Switchboards

Summary

Switchboards are freestanding sections of vertical and horizontal bus with a main device and feeder breakers or fused switches, typically used as service equipment or main distribution in commercial buildings, while heavier switchgear serves critical and large facilities. They set the size of the main electrical room, so lineup length, front and rear access, working clearances, egress doors, and delivery paths must be planned early.

Cols

Image

Dead-front: no exposed live parts on the front
Switchboard / switchgear: UL 891 switchboards versus UL 1558 metal-enclosed low-voltage switchgear with draw-out breakers
Main device: main breaker or fused switch; or up to multiple disconnects in older service arrangements
Bus: copper or aluminum bars that carry current through the sections
Front-accessible / rear-accessible: whether connections can be reached from the front only
Group-mounted / individually mounted: feeder devices in shared or separate compartments
Short-circuit current rating (SCCR): maximum fault current the assembly can withstand
Ground-fault protection of equipment: required on large solidly grounded services
Metering section / utility CT compartment

Standards

Ratings

Voltage typically 208Y/120 V or 480Y/277 V; switchboards rated up to 1,000 V
Bus ratings commonly from several hundred to several thousand amperes
Short-circuit rating must equal or exceed the available fault current
Large equipment rooms need entrances at each end of the working space

Test standards

UL 891: dead-front switchboards
UL 1558: metal-enclosed low-voltage power circuit breaker switchgear
NFPA 70 (NEC): switchboards, working space, and arc-flash labeling
NFPA 70E: electrical safety in the workplace (owner operations)

Components

Main and distribution sections: bus, main device, and feeder devices
Breakers or fused switches: molded-case, insulated-case, or power circuit breakers
Metering: utility metering and power monitoring
Surge protective device: integral or adjacent
Enclosure: steel sections on a housekeeping pad

Detailing

Size rooms for the lineup, required working space, and future sections
Provide two exits from large equipment working spaces, with doors swinging out and panic hardware where required
Keep piping and ducts out of the dedicated space above equipment
Set equipment on a housekeeping pad; coordinate conduit stub-ups and pull boxes
Plan delivery routes for shipping splits through doors and corridors

Uses

Specific uses

Commercial service entrance and main distribution
Hospitals and data centers (switchgear)
Industrial plants
Multifamily buildings with meter centers
Campus and mixed-use main electrical rooms

Pros and cons

Pros

Distributes large services in one lineup
Expandable with added sections
Monitoring and metering integration

Cons

Large footprint and clearance requirements
Long lead times for custom lineups
Arc-flash hazards require labeling and safe work practices

Manufacturing and cost

Construction

Factory-built to the one-line diagram, shipped in sections, set on pads, bolted together, terminated, tested, and energized

Main manufacturers and suppliers

Trade associations: NFPA and NECA.

Sources

Payapress: UL 891 vs UL 1558 vs UL 67, panelboard vs switchboard vs switchgear, https://payapress.com/panelboard-vs-switchboard-vs-switchgear/
ComplianceGate: UL 891 switchboards standard overview, https://www.compliancegate.com/ul-891/
University of Houston: 26 24 13 low-voltage switchboards specification, https://www.uh.edu/facilities-planning-construction/vendor-resources/owners-design-criteria/master-specs/pdf/26-24-13-low-voltage-switchboards.pdf
EC&M: NEC requirements for working clearances, https://www.ecmweb.com/national-electrical-code/code-basics/article/21259374/nec-requirements-for-working-clearances
Wikimedia Commons: STCP - Electrical Switchboard.jpg, by wdwd (CC BY-SA 4.0), https://commons.wikimedia.org/wiki/File:STCP_-_Electrical_Switchboard.jpg