Main Lug Only
Convertible
Figure 19. Main Configurations
Main Breaker Loadcenter
The incoming supply cables of a main breaker loadcenter are connected to the main breaker, which in turn feeds power to the loadcenter and its branch circuits.
Power from the main is fed through the main breaker to the bus bars, which are an extension of the utility power cables.
With the main circuit breaker in the “off” position, no power will flow to the bus bars for the branch circuit breakers. This serves as a quick and simple way to disconnect power to the entire building.
Figure 20. Simplified Main Breaker Type Loadcenter
With the main breaker in the “on” position, power is allowed to flow from the bus bars to the branch circuits.
Branch circuits run throughout the building to supply power for lighting and appliances. Because of these features, the main breaker is normally used as a service entrance panel.
Main Lug Only Loadcenter
The Main Lug Only (MLO) Loadcenter does not contain a main circuit breaker to protect the loadcenter itself. Instead, it is protected by a breaker back in an upstream panel. The supply cables from the upstream panel are connected to the main lugs and bus bars.
This type of loadcenter is used primarily used as a distribution panel.
The MLO is often called an “add-on,” “secondary,” or “downstream” panel. It is often used when the main breaker loadcenter’s circuit slots are full, or to provide power at a remote point. Also, main lug loadcenters are used in the southeastern and western parts of the country because, in these areas, local codes mandate that the main circuit breaker be located outside the home so that emergency personnel can turn off the power.
Convertible Loadcenter
The Convertible Loadcenter is supplied without lugs or breakers. Once the customer decides to use main lugs or main breakers, the appropriate kit is installed by an electrician.
In this home, a main breaker loadcenter supplies power to a main lug only loadcenter in the home’s workshop.
Using a Main Lug Only Loadcenter
Main lug only loadcenters can also be fed from metering equipment when used in apartment installations.
