Electrician's Journal-Load Centers, Panelboards, and Switchboards

Introduction

A 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. See load center below.

A 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, integrated surge suppression devices, mixing large and small frame circuit breakers, and a stacked 3-phase bus (such as I-Line Panelboards by Schneider Electric).

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. See panelboard below.

Anatomy of a Panelboard

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

The five-sided enclosure itself, commonly called the “can”, “box”, or “tub”, is typically made of galvanized steel and is either flush mounted or wall mounted. See below.

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. See below.

The “dead-front”, “front” or “trim” is 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. See below.

“Main Circuit Breaker” vs “Main Lug Only” Panelboards

Panelboards can be provided with a main circuit breaker (MCB) or with main lugs only (MLO). In a panel with a main circuit breaker, the incoming supply cables are connected directly to the line-side lugs. The main circuit breaker provides a level of overcurrent protection for all branch-feed devices as well as a single disconnecting 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.

Panelboards vs Switchboards

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 1,200A incoming current (main). Switchboards are “free-standing units” that are front connected and, like panelboards, require only front access. See panelboard above.

However, switchboards can allow for both front and rear access if desired. Switchboards can contain bussing and overcurrent devices up to 5,000A. Where panelboards are designed to UL67, switchboards are governed by UL891.

Switchboards provide power downstream in the electrical system either directly to large loads or to panelboards that will further divide the power to smaller branch circuits to feed smaller loads. In smaller facilities, the switchboard may be fed directly from the electrical utility, in which case it is referred to as a “service entrance” switchboard. Local electric utility companies typically have specific requirements for service entrance switchboards. In large commercial or industrial facilities, a switchboard will receive power from an upstream circuit breaker mounted in a switchgear assembly. See switchboard below.

Panelboard Accessories

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. See below.

Surge Protection Devices (SPDs): 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.

We hope you have enjoyed reading this post and found it helpful. Please feel welcome to visit Electrician’s Journal often to enjoy other articles and further increase your skill set. I suggest reading each post at least a few times to fully understand the basic concepts and commit them to memory.