Solar system codes

Solar system codes

Build it right!

By Jeffrey Yago, P.E., CEM

Issue #154 • July/August, 2015

Here, multiple homemade solar modules have caught fire and burned.

The further you go into the mountains or rural countryside, the more some people think building codes do not apply to them. But not following electrical codes can kill you or burn down your remote cabin, regardless if the county inspector can find you or not.

Thomas Edison built the first central generating station in 1882 to power a string of DC-powered streetlights in New York City. In 1893, the Chicago World’s Fair had the first public demonstration of public building illumination. After a large number of these early electrical systems caught fire, several independent code groups got together and published the first National Electric Code (NEC) in 1897. Since then the NEC gets revised every three years to reflect any problems that have been identified and to reflect any new electrical materials being introduced.

Unlike government-mandated codes and rules (which are sometimes the result of pressure from paid lobbyists on politicians or just plain bureaucratic stupidity), the electrical codes were developed by experts in the field and actual working electricians selected to represent all electrical installers as well as code officials. If there is one section of the NEC that has undergone the most revision, it’s Article 690, which addresses solar installations, mainly due to the significant increase in solar installations and the resulting experience identifying new wiring problems that get resolved in later code updates.

Solar power systems need to follow all wiring codes due to many of the components requiring both DC and AC electricity. The wiring in any conventional home is typically 120- and 240-volt alternating current (AC), with an interior circuit-breaker panel and an electric meter on the exterior wall just a few feet away. Since AC voltage is constantly passing through zero voltage as the power alternates polarity 60 times each second, it’s easier for safety fuses and circuit breakers to interrupt an overload. The voltage will be zero multiple times as the contacts begin to open.

It’s much harder for an electric arc to be sustained in a 120-volt AC circuit than a 120-volt DC circuit because the AC voltage is regularly passing through zero. However, unlike alternating AC electricity, direct current (DC) electricity is constant. This means it’s much harder to stop the flow of DC electricity when there is a short circuit or overload, and arcs between nearby wires or terminals can be sustained in as little as 120-volts DC over a wide gap once the arcing has started.

In fact, when inspecting a recent solar installation for a client, I witnessed a one-inch-long continuous electrical arc between the two wires in the cable routed from the solar array through an attic. This was started by a wire staple that was partially cutting into the wire insulation. Needless to say, this was starting to burn the wood framing it was stapled against, and a house fire would have started if we had not shut down the system.

Arcing between array wires at wire staple nick

Those of you familiar with DC welders know that you first touch the welding rod to the metal part and quickly pull back a small distance once the arc has started. By adjusting this distance as the rod is consumed, the arc will be continuous. This is why safety devices and circuit breakers rated for DC wiring are physically larger and more expensive than the identically-rated AC fuses and circuit breakers typically sold by building supply outlets and electrical distributors.

In an overload condition, the smaller and closer-spaced electrical contacts inside a typical AC circuit breaker will weld together as they start to open if used to interrupt a DC electrical flow. This flow will start to arc between these slowly opening contacts with the same result as the welding rod example. Circuit breakers with a DC rating will have much larger internal contacts, heavier trip springs, and mechanical linkages designed to pull the contacts further apart and at a much faster rate than an AC circuit breaker with the same amp rating.

Overheated terminals are another safety concern with DC battery and inverter power systems. An inverter connected to a 12-volt battery will require ten times the amps from the battery to supply any 120-volt AC load. For example, most manufacturers do not make a 12-volt inverter larger than 2400 watts. A 2400-watt AC load supplied by the inverter will have a 20-amp current flow at 120 volts (2400/120). However, this load results in a 200-amp current flow from the 12-volt battery to the inverter (2400/12). At these high currents, you need #4/0 size battery cables, and if these battery terminals and cable lugs are not wire-brushed clean before assembly and their bolts torque-wrench tight, overheating can occur.

I once investigated a homeowner-installed battery and inverter system that was new, yet the insulation on the battery cables caught fire and melted. The battery terminals had become so hot that the lead melted through the tops of the new batteries and you could see the battery plates down inside. The cause of this near-disaster was due to the homeowner using only his fingers and pliers to tighten all of the battery terminals, and did not use a torque-wrench per battery manufacturer’s instructions. Fortunately, the batteries were located in a well-ventilated battery room with a concrete floor and block walls that would not burn easily.

The potential for an electrical short between closely-spaced, low-voltage positive and negative wiring in DC systems is similar to the power and neutral wires in low-voltage AC systems. However, as solar arrays have gotten larger, most solar modules have continued to use the same #10 gauge wire to interconnect the modules. The more solar modules wired in series, the higher the array voltage, which increases the array wattage without increasing the current. A higher current would require larger gauge and more expensive wiring. Since most grid-tied solar systems installed today are intended to sell power back to the utility and not charge a low-voltage battery bank, solar array voltages are no longer limited by the low voltage limit of a 12- to 48-volt battery bank.

Most residential and commercial grid-tie solar systems are now designed for 350- to 600-volt DC operation. While this significantly reduces the size and cost of the wire and conduit needed to interconnect the solar array, at this high DC voltage level arcing between the positive and negative wires coming from a solar array is almost impossible to stop once the arc has started.

Keep in mind, the fuses and circuit breakers in solar array wiring are normally installed in a combiner box at the end of the wire connecting the solar array, but the solar modules are always supplying power to the section of wiring ahead of this equipment as long as the sun is shining. This means there are usually multiple long wire runs across a roof from the solar array back to any disconnect or combiner box that is not fused. If an electrical short develops anywhere along this wire, the power cannot be turned off. Recent updates to the NEC are trying to reduce this risk by now requiring DC ground-fault safety devices to be installed in above-roof combiner boxes and the wiring between multiple strings of high-voltage modules. This is just one example of how the many individuals who volunteer to be on the NEC code committees review solar installation procedures and modify the code sections for the next three-year printing cycle when they feel it is justified.

