Another specialty junction box, found especially in roof-mounted PV systems, is called a transition or pass-thru box. Some residential grid-direct systems that employ two or even three series strings may not require series fusing. This is determined by specific module and inverter operating characteristics. In these cases, installers often use pass-thru boxes to transition from the more bulky and expensive USE-2 or PV wires (rated for outdoor use without conduit) that come from the modules, to THWN-2 wire (less expensive and less bulky, but needs to be run in conduit). Most of these transition boxes are low-profile, and intended to mount directly on the roof or on an extended end of a mounting rail. They often have wire terminations already built into the box, or you may need to use your own multi-tap connector. Transition boxes provide a great spot for troubleshooting and make the transition from one wire type to another easier. These boxes must be able to endure the potentially harsh conditions found on rooftops, and should stand up to the 25-year design life of a PV system. Generally metal or fiberglass is preferred over plastic, which will degrade under long-term exposure to strong UV rays.