Solar Wire Types for Solar PV Installations

Wire types vary in conductor material and insulation.

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.

Alternating Current (AC)		Direct Current (DC)
Color	Application	Color	Application
Black, Red or Other Color	Un-Grounded Hot	Red	Positive
White	Grounded Conductor	White	Negative or Grounded Conductor
Green or Bare	Equipment Ground	Green or Bare	Equipment Ground

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.

Type Letter	Name	Max. Provisions	Application Provisions	Insulation	Outer covering
THHN .	Heat Resistant Thermoplastic	90 C, 194 F	Dry or Damp Locations	Flame retardant, heatresistant thermoplastic	Nylon jacket or equivalent
THW	Moisture& Heat Resistant Thermoplastic	75-90 C, 167-194 F	Dry or Wet Locations	Flame retardant, moisture and heat resistant thermoplastic	None
THWN	Moisture& Heat Resistant Thermoplastic	75 C, 167 F	Dry or Wet Locations	Flame retardant, moisture and heat resistant thermoplastic	Nylon jacket or equivalent
TW	Moisture resistant thermoplastic	60 C, 140 F	Dry or Wet Locations	Flame retardant, moisture resistant thermoplastic	None
UF and USE	Underground Feeder & Branch Circuit Cable- Single Conductor	60-75 C, 140-167 F	See Article 338 and 339: Service Entrance	Moisture and heat resistant	Integral with insulation and Moisture resistant
USE-2* and RHW-2*	Moisture resistant thermoplastic and Branch Circuit Cable- Single Conductor	90 C, 194 F	Dry or Wet and Service Entrance	Moisture and heat resistant	Moisture resistant with insulation
PV Wire**	Thicker insulation or jacket to provide additional protection against the abuse that USE-2 wire may receive.	90 C (194F) wet, 150 C (302 F) dry	Dry or Wet and Service Entrance	Moisture and heat resistant	Moisture resistant with insulation

*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.

Comments

Great article. Question, on a residential install, do the wires need to be labeled for the inspection?

Reply

The AC cables, metal covered, single-conductor cables, switchboard wires and mineral-insulated wires must be labeled with printed tags.

Reply

How do I know what type of insulation my cable has? Will it always be printed on the insulation itself?

Reply

According to article 310.11; the wires must be marked with "max. rated voltage", "type of the wire", "AWG size" and "an identification of the manufacturer".