Most of a PV system’s components will be installed in an outdoor location, necessitating their listing for outdoor use and exposure to the elements. Many of the enclosures used will carry a National Electrical Manufacturers Association (NEMA) rating of at least 3R—an enclosure with this rating is weather-resistant when mounted vertically.
Most NEMA 3R boxes are not considered weather-resistant when mounted at any other angle, so using a NEMA 3R junction box or disconnect mounted parallel to a roof surface is a direct violation of the Code. It is possible to use a NEMA 4 enclosure where the location requires mounting the box on an incline or even on its back. Several combiner box manufacturers use NEMA 4 enclosures, but most disconnects must be mounted on a vertical surface.
Section 690.4(D), an addition in the 2008 NEC, requires that equipment used in PV systems, including source-circuit combiner boxes, “shall be identified and listed for the application.” This restricts using on-site-manufactured combiner boxes. PV combiner boxes need to be listed to Underwriters Laboratories’ (UL) Standard 1741. Installers can choose from among several types of combiner boxes, from simple junction boxes to large multiple string combiners, that incorporate fusing or breakers.
PV systems present their own set of wiring challenges. Source-circuit wiring is often exposed to sunlight and extreme temperatures, while output circuits are often run at very high DC voltages inside potentially hot conduit. The inverter output circuit for grid-tied inverters sends electricity into the grid—in the minds of many inspectors, the wrong direction for current to flow. Live DC circuits on the roof always cause inspectors grave concern, even though these are inherently current-limited. These are all legitimate concerns that system designers and installers need to account for and be prepared to explain to permit and inspection officials.
Temperature correction. One of the first Code require-ments that PV installers need to review is Article 110, which addresses general requirements for electrical installations, including working clearances, equipment mounting, and temperature limitations associated with conductor ampacity. Since PV systems can be exposed to high temperatures, this section can have major implications.
Article 310 of the 2008 NEC (and 2011 NEC) includes Table 310.15(B)(2)(c), “Ambient Temperature Adjustment for Conduits Exposed to Sunlight On or Above Rooftops.” The adjustments in this table “shall be added to the outdoor temperature to determine the applicable ambient temperature for application of the correction factors in Table 310.16 and Table 310.18.” This adjustment can require significantly larger wire sizes, depending on the local temperatures and the height of the conduit off the roof. This is one of the areas that catches some traditional electricians, since many of them do not estimate rooftop temperatures accurately.
Equipment must be compatible with the temperatures it will experience, including properly rated conduit, wiring, and enclosures. Installers also need to be aware of temperature effects on conduit expansion and contraction, and must be installed on roofs with expansion couplings and proper support.
Section 310.15(A)(2) includes an exception allowing for a less restrictive calculation for short rooftop conduit runs (10 feet or less than 10% of the total circuit length, whichever is less). If the conduit is minimally exposed, the balance of the conduit length is able to dissipate the heat effectively, thereby reducing the heating impact. This exception may allow installers of residential PV to avoid upsizing conductors and conduit.
Color-coding. A longstanding convention in PV installations is to mark the positive DC circuit conductor red and the negative conductor black. While this may be recognizable to PV professionals, it is not a correct method per the NEC, nor is it safe, since it may lead to confusion among other tradespeople.