What is 440MCM wire?

'MCM' stands for 'thousand circular mils'. A 'circular mil' is used in North America to represent the area of a circular-section conductor. It is defined as the conductor's diameter, expressed in 'mils' (one-thousandth of an inch), squared.

Of course, squaring a diameter of a circle doesn't actually result in a true area, so the circular mil (CM) is simply a figure that 'represents' an area.

Somewhat confusingly, in this context, 'M', means one-thousand (from the Latin, 'mille'), not one-million (M = 'mega') as it would in the metric system.

For measuring rectangular-section conductors, the 'square mil' is used -this is the product of the two sides of the rectangular, each expressed in mils.

This system of measurement is unique to North America; most other countries use the square millimetre to represent conductor sizes.