The Cable Chronicles: History of Solar Power and Solar Cable Standards

Cable ChroniclesHumans have been harnessing renewable energy sources, like solar power, longer than most people realize. As early as 7th century BC, people were intensifying the rays of the sun with the help of a magnifying glass to build fire. In modern times, solar energy is harnessed on a much larger scale and there is an extensive history behind the technological advances that have brought us to the solar cells we use today. Although standards for the solar wire used in these applications are relatively new, they have already seen quite a progression of their own.

Before we could make it to solar cable and solar cable standards, we needed to work our way up to modern solar technology – the technology we’d need to harness electrical power via the sun’s rays.

In 1839, French scientist Edmond Becquerel discovered the photovoltaic effect: the creation of voltage or electric current in a material upon exposure to light. For the next century or so, scientists tinkered with the concept and photosensitive materials like selenium, but in 1908, William J. Bailey of the Carnegie Steel Company invented a solar collector with copper coils and an insulated box, which was essentially a blueprint for the modern solar cell. By 1950, three engineers at Bell Telephone Laboratories created the first solar cell powerful enough to convert the energy needed to run standard electrical. Since then, scientists have continued to work to improve the conversion rate, equipping us to power special cars, aircrafts and even entire buildings with the use of modern solar arrays.

For a long time, THHN wire was used as solar power cable in these arrays. THHN is “Thermoplastic High Heat-resistant Nylon-coated” general purpose building wire. It is used in a wide range of industries and applications, and its heat resistant properties made it more suitable for solar applications than other types of wire and cable. It was a sensible choice for use in evolving solar technologies. Historically, UF, SE and USE wire were also used in various components and were covered in sections of the National Electrical Code dealing with photovoltaic systems.

As of the 2008 version, the NEC only lists PV Wire and USE-2 Solar Wire (commonly dual-rated as RHH/RHW-2 wire) as acceptable options. They are safer, sturdier and more efficient than their predecessors.

PV Wire

PV Wire

PV wire meets current UL standard, UL 4703. Photovoltaic wire is rated to 90°C in wet and 150°C in dry conditions, and to 600, 1000, or 2000 volts. It is known for its use as underground service entrance cable and can be used in either grounded or ungrounded arrays. It is often used in exposed applications and has thicker insulation and jacketing, better sunlight resistance, flame resistance, and flexibility at low temperatures than USE-2 cables.


USE-2 Wire

USE-2 Wire

USE-2 cable is regulated by the UL 854 standard. According to UL experts, it is popularly chosen for underground applications, although it is also appropriate for exposed use according to Sec. 690.31 (B) of the NEC. It is rated to 90°C in wet and dry conditions, and to 600 volts. USE-2 wire is more crush and impact resistant than PV wire.


Although USE-2 and PV wires have been developed specifically for modern solar power cable applications, it isn’t to say that other wire types have completely outlived their usefulness in the solar industry. Wires like THHN can still be used in certain PV functions where it is safe to do so. However, it should never be used to replace PV or USE-2 wire where they are specified, as it does not include all of the specialized features incorporated into PV wire and USE-2 wires, and could lead to failures.

The history of solar power and solar power cable is rich, despite the relatively short time cabling standards like those for USE-2 wire and photovoltaic wire have had to develop. As we continue to refine solar energy processes and the materials we use in building the arrays, it is almost certain that our standards will also continue to evolve.

To learn more about the current uses of popular solar cables, watch our PV Wire vs. USE-2 Wire video.

About awcwire
Allied Wire & Cable is a value-added manufacturer and distributor of electrical wire and cable, tubing, and more. We are a family owned and operated company, serving a wide range of industries, including the military, automotive, aerospace, and telecommunications markets. Allied is headquartered in Collegeville, PA. Additional locations can be found across the US, in Merrimack, NH, Tampa, FL, Pewaukee, WI, and Las Vegas, NV. For more information on Allied, visit our main website at

One Response to The Cable Chronicles: History of Solar Power and Solar Cable Standards

  1. I really like your informative Cable Chronicles series. .

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