Letter M

Main Breaker
The circuit breaker in or upstream from a load center,
panelboard, switchboard, switchgear or other equipment that
supplies the full current for that equipment.


Main Lug
Only A designation given to indicate that a main breaker or main
switch is not included.


MCM
Abbreviation for a thousand circular mils. Circular mills are used
to designate the cross-sectional area of a round conductor.
One mill is equal to 1/1000 of an inch. The circular mill area of a
solid, round conductor is calculated by squaring the conductor’s
diameter (in mills). 1 MCM is 1000 circular mils (also shown as
1 kcmil).


Metric Unit Prefix
A prefix added to a unit of measure to increase or decrease the
size of that unit of measure. For example, the metric unit prefix
kilo can be added to meter to form a unit of length (kilometer)
equal to 1000 meters. Metric unit prefixes are associated with
powers of ten.