These melted battery cables were caused by a loose-bolted connection.

While all electrical wiring, including solar wiring in remote off-grid homes and cabins should follow the NEC, there are other safety concerns for homes with a solar system that are not found in a conventional home. For example, the first thing your local volunteer fireman is instructed to do when responding to a house fire is to pull the electric meter which shuts off all electricity inside the home. This not only protects the rescue workers holding water hoses, but also protects the residents from touching live wires that may be exposed or hanging from fallen electrical fixtures as a result of fire damage.

However, pulling the electric meter on any home having a backup solar-power system may not turn off power to the electrical circuits supplied from an inverter and batteries. In addition, a totally off-grid solar home will have no electric meter to pull. This is why most updated building codes now require locating a clearly-labeled safety disconnect for the solar system near the electric meter so both systems are easily recognized and can be de-energized at the same time by emergency responders.

Another code area undergoing review relates to firemen taking an ax to the high point of a roof to vent smoke out of a burning building. Codes requiring more open spaces and walkways between groups of solar modules are starting to be enacted for large roof-mounted solar arrays.

Most people would never believe a solar module made from glass, aluminum, and copper components could actually catch fire and melt, but DC electricity is different and this really does happen on systems improperly wired or using materials that are not code-compliant.

The solar system you plan to install may be only a fraction of the size of the commercial solar systems, but the wiring issues are the same. There are reports of low-cost solar components being sold to unsuspecting homeowners that were never subjected to the rigorous safety testing required to receive an Underwriters Laboratories (UL) label. Not only should any do-it-yourself solar installation be wired according to the NEC, but every electrical component should be stamped with the UL symbol, even if an inspector will never see the work. Note that some jurisdictions in the United States also accept electrical components tested in Canada with the CE symbol.


A large part of the NEC is devoted to electrical system grounding and grounding materials. There are actually multiple grounding issues related to a solar system installation, and there are very specific codes describing how these separate grounding systems can be, or cannot be, interconnected. For example, a conventional home in the suburbs may not have the same risk of a lightning strike as a roof covered by metal solar modules located on a remote, treeless mountaintop. Lightning protection systems are normally a totally separate grounding system from the electrical system grounding and will use their own ground rods and wires.

A home or cabin having both AC wiring and DC solar wiring is required by the NEC to separately ground both systems. However, since an inverter is connected to both the AC and DC wiring systems, and since the NEC normally only allows one ground connection to each electrical device to prevent power flowing through the ground wires, there are separate grounding rules covering inverters. In addition, to prevent shock and insure all GFI circuit breakers will function properly, all metal enclosures (including junction boxes, metal conduit, and electrical equipment housings) are required to be separately bonded to each other and to the grounding system.

The “neutral” leg of any AC power source and the negative of any DC power system over 24 volts are required to be grounded at only one point, which prevents metal parts from using your body as a deadly path to ground between these separate systems in the event of a grounded power conductor.

Backup generators also pose their own unique grounding issues as some will have their “neutral” wire bonded to their metal frame, which may or may not be grounded, and other generators may require the neutral wire from the generator to be switched on a separate pole in the transfer switch to avoid the double grounding problem. Since many battery-backup and off-grid solar systems also include a generator, this means how the generator is grounded to your electrical system will depend on how its neutral wire was wired at the factory. While I am sure this sounds confusing, your generator installation guide should indicate which way it needs to connect to the home’s electrical panel and grounding system.

Properly grounding any electrical system with both AC and DC wiring to meet the NEC code can be difficult to sort out, which is why I always recommend using a licensed electrician to help with this work.

The NEC also specifies what materials can be used for electrical wiring and wiring terminals. For example, most homes built after the 1960s are wired using copper wire and plastic or steel junction boxes, but almost all solar module frames are aluminum and most array mounting racks are aluminum. Copper and aluminum create damaging electrolysis action when bolted together, especially when wet. Since the NEC requires every individual solar module and each section of array mounting rack to be grounded, connecting all these aluminum parts together with a bare copper ground wire requires special dielectric connectors.

I have observed a solid-copper ground wire almost dissolved in less than a year where it was bolted to the aluminum supports of a solar array using an indoor type aluminum lug and a galvanized-steel bolt. The code requires these ground lugs and attachment bolts to be cadmium-plated solid copper and stainless steel for a reason. The code even specifies how many threads must make contact between this ground screw attaching the ground lug and the tapped hole in the aluminum frame.

Left: Group all exterior home power disconnects.
Right: AC and DC breakers may have the same amp rating, but they are not the same. There have been many electrical fires caused by using AC breakers in DC wiring.


While the NEC can be confusing at times, even for licensed electricians, a more “user-friendly” NEC handbook is available that includes extensive use of graphics and simple text to explain each code section. In addition, there are several “inspector guides” available online to help local code officials who may be less familiar with solar systems. These are an excellent source of dos and don’ts.

If you are planning to install any solar or backup power system and will not be hiring a licensed electrician to do the wiring, I strongly encourage you to do your research and to reference the links above.

This brief introduction to the NEC is not intended to be an installation guide on how to properly wire or ground a do-it-yourself solar project. It should, however, let the reader see that installing any electrical system requires following the NEC, and understanding that it was written to reduce the risk of property damage and even death. Building in a remote area where code inspectors as well as rescue workers would have a hard time finding you is even more reason to make sure you do it right.

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