Load centers are the gateways for a residential electrical distribution system, housing the breakers that protect connected circuits from the damage electrical faults can cause. Load centers come in two varieties – main breakers and main lugs – with an important difference in capabilities that contractors and distributors should understand.
Main breakers are located at the electrical-service entrance and, along with individual circuit breakers, these units feature a main breaker capable of disconnecting the entire load center, including all connected circuits, at a single switch point. Main lugs, however, don’t include a main breaker switch, only individual circuit breakers.
Because main lugs lack primary overload protection, they’re typically used as distribution panels downstream of a main breaker. As such, manufacturers and contractors alike might also refer to them as add-on, secondary or downstream panels. Applications might include a home workshop or other discreet area of a home where connected loads are large enough to require their own circuits. Additionally, main lug load centers can be fed from metering equipment in multi-unit apartment buildings, so renters could have access to branch circuit breakers within their individual units.
In some cases, it might be possible to use a back-fed circuit breaker and retainer clip to convert a main lug load center to serve as a main breaker. Alternatively, some manufacturers offer load centers designed for easy conversion from one style device to the other. For example, Siemens PL series load centers can be converted from main lug to main breaker, and back again. The company’s ES series load centers also can be converted to main lugs (but main lugs in this series cannot be converted to main breakers).
Photo courtesy of Siemens