MLO, main lugs only, describes a panelboard whose incoming feeder lands directly on lugs rather than on a main breaker, so its overcurrent protection sits in another panel or switchboard. MLO panels are common as subpanels and in multi-section boards. Their bus rating must be at least the size of the upstream device.
Sub on the second floor is MLO, fed from a hundred-amp breaker in the main, so there's no main to shut off up here.
Also heard as
- main lug only
- main lug panel
- lug-only
Related terms
Tub
Empty panel enclosure, the steel back box that goes in at rough-in before the interior and cover arrive.
Guts
Interior of a panel: the bus bars, breaker mounting and neutral assembly that bolt into the enclosure.
Home run
Part of a circuit running from the first box back to the panel, or the arrow on a drawing that shows it.
Bond
Connection that ties metal parts together so they stay at one potential and give fault current a path home.