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Panelboards: fundamentals of electrical panelboards

What is an electrical panelboard?

Panelboards are used to safely distribute electricity throughout commercial and industrial facilities. A panelboard is a component of an electrical distribution system which divides an electrical power feed into branch circuits, while providing a protective circuit breaker or fuse for each circuit, in a common enclosure. A panelboard services to protect branch circuits from overloads and short circuits. Panelboards are designed to meet UL 67 and NEMA Standard PB1.  


What is the difference between a switchboard and a panelboard?

Switchboards and panelboards provide a similar functionality in a power distribution system. Panelboards are typically flush mounted or surface mounted and are limited to a maximum of 1,200 A incoming current (main).  Switchboards are free-standing units that are front connected and, like panelboards, require only front access. 

However, switchboards can allow for both front and rear access if desired. Switchboards can contain bussing and overcurrent devices up to 5,000 A. Where panelboards are designed to UL 67, switchboards are governed by UL 891.

What is a load center?

load center (i.e., breaker box, fuse box) takes electricity from the utility source and distributes it throughout a facility to support reliable electrical distribution.

The distribution of power to the various circuits is protected from over-current by the use of circuit breakers or fuses.

Once you open the door to the panel you can access all the circuit breakers or fuses. Usually one of these panels distributed power to all of the circuits within a home or facility, but there may be a situation where another "sub-panel" is utilized to support a dedicated region within a building.  

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

load center provides similar functionality in a power distribution system as a switchboard and 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-in breakers, whereas a load center is limited to plug-in breakers. 

Load centers are also limited to 240 V or less and typically are provided at 100 A or less, with a maximum current rating of up to 400 A. Panelboards can be applied at up to 600V and with a maximum current rating of 1,200 A. Load centers are typically offered with no or minimal options where panelboards can be provided with various optional features including main and branch circuit energy monitoring, and integrated surge suppression devices.  

 What's the difference between lighting and appliance panelboards compared to power panelboards?

Prior to 2008, the National Electric Code (NEC) distinguished between ‘lighting and appliance’ panelboards and ‘power’ panelboards. In 2008 the NEC distinctions and terms were removed and a limitation of 42 circuits (poles) per panel was also relaxed. Although these terms no longer exist in the NEC it is still common to see references throughout the industry.
Traditionally, the lighting and appliance panelboard is viewed as the power distribution component that is closest to the load and contains mostly single-phase overcurrent devices feeding lights and receptacles, hence the name. The power panel is more of an intermediary component of the power distribution system and contains mostly three-phase loads, whether connected directly to a large 3 phase piece of equipment or as a three-phase feeder to a downstream panelboard.

 General purpose panelboards

Eaton offers general purpose panelboards for both circuit breaker (Pow-R-Line 1X, 2X, 3X) and fusible switch (Pow-R-Line 1XF, 2XF) applications.

The choice of specific panelboard is dependent on:

  1. Voltage rating
  2. Current (ampere) rating of the main device or of the incoming main on a ‘main lugs only’ application
  3. Maximum current rating of the feeder devices required
  4. Required short circuit rating

 Types of special purpose panelboards:

Branch circuit monitoring panelboards provide integrated energy monitoring for both the main incoming power to the panel as well as some, or all, of the branch circuits. Branch Circuit Monitoring panelboards are often used in commercial buildings to monitor common loads and individual tenant loads for tenant billing or to meet emerging energy code requirements. Branch circuit monitoring is also often commonly found in Datacenter and Industrial applications. 
Column width panelboards are typically found in industrial or warehouse locations and are designed specifically to fit within standard I-beam support columns. Eaton’s Pow-R-Line 1X-LX and Pow-R-Line 2X-LX column width panelboards measure a compact 8.63” W x 6.0” D. Standard panel heights range between 69” and 90”.
Fusible lighting panelboards are designed for industrial, commercial or institutional applications with very high fault currents and where a need to meet NEC Selective Coordination requirements exist. Eaton’s Pow-R-Line 3FQS panel uses the Bussmann Series Cube fuses to easily selectively coordinate with a variety of upstream circuit breakers or fuses. 
Lighting control panelboards are specialized panelboards that include both circuit breaker overcurrent protection and the ability to control lighting and receptacle loads per circuit, all in one compact panelboard. The panelboard has a digital controller that can be programmed with desired schedules and/or trigger inputs, or can be pass thru control signals generated by other control systems, like an HVAC Building Automation System or SCADA system. Similar to the branch circuit monitoring panelboard, you do not have to control all circuits, you can pick and choose which circuits you want to control, to keep cost down. This type of panelboard is commonly used for complying with local energy codes or controlling lighting at remote outbuildings and sports fields.
Elevator control panelboards are designed to integrate electrical power distribution, elevator control and fire alarm system components in a common, standard enclosure. Eaton’s elevator control panelboards can provide integrated power distribution and control for up to four elevators in a common shaft.  

