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MC4 connectors are single-contact connectors commonly used for connecting solar panels. MC4 stands for the manufacturer Multi-Contact USA and a 4 mm2 contact assembly pin. The connector was introduced by Multi-Contact in 2008 to replace the MC3 in response to the National Electrical Code 2008 wiring connector that required that the plugs lock together. The MC4 connector is UL rated at 20A and 600V maximum, depending on the conductor size used. Standards efforts in Europe also allow 1000V versions.
While small solar panels used for battery charging for example, may not require special connectors, larger terrestrial arrays for power generation involve higher currents and voltages, and place special demands on both cables and connectors for safe operation. The MC4 connector incorporates a flexible watertight seal and is supplied as 'male' and 'female' type to minimise the chance of wrong connections.
For a proper seal, MC4s require the usage of a cable with the correct diameter. Normally double-insulated (insulation plus black sheath) and UV resistant (most cables deteriorate if used outdoors without protection from sunlight). Connection is made by use of a special crimping tool, alternatively by soldering.
Application and safety
MC Multilam Technology claims that constant spring pressure provides reliable low resistance contacts. 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 17 to 34V each, with overall voltages up to 600V per string.