Small electrical installations normally have only one distribution board, connected directly to the main service, and appliances are powered with branch circuits protected by breakers. However, powering all loads from the same distribution board is impractical in larger installations, since the amount of wiring required would be excessive. In these cases, secondary load centers are powered from the main distribution board, and in turn these load centers supply electricity for individual appliances.
Feeder circuits connect the main distribution board to secondary load centers. Since these circuit power load centers, they use large conductors and can become very expensive. For this reason, it makes sense to minimize the number of feeder circuits and their length.
In some cases, load centers must be installed close to each other. For example, this can occur in a building area where many appliances require individual branch circuits, and one load center does not provide enough space. When load centers are equipped with sub feed lugs or feed through lugs, a single feeder circuit can power two or more of them.
Sub feed lugs have two connection spaces: one for the incoming feeder circuit, and another to extend the connection for another load center. Sub feed lugs are located on the upper side of a load center, just like main lugs, which means the incoming and the outgoing connection are on the same side.
This configuration is only available for main lug panels, not main breaker panels, which means an external main breaker is required. However, since the external breaker is located upstream from the first panel, any secondary panels connected through sub feed lugs are also protected.
It is important to note that sub feed lugs add height to load centers, but they allow the connection of additional panels to the same feeder circuit. Any potential space lost due to the sub feed lugs is compensated by the space gained with additional load centers.
Feed through lugs accomplish the same function as sub feed lugs, but the connection layout is different:
This configuration is available for both main lug panels and main breaker panels. Any main breaker protecting the first load center also protects the second one, since the connection is downstream.
If the panels are installed side by side and feed through lugs are used, the wiring between them is longer compared with a connection using sub feed lugs. This is because the wiring must travel from the bottom of the first panel to the top of the second, while it must only travel horizontally between the top of both panels with sub feed lugs.
The layout is very similar to that of a load center with feed through lugs, where the connection to the second panel is at the bottom of the first. However, in this case there is a circuit breaker built into the first panel, providing not only a connection point but also fault protection.
This configuration is available for both main lug and main breaker panels, just like feed through lugs. If the first panel has a main breaker, the secondary panel is protected by both the main breaker and the sub feed breaker.
To make sure your electrical installation has an optimal layout, the best recommendation is working with qualified design professionals. In many cases, it is possible to achieve drastic savings with smart design decisions, without compromising performance. Ideally, your installation should consolidate electrical wiring as much as possible - long and redundant feeder circuits represent a considerable waste of capital in a large installation.