By Mike Holt, NEC Consultant

Though it covers solar power systems, Article 690 is not light reading.

Article 690, consisting of eight Parts, applies to Photovoltaic (PV) electrical energy systems, array circuit(s), inverter(s), and charge controller(s) for PV systems. The requirements of Chapters 1 through 4 apply to these installations, except as specifically modified by Article 690.

These systems may be interactive with other electrical power sources or stand-alone with (or without) energy storage (batteries).

Article 690 might be the most difficult NEC Article to navigate. For several cycles, Article 690 requirements didn’t really change that much. Then a huge influx of changes occurred due to the sudden popularity of these systems.

The good news is the 2014 NEC cleaned up many areas of confusion. The bad news is the 2014 NEC introduced new technical changes (to keep up with changing technology) and other changes that could be confusing. But considering what a moving target solar has been lately, CMP 4 has done an amazing job.

What does that mean?

Most of us don’t enjoy reading definitions. But the specialized terminology in Article 690 means you must do exactly that. Let’s get straight what these key terms mean.

Alternating-Current PV Module. A PV module unit consisting of solar cells and an integral micro-inverter that changes dc power to ac power when exposed to sunlight, and that is listed as an ac module.

Array. An electrical, mechanically integrated assembly of PV modules or panels with a support structure and foundation, tracker, and other components that form a dc power-producing unit.

Bipolar Photovoltaic Array. A PV array that has two outputs, each having opposite polarity to a common reference point or center tap.

Building Integrated Photovoltaics. PV cells, devices, modules, or modular materials designed to integrate into the outer surface or structure of a building (e.g. roof shingles that are the actual modules).

Direct Current (dc) Combiner. It combines two or more dc circuit inputs (in parallel) to provide one dc circuit output. DC combiners can also combine two or more output circuits into another “output circuit.” These “array combiners” are used on large inverter systems with several tiers of combiners. The term “DC combiner” is new with the 2014 NEC. The terms “dc combiner,” “source circuit combiners,” “recombiners,” “subcombiners,” “string combiner,” and “array combiner” are often used in the industry to describe the same equipment. This inconsistency can easily cause confusion. Use the NEC word choice to help avoid confusion.

DC-to-DC Converter. A device (installed in the PV source circuit or PV output circuit) that provides output dc voltage and current at a higher or lower value than the input dc voltage and current. These components are intended to maximize the output of independent modules and reduce losses arising from variances between module outputs. They're directly wired to each module and bolted to the module frame (or PV rack).

Interactive System. A PV system that operates in parallel (interactive) with electrical utility power (or other power source, e.g., generator or wind system) through a utility-interactive inverter. Listed utility-interactive inverters automatically stop exporting ac power upon loss of the utility (or other source) power, and automatically resume exporting ac power to the utility source once it’s been restored for at least five minutes [705.40 Ex].

Inverter. Electrical equipment that changes dc power from the PV system to grid-interactive ac power. Inverters change direct current produced by the PV modules or batteries into alternating current. Grid-tied inverters synchronize the ac output current with the utility’s ac frequency, thus allowing the PV system to transfer unused PV system current to the utility grid. Battery-based inverters for stand-alone systems often include a charge controller, which is capable of charging a battery bank from a generator during cloudy weather.

Inverter Input Circuit. The dc conductors between the battery and inverter of stand-alone systems or PV output circuits and an inverter for a utility-interactive system.

Inverter Output Circuit. The ac circuit conductors from the inverter output terminals that supply ac power to premises wiring, including conductors from ac modules [690.6(B)].

Module. A PV unit designed to generate dc power when exposed to sunlight. PV modules use sunlight to make direct current (dc) electricity by using light (photons) to move electrons in a circuit. This is known as the “photovoltaic effect.”

Monopole Subarray. A PV subarray that has two conductors in the output circuit, one positive (+) and one negative (−). Two monopole PV subarrays are used to form a bipolar PV array. Every module is monopole (two wires).

Multimode Inverter. An inverter that has the capabilities of both stand-alone and utility-interactive systems (Sidebar).

PV Output Circuit. Circuit conductors between the dc combiner and the dc input terminals of the inverter or dc disconnect.

PV Power Source. One or more arrays of PV modules that generate dc voltage and current power.

PV Source Circuit. The circuit conductors between the PV modules and the terminals of the dc combiner, or the inverter dc input terminals if no dc combiner is used. PV Source Circuits are often called “strings.”

PV System Voltage. The direct current (dc) voltage of any PV source or PV output circuit. For multiwire installations, the PV system voltage is the highest voltage between any two dc conductors.

Solar Cell. The basic building block of PV modules, these generates dc power when exposed to sunlight.

Stand-Alone System. A PV system that supplies power without an interconnection to another electric power source.

General requirements

A PV system can supply power to a building and to any other electrical supply system(s) [690.4(A)]. It’s important to note that equipment for PV systems (e.g., inverters, PV modules, dc combiners, dc-to-dc converters, and charge controllers) must be listed for PV application [690.4(B)].

PV systems, associated wiring, and interconnections must be installed by a qualified person [690.4(C)]. A “qualified person” has the knowledge related to construction and operation of PV equipment and installations plus the safety training to recognize and avoid hazards to persons and property [Article 100].

Where multiple utility-interactive inverters are remote from each other, a directory is required at each dc PV system disconnecting means, ac disconnecting means (for mini- and micro-inverters), and service disconnecting means. It must show the location of all dc and ac PV system disconnecting means in the structure [690.4(D), 705.10]. A directory isn’t required where all PV system disconnecting means are grouped at the service disconnecting means.

Solar success

Because Article 690 is so difficult to navigate and solar in general is so complicated to install, your solar installation could easily be a failure. A key step to avoiding that outcome is to take the time to learn the definitions in 690.2.

In Part 2 of this series, we’ll look at ground fault protection, circuit requirements, and other requirements for these installations.

Sidebar. Multimode Inverters

Multimodes can be used for either stand-alone or utility-interactive systems, because they have the capabilities for both. They address a serious drawback that people have with PV systems. Many customers believe that putting a PV system on their roof will allow their lights to stay on when the utility power goes out. With a typical utility-interactive inverter this isn’t the case, because when the “other” power sources (e.g. utility power, generator, or wind) shuts off, so does the inverter.

A multimode inverter solves this concern because, under normal conditions, it acts just like a standard utility-interactive inverter. When the “other” power source shuts off, the inverter automatically switches modes and signals a relay in the inverter to break connection to the other source. This acts as a sort of transfer switch in the effect it has of proving safety to utility workers who are anticipating working on a de-energized line.