 Hazardous rated panelboards

Hazardous rated panelboards are for use in areas where flammable gases and vapors or combustible dusts may be present. Selecting the appropriate hazardous area panelboard is dependent on:

  1. Hazardous area classification
  2. Voltage Rating
  3. Short Circuit Rating
  4. Factory vs. Non-Factory sealed type


Hazardous rated panelboards are available with the following NEC hazardous area certifications:

  • Class I, Division 1
  • Class I, Division 2
  • Class II, Division 
  • Class II, Division 2
  • Class III
A description of hazardous area classifications can be found in our 2017 Code Digest.

 What are the elements of panelboard enclosures?

Panelboards are constructed, and typically shipped, as three separate primary components.  

The five-sided enclosure itself, commonly called the ‘can’ or ‘box’ is typically made of galvanized steel and is either flush mounted or wall mounted.  

The ‘chassis’ or ‘interior’ is a rigid subassembly consisting of the main and branch overcurrent devices and busbars mounted on a support rails. The chassis is screw-mounted to the box.
The ‘fronts’ or ‘trims’ are available in both surface-mounted and flush-mounted design. Fronts for smaller lighting and distribution panelboards are often one piece and include a ‘door-in-door’ design with a flush latch and lock assembly. Larger panelboards may use a three-piece design. Both the door-in-door and three-piece design provide a ‘dead-front’ to prevent exposure to live components.    

Eaton’s EZ-trim incorporates a unique, one-piece design that allows the trim to be installed in seconds with no additional hardware or tools.   

To match the construction process, the panelboard cans are typically shipped first, followed by the interiors and the trims. 

In addition to these primarily components, panelboards also include a label that provides information regarding the unit’s electrical ratings and manufacturing date, filler plates used to cover any unused spaces, and a circuit directory card located on the inside of the door. Get your blank panelboard circuit directory template.


What is the difference between a main lug only vs. main circuit breaker?

Panelboards can be provided with a main circuit breaker or with main lugs only (MLO). In a panel with a main circuit breaker 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 fed devices as well as a single disconnect means for all loads being fed by the panel. Main lug only panels are typically applied downstream of a main circuit breaker panel and are often referred to as secondary or add-on panels. For MLO panels, the incoming cables are terminated on line side lugs that are attached directly to the bus, no main overcurrent device exists in the panel.

 Learn more about branch devices:

The branch devices in a panelboard provide overcurrent protection for downstream devices. Most often both functions are provided by a molded case circuit breaker. However, fusible switches can also be provided.  

Eaton’s panelboards are built around Eaton’s Power Defense circuit breaker technology and Bussmann Series fuse technology.  

 Learn more about meters:

Panelboards can be supplied with electronic metering devices to monitor both incoming power to the panel as well as the power distributed through individual branch devices.

These meters can communicate, via common industry protocols, as part of an overall building energy management system. They can aid in complying with local energy codes or be part of your overall IoT/Industry 4.0 strategy.


 What are surge protection devices?

Voltage spikes and surges can find their way into your facility from an outside disturbance or can manifest itself from equipment within your facility.  Surge protection devices (SPDs) protect sensitive downstream electronic devices from the damaging effects of voltage surges or spikes.

Panelboard mounted SPDs can either be side-mounted or integrated design. Side-mounted units come pre-wired in a separate enclosure and are field-mounted and wired to the panel in the field.   

Integrated Surge Protection Devices, like Eaton’s SPD Series, are factory mounted directly to the panelboard chassis. In addition to time and space savings, an integrated SPD increases system protection by reducing let-through voltage due to the zero-lead length design.  

Learn about the many advantages of integrating surge protection inside power distribution equipment.

 Short-circuit rating and series ratings definitions:

Panelboards, as well as other power distribution assemblies like switchboards and switchgear, carry a short-circuit current rating that defines the amount of fault current that the equipment is designed to withstand. There are three protection systems used to protect low voltage power distribution systems.  They are:

Fully rated protection — where all overcurrent devices are rated for the full prospective short-circuit current at their line side terminals throughout the system.  In a fully rated system, the short-circuit current rating of the panelboard is determined by the short-circuit current rating of the lowest rated overcurrent device in the panelboard 

Selectively coordinated protection — A fully rated system where the overcurrent device closest to the fault will open first, thus isolating the faulty circuit.  

Series rated protection — A short-circuit interrupting rating assigned to a combination of two or more overcurrent protective devices that are connected in series and in which the rating of the downstream device(s) in the combination is less than the series rating. Series ratings are also known in the industry as integrated ratings, series combination ratings and series connected ratings.  

In a series rated system, the overcurrent devices in series in the protective scheme must have been tested and listed by Underwriters Laboratories (UL), for series combination use in the system.

Eaton’s currently published series ratings can be found in the current version of the panelboard and switchboard series rating information manual.

 Interested in learning more?

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