27 June 2012
Wire and connectors are not often high on the list of concerns for those in the solar industry. Modules, inverters, and racking seem to steal the limelight. However, all of the precious electricity a PV module creates will be for naught without wire to take it somewhere to be used.
Proper wiring is not only of immense importance to the success of any solar project, but mismanagement can create significant problems with performance, efficiency, and most importantly, safety. It is for this reason that code inspectors dedicate most of their attention to wiring and connections during system inspections.
Types of systems
It is almost always quicker to take your time and install a PV system correctly the first time.
There are two relatively distinct wiring areas of a PV system: the array and the balance of system, or BOS, which refers to all of the components other than the modules and racking. This includes the inverters, the disconnects, the combiner boxes, etc., that connect the array to a load. In a residential case, the load would be the main distribution panel that feeds electricity to the home. Both of these areas of wiring have unique challenges.
Here is an example of array wiring getting pinched between the module and the racking
In my experience, the most common problem on an array is improperly secured wire, which makes it much more susceptible to damage from the elements. For instance, a wire on a roof-mounted array that is touching the roofing material will sustain damage as the wind blows. Eventually, this will expose the wire and cause a short in the system, as will pinching a wire between a module and the racking.
Wiring in this combiner box is neatly installed, and the positive and negative are differentiated with black and white wire, however there is no way to determine which positive and negative belong to the same string. Labeling the wire would remove the guesswork.
PV installation can become repetitive (especially on larger systems), and mistakes can and do occur. For example, a simple task such as connecting modules is a common mistake made by PV installers. Although easily avoided with frequent string voltage testing, this issue can become a major headache when commissioning a system (and is almost always located at the least accessible part of the array). While checking the voltage, an installer can also check for polarity to ensure that has not reversed. However, not labeling and keeping track of the positive and negative homeruns can lead to problems, thus underlining the importance of making sure the polarity is correct to prevent damage to equipment in the rest of the system. Factors including hastiness, working in difficult weather conditions (either hot or cold), in tight work areas, or on roofs, are the most common reasons for these types of issues.
Marking each wire with colored tape is a simple way of differentiating wire. One will need to mark at every point of connection so it is clear all the way through the system.As for wiring the remaining balance of the system, the most common problem pertains to installations that do not meet the National Electric Code (NEC). A strong understanding of the code sections that pertain specifically to PV is vital for a proper installation, as is staying current with the latest NEC edition since the PV section (690) changes often. It is common to find loose wire connections when inspecting a PV system, as junction boxes, combiner boxes, disconnects, and inverters not only have multiple connection points in them but limit the working area needed to complete connections. These connections should be torqued properly and rechecked prior to moving on. Again, reversing the polarity is an easy mistake, and it is most likely to occur while wiring the balance of system, given the number of connection points between the array and the inverter. As previously mentioned, it is of critical importance that positive and negative homeruns be recorded, to ensure that system equipment is not damaged.
Another common mistake made by PV installers involves forgetting the orientation of the line side when connecting wires in the AC and DC disconnects. Electricians who are not familiar with PV are accustomed to the line side or source wires coming from the distribution panel. However, in PV installations, the array is the line side. The following can be a problem for code inspectors as well, as unfamiliarity with PV installations can result in perceptions of improperly installed equipment. The installation may be properly installed, but the inspector thinks that it is done incorrectly.
None of these problems is easy to track down or to solve after an installation is complete. It is almost always quicker to take your time and install a PV system correctly the first time. The first step in avoiding these and other mistakes is to understand the assembly and operation of a PV system. Getting hands-on experience is certainly the best way to learn; there are really no substitutions. However, taking classes to learn and understand the fundamentals of photovoltaics, the components that make up a PV system, and the purpose of each of those components will help an installer excel in their trade.
This is an example of the wire being very nicely secured away from the elements, though zip ties were used as the method of securing. Zip ties are unlikely to remain in place for the operating life of the PV array
About the Author
John Conde is a NABCEP-certified installer and instructor of solar photovoltaic classes for Everblue Training Institute.
All photos are the property of John Conde.