In electrical wire sized 350mcm, what does the mcm stand for. kgb answers » Local & Lookup » Definitions & Word Origins » In electrical wire sized 350mcm, what does the mcm stand for. MCM stands for Thousand Circular Mils. For very big wire sizes (fatter than 4/0), the wire gauge system is usually abandoned for cross-sectional area measurement in thousands of circular mils (MCM), borrowing the old Roman numeral M to denote a multiple of thousand in front of CM for circular mils..