Is THHN wire compatible with SolarAPP+?

SolarAPP specifies AC and DC conductor types within raceway and requires that such wire be 90°C, wet rated, and suitable for the application per the National Electrical Code, such as THWN-2.  Contractors and Inspectors should pay close attention to the rating of installed conductors to ensure compatibility with the SolarAPP+ question and the code:

Are DC and AC conductors copper, Class B or Class C, and THWN-2, NM-B, USE-2, PV Wire, or jacketed multi conductor cable assembly listed and identified for the application? [NEC 690.8(B); 690.31 (A) & (C), 310.15(A) and (B)] NEC Section 334.112 Informational Note: Types NM, NMC, and NMS cable identified by the markings NM-B, NMC-B, and NMS-B meet this requirement.

The most popular type of building wire used in construction is Thermoplastic High Heat-resistant Nylon coated (THHN) wire. THHN wire is also commonly used in machine tools, control circuits and some appliances.

THHN wire is approved for up to 600V and is listed by Underwriters Laboratory (UL), as rated for 90° C (194° F) in dry and 75° C (167° F) in wet environments. 

THHN wire comes with a dual rating: THHN / THWN, which means it’s appropriate for both dry and wet applications per the table below (an example product label is also in the picture below). In short, the wire used, must carry a dual rating of THHN/THWN in order to be 90° C (194° F) rated in wet environments. 


T = ThermoplasticHH = High Heat ResistanceN = Nylon Coated

Temp Rating: 90° C in dry locations

T = ThermoplasticH = Heat andW = Water Resistance

N = Nylon Coated

Temp Rating: 90° C in dry locations and 75° C in wet locations

T = ThermoplasticH = Heat andW = Water Resistance

N = Nylon Coated

Temp Rating: 90° C in both dry and wet locations

THHN wire can have the following characteristics depending on the application:

  • Conductors: Depending on the size can be either stranded or solid
  • Wire material: Copper or aluminum
  • Jacket: Polyvinylchloride (PVC) insulation with a nylon jacket

Aside from the mechanical protection, a nylon jacket protects the wire from abrasion as it’s pulled through conduit. In addition, the jacket protects the wire from hydrocarbons such as oil, gasoline and grease. The nylon jacket also protects the wire from being puncture or accidentally cut as it protects the insulation.

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