Industrial Switchboard (UL 891) Guide: Sizing & NEC 408 | PAYAPRESS Busbar Processing Machine

If you work with low voltage power distribution, you will meet the industrial switchboard very often.

This guide explains the basics in simple language so you can choose, size, and specify correctly.

We cover UL 891, the key NEC Article 408 ideas, and the differences between switchboard, switchgear, and panelboard.

Table of Contents

What is an industrial switchboard?

An industrial switchboard is a floor-standing enclosure that takes one incoming power feed and splits it into many branch circuits.

Inside, you usually have a steel frame, copper or aluminum busbars (horizontal and vertical), an incoming section, meters, surge protection, and overcurrent devices like molded-case circuit breakers (MCCBs).

Dead-front covers allow safe access to devices while keeping live parts behind barriers. Typical ratings are up to 600 V and bus up to 6000 A.

Standard short-circuit withstand for many designs is 65 kA at the incoming terminals, and higher (for example 100–200 kA) is possible with approved main devices and tested combinations.

UL 891 in simple words

UL 891 is the U.S. product standard for dead front distribution switchboards rated 600 V or less.

It defines construction and test requirements so the assembly is safe and robust.

Manufacturers perform Factory Acceptance Tests (FAT) to check wiring, clearances, insulation, mechanical strength, and operation of devices.

When you request a quote (RFQ), say clearly that you need a “UL 891 listed and labeled switchboard.” This helps the vendor design and document correctly from day one.

NEC Article 408 — site rules that matter

NEC 408 covers panelboards, switchboards, and switchgear. For day to day work, focus on three ideas:

(1) install straight, plumb, and accessible with proper working space;

(2) avoid hazardous or corrosive locations unless the enclosure is rated for that environment;

(3) for service-entrance switchboards (fed directly by the utility), follow the utility’s grounding and bonding requirements.

Good layout now saves headaches during inspection and maintenance later.

Switchboard vs switchgear vs panelboard

You choose switchgear when you need higher continuity of service and easier breaker maintenance under energized conditions.

*Panelboard vs switchboard:* A panelboard is wall mounted, smaller, and often ≤1200 A; breakers usually snap onto the bus.

A switchboard is floor-standing, handles higher amperage up to 6000 A, and often uses bolted-on breakers.

Pick based on load level, maintenance approach, and space in the electrical room.

Ratings that really matter

SCCR (Short-Circuit Current Rating) is the maximum fault current the entire assembly can safely withstand at its incoming terminals.

Common labels are 42 kA or 65 kA, and some projects require 100–200 kA. The right value depends on your short-circuit study.

AIC (Ampere Interrupting Capacity) is a breaker rating; each breaker must be able to interrupt the available fault current at the point where it is installed.

Always match the breaker’s AIC to the calculated fault current so it can clear safely.

Enclosures: NEMA vs IP

For indoor dry locations in the U.S., NEMA Type 1 is typical. For outdoors, NEMA 3R is common.

If the site is dusty or wash-down, look at higher NEMA types or IP66.

Choose the enclosure rating to fit the environment and local code.

For low-voltage power distribution, get the switchboard spec right by aligning the product with its standard and the site code: state “UL 891 listed & labeled” in your RFQ, match the enclosure rating to the environment (NEMA/IP), and confirm the assembly SCCR meets or exceeds available fault current while each breaker’s AIC is adequate. For an authoritative source, see UL’s overview of the UL 891 switchboard standard, which explains the scope and intent of the listing and testing requirements.

Sizing and arc-flash basics

Start with a load calculation. Select the main device main breaker or main lugs for the total calculated load, and leave space for future expansion.

For arc-flash safety, follow your site study or NFPA 70E-CSA Z462 tables, use the correct PPE, and consider features like energy-reduction maintenance settings, IR windows, and remote operation where available.

Good documentation single-line, layout, nameplate data makes installation and commissioning smoother.