Solar Wire Types for Solar PV Installations

Wire types vary in conductor material and insulation. This is an overview article for wires and conductors that are commonly used in solar pv installations.

Aluminum or Copper: The two common conductor materials used in residential and commercial solar installations are copper and aluminum. Copper has a greater conductivity than aluminum, thus it carries more current than aluminum at the same size.

Aluminum may be weakened during installation especially during bending, however it is less expensive than copper wires. It is not used (not permitted) for interior home wiring, as they are used in larger gauges for underground or overhead service entrances and for commercial operations.

Solid or Stranded: The cable could be solid or stranded, where stranded wires consist of many small wires that allow wire to be flexible. This type is recommended for larger sizes. The current tends to flow on the outside of the wire, thus stranded wires have slightly better conductivity as there is more wire surface.

Insulation: The insulation covering wire can protect the cable from heat, moisture, ultraviolet light or chemicals. 

• THHN is commonly used in dry, indoor locations.
• THW, THWN and TW can be used indoors or for wet outdoor applications in conduit.
• UF and USE are good for moist or underground applications.
• PV Wire, USE-2 and RHW-2 cables can be used in outdoor and wet conditions where their outer cabling is UV and moisture resistant. They must be sunlight resistant.

Color: Electrical wire insulation is color coded to designate its function and use. For troubleshooting and repair, understanding the coding is essential. The wiring label differs according to AC or DC current. 

Here is a simple table for color coding.

For your convenience I have added the table from National Electrical Code that shows conductor applications and insulations. Please refer to NEC Article 310, page 15 - Table 310.13 (.PDF document) for the expanded version of the table.

*Mentioned as a page note: "Wire types designated with the suffix "2," such as RHW-2, shall be permitted to be used at a continuous 90°C (194°F) operating temperature, wet or dry."

**NEC finds the use of PV Wire adequate and necessary in ungrounded systems.