MC4 connectors are single-contact electrical connectors commonly used for connecting solar panels. The MC in MC4 stands for the manufacturer Multi-Contact (now Stäubli Electrical Connectors) and the 4 for the 4 mm diameter contact pin. MC4s allow strings of panels to be easily constructed by pushing the connectors from adjacent panels together by hand, but require a tool to disconnect them to ensure they do not accidentally disconnect when the cables are pulled. The MC4 and compatible products are universal in the solar market today, equipping almost all solar panels produced since about 2011. Originally rated for 600 V, newer versions are rated at 1500 V, which allows longer strings to be created. Background While small solar panels used for battery charging and similar tasks may not require special connectors, larger systems normally connect the panels together in series to form strings. In the past this was accomplished by opening a small electrical box on the back of the panel and connecting user-supplied wires to screw terminals within. However, in the USA, bare terminals of this sort are limited to 50V or less by the National Electrical Code (NEC). Above 50V only a licensed electrician can make the connections . Additionally, these sorts of connections were subject to problems caused by water leakage, electrical corrosion and mechanical stress on the wires. Starting in the 2000s, a number of companies introduced products to address these issues. In these systems, the junction box was sealed and two wires were permanently attached using strain reliefs. The cables ended with push-fit connectors that met the definition of a convenience receptacle, meaning they could be (legally) connected together by anyone. Two connectors became somewhat common during this period, the Radox connector and MC3 connector, both of which essentially looked like weather-sealed phono jacks. In 2008 the US Electrical Code was updated to require solar panel connectors to offer "positive locking", so that they were able to be plugged together by hand but only separated again using a tool. Radox, a European manufacturer, did not respond to this specification and has since disappeared from the market. Two US-based companies, Tyco Electronics and Multi-Contact, responded by introducing new connectors to meet this requirement. Tyco's Solarlok became a market leader for a period in the late 2000s, but a number of factors conspired to push it from the market. Among these was the fact that the system had