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Differences between USE-2 wire and PV wire

2022-09-29

USE-2 wire has long been the interconnection cable of choice for solar modules, while PV wire was first mentioned in the 2008 edition of the National Electrical Code. Although both cables share similar structural and performance requirements, they remain distinct due to their unique installation conditions. Below is a comparison of their applications, construction, and testing requirements.

USE-2 wire has long been the interconnect cable of choice for solar modules, while PV wire was first mentioned in the 2008 edition of the National Electrical Code. Although both cables share similar structural and performance requirements, they remain distinct due to their unique installation conditions. Below is a comparison of their applications, construction, and testing requirements.

In terms of applications: The USE-2 cable, designated as an underground service entrance wire, is typically used for connecting power supply equipment—but it’s intended solely for installation in damp or dry environments where temperatures do not exceed 90°C. In contrast, PV cables are rated for higher temperatures, with a maximum of 90°C in humid conditions and up to 150°C in dry environments, making them specifically designed for connecting solar modules. Regarding voltage ratings, the USE-2 cable is approved for 600V systems, while PV cables can accommodate voltages of 600V, 1000V, and even 2000V, allowing them to seamlessly integrate into solar systems designed for voltages exceeding 600V. According to the National Electrical Code (NEC), USE-2 cables are permitted only in grounded solar photovoltaic arrays; however, PV cables can be safely used in both grounded and ungrounded solar PV systems.

Structurally, USE-2 wires typically feature mechanical protection to prevent physical damage, while PV wires incorporate thicker insulation layers or outer jackets for added safeguarding. USE-2 wires are generally installed in locations where movement is minimal or where they’re unlikely to encounter mechanical stress, allowing for either solid or stranded conductor designs—materials for which copper, copper-clad aluminum, or aluminum are suitable choices. In contrast, PV wires must use stranded copper conductors to ensure flexibility meets the specific requirements of their intended application. The minimum conductor size for USE-2 wires is 14 AWG, whereas PV wires can utilize conductors as small as 18 AWG.

In the insulation and jacket materials section, while USE-2 and PV wires commonly use XLPE or EPCV thermoset insulation and jackets, other thermoset materials are occasionally employed as well, such as CP-coated EP, CP-coated, EPCV, SBR/IIR/NR, and EP. The design of PV wires can follow the UF wire type, but must incorporate a full PVC insulation and jacket capable of withstanding high-temperature, humid environments at 90°C or higher—while also requiring an additional 15 mils of thickness.

Regarding testing requirements: USE-2 wires are typically installed underground or in similar environments, so they do not need to meet combustion-level standards. However, PV wires, being exposed to outdoor conditions, must pass combustion tests. That said, mechanical durability tests such as overload testing, as well as resistance to compression and impact, apply only to USE-2 wires. As for resistance to sunlight exposure and low-temperature flexibility tests, USE-2 wires only require 300 hours of treatment in a weathering chamber and exposure to a temperature of -25°C. In contrast, PV wires, to comply with even stricter specifications, must undergo 720 hours of weathering chamber testing and be subjected to a chilling temperature of -40°C.

Overall, PV cables exhibit superior resistance to sunlight and low-temperature flexibility compared to USE-2 cables, and they feature thicker insulation layers or sheaths along with enhanced flame-retardant ratings. Additionally, since these cables can be used in ungrounded solar photovoltaic arrays and inverters without built-in transformers, they are expected to become the primary interconnection wires for solar modules in the future.

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