Ungrounded PV Power Systems in the NEC: Page 10 of 12
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Overcurrent protection. NEC Section 690.35(B) applies to overcurrent-protection requirements for ungrounded PV systems. The language used in this section parallels that found in Section 690.35(A) but refers back to Section 690.9, which in turn refers back to the general overcurrent-protection requirements found in Article 240. This means that the dc fuses or circuit breakers used in an ungrounded PV system are sized and located precisely as they would be in a grounded PV system. (See QA “Series String OCPD Requirements for Grid-Direct Inverter Applications,” December/January 2009, SolarPro magazine, and “DC Combiners Revisited,” February/ March 2011, SolarPro magazine.)
There is, however, a critical difference in the way that the general overcurrent-protection requirements are applied to ungrounded PV systems. If overcurrent protection is required, then both the PV positive and the PV negative conductor need to be protected. The rationale is similar to the disconnect requirements described previously. In this case, Article 240.21 requires that overcurrent protection “be provided in each ungrounded circuit conductor.” Since both PV positive and PV negative conductors are ungrounded current-carrying conductors, both require overcurrent protection.
In practice, this means that source-circuit combiners in ungrounded PV systems have twice as many fuse holders as they would in grounded applications. One set of fuses is dedicated to the PV positive conductors and the other set is dedicated to the PV negative conductors. The same is true of subarray combiners. It does not matter whether these dc combiners are external or internal to the inverter. If overcurrent protection is required, it is required in both polarities of the circuit. If a servicing disconnect is required for these fuses, then that device needs to switch both conductors. When it comes to disconnects and fuses in ungrounded PV arrays, make sure you are prepared for double the fun.
Source-circuit conductors. For many years, PV manufacturers and system integrators have used sunlight-resistant, wetrated single conductors for PV source circuits. The cable type most commonly used in these applications is underground service entrance cable (USE) marked USE-2. While the insulation for both USE and USE-2 cables is rated for use in wet locations and exposure to sunlight, USE-2 cable has a higher temperature rating when wet (90°C) than does USE (75°C). However, despite its suitability for use in grounded PV systems, the NEC does not allow the use of type USE-2 cable as an exposed single-conductor wiring method for source circuits in ungrounded PV systems.
NEC Section 690.35(D) specifies the requirements for PV source-circuit conductors in ungrounded PV systems. Nonmetallic, jacketed multi-conductor cables are allowed, as are conductors installed in raceways. However, to install exposed single conductors for the PV source circuits in an ungrounded PV system, Section 690.35(D)(3) requires the use of “conductors listed and identified as Photovoltaic (PV) Wire.”
As the name suggests, PV Wire is designed for making PV module interconnections, specifically where single conductors are run outside raceways or conduits and exposed to a unique set of physical and environmental abuses. PV Wire differs from USE-2 in that it uses thicker insulation or a jacket to provide additional mechanical protection. As a result, PV Wire has greater sunlight resistance and can better withstand extreme cold than USE-2. Because of these improved physical properties, PV Wire can be rated for use at higher voltages than USE-2, such as 1,000 volts versus 600 volts. PV Wire can also be used in applications where USE-2 cannot, such as in ungrounded PV array source circuits.
PV modules. The PV Wire requirement in NEC Section 690.35(D)(3) has implications beyond the cable type used to connect a string of modules to a junction or combiner box. The listed PV modules themselves must be constructed using PV Wire for the module interconnection cables. This means that not all PV modules sold in North America can be used in ungrounded PV systems—only modules constructed with listed PV Wire.