Switchboard vs. Switchgear: What Actually Differs

Two products, two standards

The terms get used interchangeably on job sites, but they describe different assemblies. A switchboard is typically listed to UL 891. Low-voltage power switchgear is built to IEEE C37.20.1 and typically listed to UL 1558. The two standards differ in construction requirements, test durations and the breakers they are designed around, and those differences drive nearly everything else in this comparison.

Both serve distribution at 600 V and below, and both can be applied at service entrance, as distribution boards or as the low-voltage side of a unit substation. Which one belongs on the one-line diagram depends on fault duty, coordination requirements, expected maintenance practice and budget.

Breaker types and how they are mounted

Switchboards use molded-case circuit breakers (MCCBs) and insulated-case circuit breakers (ICCBs), both listed to UL 489. Feeder MCCBs are commonly group-mounted on a shared panel bus; mains and large feeders are individually mounted. Devices are usually fixed-mount and bolted to the bus, although drawout ICCBs are available from some product lines as an option.

Low-voltage power switchgear is built around drawout low-voltage power circuit breakers (LVPCBs) listed to UL 1066 and designed to IEEE C37.13. Each breaker sits in its own compartment on a racking mechanism with connected, test and disconnected positions. The breaker can be electrically tested in the test position and removed for service without de-energizing the bus above it.

Continuous ratings: 80 percent versus 100 percent

The NEC requires an overcurrent device supplying continuous load to be rated at not less than the noncontinuous load plus 125 percent of the continuous load, unless the assembly and device are listed for continuous operation at 100 percent of rating. For a standard MCCB this works out to roughly 80 percent of the breaker rating on continuous loads. Switchboard mains and feeders are commonly 80%-rated devices for this reason, and the extra margin has to be carried in the device and conductor sizing.

100%-rated MCCBs and ICCBs exist, but their listing usually carries conditions: a minimum enclosure volume, ventilation requirements and 90 degree C rated conductors sized at 75 degree C ampacity. LVPCBs in switchgear are 100%-rated by design because the compartmentalized, ventilated construction is part of the tested assembly. On a 4,000 A service the difference between 80 and 100 percent is not academic; it decides frame size, bus size and sometimes the number of sections.

Short-circuit and short-time withstand

Both assemblies carry a short-circuit current rating for the bus, but the withstand test durations differ. Switchboard bus bracing under UL 891 is verified with a short-duration test, typically 3 cycles, on the assumption that the main device clears quickly. Switchgear under UL 1558 and IEEE C37.20.1 carries a 30-cycle short-time withstand rating, and the LVPCBs carry a matching short-time rating under IEEE C37.13.

That 30-cycle capability is what allows a switchgear main to run with the instantaneous element turned off and a short-time delay set to let downstream devices clear first. It is the mechanism behind true selective coordination in large low-voltage systems. A switchboard main protected by an instantaneous trip may have to sacrifice some coordination to protect its own bus, or rely on zone-selective interlocking to recover it.

Access, construction and maintainability

Switchboards are front-accessible or front and rear accessible, with devices, bus and cable terminations often sharing space within a section. That compactness is the point: shorter lineups, lower cost and easier fit in electrical rooms with a wall behind the gear. Servicing a fixed-mount main, however, generally means an outage on everything it feeds.

Switchgear is compartmentalized and rear-accessible. Breakers, bus and cable terminations are in separate compartments with barriers, and each drawout breaker can be racked out, inspected, tested and replaced with a spare while the rest of the lineup stays in service. The cost is footprint, rear clearance, weight and price.

• Switchboard: fixed MCCB/ICCB, front or front/rear access, shared sections, smaller footprint, lower cost
• Switchgear: drawout LVPCB, rear access, individual compartments, 30-cycle bus, higher availability
• Both: 600 V class, bus short-circuit ratings, options for metering, surge protection and communications

When to specify each

Specify a switchboard when the service is moderate in size, the fault current is within MCCB and ICCB capabilities, coordination can be achieved with the available trip units, and a planned outage for maintenance is acceptable. Commercial buildings, light industrial plants and secondary distribution in larger facilities fit this profile well.

Specify low-voltage power switchgear when the load cannot tolerate an outage for breaker maintenance, when 100%-rated mains and feeders are needed, when the coordination study requires short-time delay on the mains, or when high fault currents and large frame sizes push beyond what a switchboard handles comfortably. Data centers, hospitals, process plants and utility-scale unit substations are typical. Applicable listings, standards and design requirements depend on equipment type, configuration, project specifications and jurisdiction.