listed with a relatively high series fuse rating that may allow three or more strings in parallel without the need for OCPDs. Consider three strings of modules, each with an Isc of 4.4 A and a series fuse rating of 15 A. In this scenario, the amount of potential external fault current for the two external strings 11 A (4.4 A x 1.25 x 2). Since this is less than the 15 A series fuse rating, OCPDs would not be required here. Most system designers and installers stick with no OCPD for one and two strings—but as soon as a third string is added, OCPDs are added as well.
If your PV circuits require OCPDs, you need to use 690.9(B) to determine their amperage rating. Although this shows up as a change in 2014, the only real change is the relocation from 690.8. The NEC requires that you multiply the maximum circuit current calculated in 690.8 by 125% to account for the continuous nature of PV circuits. This results in an OCPD that is 156% larger than the Isc rating.
This calculation will dictate the minimum OCPD size required to protect the circuit. In the earlier example, the module has an Isc of 8.8 A, so the smallest OCPD we can use to protect a circuit with a single string of modules would be 8.8 A × 1.25 × 1.25 = 13.75 A. This isn’t a standard OCPD size, so we would round up to the next standard size, which would be 15 A (based on 240.6). This corresponds with the maximum OCPD size listed by the module manufacturer. In this case, the minimum and maximum size is the same.
The final check is to ensure that the conductor chosen per 690.8 is properly protected by the required OCPD. This means that after conditions of use are applied, the conductor’s ampacity needs to comply with the rules of 240.4.
The OCPDs in the PV source and output circuits must also have the proper DC voltage ratings. Verify that the DC rating of the OCPD will exceed the maximum system voltage of your system. For any residential PV applications, this will result in installing 600 VDC fuses in the combiner boxes.
One final note is the accessibility of PV circuit OCPDs. Many electricians and authorities having jurisdiction are familiar with the requirement that OCPDs be readily accessible (per 240.24). But in 690.9(D), the rule is that the OCPDs “shall be accessible but shall not be required to be readily accessible.” This allows placing combiner boxes with OCPDs on the roof and/or behind modules if so desired.
When sizing conductors and OCPDs for PV systems, consider both 690.8 and 690.9 to make the proper calculations. Walking through each section step-by-step will help eliminate mistakes and help determine proper equipment ratings. In Code Corner 163, I will go through detailed examples to help clarify this all-too-confusing process.
Ryan Mayfield is the principal at Renewable Energy Associates, a design, consulting, and educational firm in Corvallis, Oregon, with a focus on PV systems.