Solar Photovoltaic Systems and the 2014 NEC

solar.pngAlthough understanding of solar photovoltaic systems in the 2014 National Electrical Code (NEC) requires an understanding of the basic principles first, including a clear knowledge of definitions as used in Article 690. An apprentice or a journeyman electrician cannot begin to comprehend and deal with the intricacies of a photovoltaic (PV) installation without knowledge of the intent, purpose, and design requirements involved with these systems. A brief explanation on some of the major sections or subsections may help with the understanding of these systems.

Solar PV Definitions

The scope of Article 690 covers the provisions of this article that apply to solar PV electrical energy systems. These systems originate with solar cells installed in a series configuration in what are called modules. A module is defined as a complete, environmentally protected unit consisting of solar photocells, where sunlight is turned into direct current (dc) energy, optics that help focus and direct the sunlight into the photocell, as well other components of the PV system, designed to generate and process dc power when exposed to sunlight.

The dc output from a module can be connected to other modules connected in series to increase the voltage output and then in parallel to increase the current, which are then used to power dc loads, charge batteries, or be converted into alternating current through a dc to alternating current (ac) inverter or converter. Modules can be combined into panels, and panels can be combined into subarrays or arrays. A panel is defined as a collection of modules mechanically fastened together, wired, and designed to provide a field-installable unit. A subarray is an electrical subset of a PV array, and an array is a mechanical integrated assembly of modules or panels with a support structure and foundation, tracker, and other components, as required, to form a dc power-producing unit. Field people often mistakenly call a "module" by the term "panel" causing confusion, especially with apprentices. Then both journeymen and apprentices have problems understanding the requirements in the National Electrical Code. Care should be taken to always use the proper terminology to avoid confusion.

An inverter is equipment used to change voltage level or wave form, or both, of electrical energy. An inverter can be called a power conditioning unit (PCU) where it is used to match the voltage levels, phase angles, and other utility company power characteristics. An inverter may also be called a power conversion unit where it is used to change dc to ac, with the additional function of power conditioning. Inverters may also function as battery chargers by converting ac power into dc and then acting as a charge controller or regulator so the batteries are not overcharged.

There is also a new definition in 690.2 of the 2014 NEC that covers “dc to dc converters,” a device installed in the PV source circuit (the output of the modules) or the PV output circuit (usually from a combiner box to the inverter) that can provide a dc output voltage and current at a higher or lower value than the input dc voltage and current into the converter.

There are three different types of inverters: The standalone inverter, the utility interactive inverter, and the multimode inverter. The standalone inverter is a solar PV system that supplies power independently of an electrical production and distribution system, such as the utility power grid. A utility interactive inverter is an inverter that operates in parallel with and may deliver power to an electrical production and distribution system. A multimode inverter has the capabilities of both the interactive inverter and the standalone inverter, since there can be a branch circuit or feeder at the inverter that supplies loads not interconnected with the utility, plus feeders that are interactive with the utility supplied loads.

There are some additional definitions that are critical to understanding PV systems and the requirements in the NEC. A PV power sour