Load centers: Fundamentals of electrical load centers

Load center fundamentals:
Know your load center

Every building and home utilizes electricity for lighting, receptacles, and appliance loads.  A load center is used in residential and light commercial applications to distribute electricity supplied by the utility company throughout the home or building to feed all the branch circuits.

Each branch circuit is protected by the circuit breaker housed in the load center.  In the event of a short circuit or an overload on a branch circuit, the circuit breaker will cut the power before any potential property damage or personal injury can occur.

Load centers are designed, manufactured, and tested in accordance with the latest applicable standards including:

  • Underwriters Laboratories (UL)
  • Canadian Standards Association (CSA)
  • National Electrical Manufacturers Association (NEMA)
  • National Electrical Code (NEC)

What is the difference between a load center and a panelboard?

A load center provides similar functionality in a power distribution system as compared to a panelboard. As far as UL and the NEC standards are concerned, there is no difference between a panelboard and a load center. In North America, the electrical industry refers to smaller, lower cost panelboards sold primarily in residential applications as load centers. Panelboards are typically deeper than load centers and can accommodate both bolt-on circuit breakers as well as plug-on breakers, where a load center only uses plug-on breakers. 

The history and purpose of your load center

A load center helps to provide safety to the homeowner, and maintenance personnel by housing all the branch circuits in one enclosure, helping to prevent coming in contact with energized electrical parts.

Load centers use plug-on circuit breakers to reliably distribute the electricity to circuits throughout a home or small building.  Plug-on, refers to how the circuit breaker connects to the bus bar of the load center.  The load center can provide safety from ground and arc faults by using specialty, or electronic circuit breakers.

Load centers have been called different names over the years.  For example, a fuse box, breaker box, panelboard or a distribution panel.  Historically, homes used fuse panels to distribute power, but today load centers with enclosed circuit breakers are the industry standard.

Where are load centers used?

Load centers have a wide range of applications from single family homes to original equipment (OEM) products.  They range from 40 – 600 amps, and 2- 120 circuits.

Residential single family
Typically have one or more load centers to distribute power inside the home.  Commonly located in the garage, basement or outdoors depending on the geography.

Residential multi-family
Apartments, condominiums and townhomes are most common for the use of a load center to distribute power.  Commonly installed within group metering products.

Light commercial 
Load centers provide power distribution in light commercial buildings such as strip malls, office buildings, and warehouses.

Load centers can also be used to upgrade and replace older distribution equipment.

Original equipment (OEM)
Machinery OEMs use load centers as a means to distribute power in their electrical systems.

There are multiple ways to install a load center


Service entrance or main breaker panel

  • The point at which the power enters the home or building
  • There can only be one service entrance panel per home or building
  • Neutral and ground bars can be tied together

Sub panel or main lug only

  • Used downstream from the service entrance panel
  • Ideal when adding additional electrical services to an existing home or building
  • Typically powered from a branch circuit housed in the service entrance panel
  • Isolated and insulated neutral bar, and dedicated ground bar is required

Flush mounted

  • Typically recessed between studs in the wall to save space

Surface mounted

  • Mounted to the surface of the wall
  • Typically applied in buildings, basements or garages

What is the difference between a main lug only and main circuit breaker load center?

Load centers can be equipped with a main breaker or main lug only (MLO) design.  In a load center with a main breaker design, the incoming supply cables are connected directly to the main circuit breaker.  The main circuit breaker provides a level of overcurrent protection for all branch circuits, as well as a single disconnect means for all loads being fed by the load center.  
Main lug only load centers are typically applied downstream of a main circuit breaker panel and are often referred to as a sub panel. For main lug only load centers, the incoming cables are terminated on the line side of the lugs that are attached directly to the bus, no main overcurrent device exists within the panel.

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