MC4 connector

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 compatible 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 National Electric Code (NEC) and the UL6703 standard for PV connectors specify that connectors have to be from the same type and brand to avoid the dangers of cross-mating. In addition, IEC 62548 ‘design requirements for PV Systems' require PV connectors to be of the same origin. Originally rated for 600V, newer versions of the MC4 connector are rated at 1500V, which allows longer series strings to be created.

Background
MC4 connectors are successors of the MC3 Connectors, which were developed by Multi-Contact in 1996. MC3 is the abbreviation of Multi-Contact and its size 3mm PV connector with 3 mm contact pin. MC3 had certification ratings of (1000VIEC/600V UL) and 30A (10AWG PV Cable). The MC3 connectors were discontinued in 2016. MC4 name stood for Multi-Contact and its size 4mm PV connectors, introduced by Multi-Contact in 2004.

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 PV 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 NEC banned the opening of PV connectors under load. PV connectors were required to contain positive locking mechanisms controlled by certified, product specific tools. Radox, a European manufacturer, did not respond to this specification and has since disappeared from the market. A US-based and a Swiss company, Tyco Electronics and Multi-Contact (purchased by Staubli in 2002), responded by introducing new connectors to meet this requirement. The MC3 PV connector lacked these requirements. Later, the MC4 PV connector, launched in 2004, provided these positive locking mechanisms, leading to widespread market acceptance.

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 two sets of cables and wires, which led to considerable annoyance in the field when equipment from different vendors could not be plugged together. By 2011, the MC4 was already in a strong leadership position, which led to the introduction of products from a variety of other connector vendors. As of 2022, Staubli Electrical Connectors possesses 50% of its global market shares.

Description
The MC4 system consists of a plug and socket design. The metal contacts of the plugs and sockets are inside plastic insulators; the plug's metal contact is inside a cylindrical insulator that looks like a socket, and the socket metal contact is inside a square probe that appears as a plug. The socket has two plastic locking tabs that have to be pressed toward the central probe slightly to insert into holes in the front of the plug connector. When the two are pushed together, the fingers slide down the holes until they reach a notch in the side of the plug connector, where they pop outward to lock the two together.

For a proper seal, MC4s must be used with cable of the correct diameter. The cable is normally double-insulated (insulation plus black sheath) and both UV and higher temperature resistant (most cables deteriorate if used outdoors without protection from sunlight). Connectors are typically attached by crimping, though soldering is also possible.

The MC4 connector is UL rated at 1500V DC and up to 95A with the 6AWG PV Cable. MC4-Evo 2 has both UL and IEC certification ratings of 1500V DC and up to 70A with 10mm² PV Cable, depending on the conductor size used in pairs.

Application and safety
Staubli's (formerly Multi-Contact) MULTILAM Technology claims that constant spring pressure provides reliable low contact resistance.[citation needed] However, it is very important to never connect or disconnect them under load, even on low-voltage (12–48V) systems. An electric arc may form which can melt and seriously damage contact materials, resulting in high resistance and subsequent overheating. This is partly because direct current (DC) continues to arc, whereas commonly used alternating current (AC) more readily self-extinguishes at the zero-crossing voltage point.

Large arrays of panels are commonly interconnected in series, made of strings of panels generating 17V to 50V each, with overall voltages up to 600V per string or 1500V in special large arrays with high voltage panels.

The installation manuals on MC4 connectors instruct to avoid using non-Staubli connectors with MC4 connectors. Product certifications and the manufacturer's warranty only apply if MC4 connectors are mated to connectors from the same type made by Staubli Electrical Connectors and the instructions in the manual were followed.