MC4 connector

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MC4 connectors are weatherproof plug-and-socket parts used to connect solar panels in series. “MC” comes from Multi-Contact (now Stäubli Electrical Connectors) and “4” refers to the 4 mm contact. They snap together by hand for easy panel strings, but require a certified tool to disconnect to prevent accidental loosening. For safety, codes require using the same brand and type of connector to avoid cross-mating, per NEC UL6703 and IEC 62548.

History and ratings in brief:
- Introduced in 2004 as a successor to MC3 (1996). MC3 (3 mm) was rated 1000 V IEC / 600 V UL and 30 A with 10 AWG PV cable; MC3 was discontinued in 2016.
- MC4 became the market standard by the early 2010s. Other brands faded or faced compatibility issues; as of 2022, Stäubli holds about half the market share. MC4-Evo2 is a backward-compatible upgrade with higher ratings.

How they work and safety:
- The plug and socket hold metal contacts inside plastic insulators. The socket has two locking tabs; when pushed together, they lock with a click.
- The cable must match the connector’s size. Cables are usually double-insulated and UV/temperature resistant; most MC4s are crimped to the cable.
- UL rated up to 1500 V DC and 95 A with 6 AWG PV cable; MC4-Evo2 is rated up to 1500 V DC and up to 70 A with 10 mm² cable (depending on conductor size).
- Never connect or disconnect under load, as arcing can damage contacts. In large arrays, strings run from about 17–50 V per panel to up to 600 V per string or 1500 V in very large systems.
- For warranties and safety, use MC4 connectors from the same maker; mixing brands or types can cause issues.