What is panelboardin construction?

A panelboard is wall-mounted (flush or surface), accessed and serviced from the front only, and listed to UL 67. A switchboard is free-standing floor gear, often rear-accessible, listed to UL 891, and built for larger services. On a one-line they can look similar, so check the equipment schedule and specifications before pricing: a switchboard costs several times what a panelboard does and needs a housekeeping pad and rigging.

MLO stands for main lug only: the feeder conductors land directly on the panel bus lugs with no main breaker inside the panel. Protection comes from the feeder breaker upstream. A main breaker panel costs more than the equivalent MLO panel and is required in some applications, such as service entrance use, so the takeoff should record which type each panel schedule calls for.

Older editions of the National Electrical Code capped lighting and appliance panelboards at 42 overcurrent devices, and manufacturers standardized on 18, 30, and 42 circuit products. The hard limit was later removed from the code, but 42-circuit panels remain the standard commercial size, so panel schedules and price books are still built around them. Larger circuit counts exist but price as special order.