What are the differences between PV wire and USE-2 wire?
Whereas USE-2 wire has long been accepted for PV module interconnections, PV wire is newly addressed in the 2008 edition of the NEC. Though their construction and performance requirements are similar, some variations between PV wire and USE-2 wire do exist due to their unique installation conditions. Below is a comparison of these two types of wires, based on their usage, construction and testing requirements:
PV wire, with 90ºC wet rating and up to 150ºC dry rating, is dedicated for interconnecting PV modules. USE-2 wire is designated as underground service entrance cable typically for connecting to the terminals of service equipment. It is limited to installations in maximum 90ºC wet and dry conditions.
Both USE-2 wire and PV wire can be rated 600 V. However, PV wire can also be rated 1000 V and 2000 V to accommodate photovoltaic modules intended for use in systems with a system voltage greater than 600 V. Per the NEC, USE-2 wire is suitable for use in grounded PV arrays only but PV wire can be used within both grounded and ungrounded PV arrays.
PV wire comes with a thicker insulation or jacket to provide additional mechanical protection against the physical abuse that USE-2 wire typically receives. PV wire employs stranded copper conductors to make it flexible enough for the intended application. On the contrary, USE-2 wire can employ either solid or stranded conductors made of copper, copper-clad aluminum or aluminum since it is typically installed in locations not subject to movement or mechanical damages. The minimum conductor size for USE-2 wire is 14 AWG, but PV wire can employ a smaller conductor size down to 18 AWG.
Insulation and jacket materials – both PV wire and USE-2 wire use thermoset insulation and jacket typically made of XLPE or EPCV. Nevertheless, other thermoset materials such as CP over EP, CP over EPCV, SBR/IIR/NR and EP are also available. PV wire may also be constructed similar to Type UF but uses an additional 15 mils of 90ºC wet and dry rated integral PVC insulation and jacket.
Because USE-2 wire is typically installed underground or in similar locations where a flame may not propagate but PV wire can be exposed in an installation, a flame test is required only for PV wire. On the other hand, an overload test and mechanical abuse tests including crushing resistance and impact resistance are applicable only to USE-2 wire.
As for sunlight resistance and flexibility at low temperature tests, PV wire must comply with more stringent requirements. It undergoes a 720-hour weatherometer and -40ºC cold chamber conditioning. The requirements for USE-2 wire are 300-hour weatherometer and -25ºC cold chamber conditioning.
In summary, when compared to USE-2 wire, PV wire has superior sunlight resistance and low-temperature flexibility in addition to a thicker insulation or jacket and a proven level of flame resistance. Given that PV wire can facilitate the use of ungrounded PV arrays as well as transformerless inverters, it is anticipated to grow in popularity for module interconnections.