For those of who don't know what SPT-1 wire is - it is also known as lamp wire or zip cord, and is commonly used by decorators to build custom lengths of extension cords. It allows for a clean display installation by elimination excess extension cords that can be unsightly. So what are the differences? For some reason this has become a somewhat confusing topic, but really, it's quite simple: They are exactly the same with the exception of the thickness of the insulation covering the wire. Both SPT-1 wire and SPT-2 wire are made using 18 gauge copper wire at their core. The difference is that SPT2 has a thicker insulation around the wire. Thereby making it slightly larger around than SPT1 and giving it the ability to carry a slightly higher electrical load.
Some folks will say that an SPT1 18awg wire is rated to carry a 7amp load and an SPT2 18awg wire is rated to carry a 10am load. While this is true under certain conditions, there are other variables that also affect the safe use of any wire type - including the length of the run (voltage drop over distance), the thickness of the insulation, the quality of the copper wire, etc. We always recommend that you seek the services of a licensed electrician if you have any question at all about the safety of your installation.
Visit The Christmas Light Emporium at the links you see here to learn more about all of the installation accessories we offer including a variety of colors and lengths of SPT wire and other professional installation accessories: SPT-1 and SPT-2 Wire Installation Accessories Happy Decorating!