You've hit upon a common source of confusion and sometimes... vendor creative interpretations of standards. Here's a breakdown of what's likely happening and how to proceed

The Core Issue: UL Standards and Stacked Breakers
UL 891

This is the standard that covers dead-front switchboards. It's the key document in your scenario.

"Stacked Breakers" Definition

The term "stacked breakers" is often misused. Are you referring to:

Physically adjacent breakers in a vertical (or horizontal) configuration sharing a common mounting structure (bus)? This is what most people mean by "stacked." Multiple breakers being fed from a single set of main bus bars? The Confusion/Change

There has not been a fundamental change in UL 891 that outright bans all vertically stacked breakers, or breakers in close proximity. What has happened over time are:

Increased Scrutiny on Thermal Management

UL standards, including UL 891, are increasingly focused on ensuring adequate thermal management within enclosed switchgear. Higher amperage breakers in close proximity can generate more heat, potentially leading to derating requirements.

Clarification and Evolution of Testing Requirements

The testing requirements for UL 891 have become more rigorous regarding temperature rise and short-circuit withstand. This can indirectly impact how closely breakers can be placed.

Why the Vendor Discrepancy?

Here's why Vendor A and Vendor B might be giving you different answers:1. Interpretation of UL 891 and the Design's Impact

Vendor A is likely taking a more conservative approach, or their specific switchboard design, combined with your chosen breaker ratings (4000A and 3000A in close proximity), might trigger thermal concerns based on their calculations and UL requirements. Vendor A might be using historical or older data.2.

Vendor B's Design

Vendor B's design may have better thermal management, or they may be willing to accept the risk that their design will pass UL testing. It's also possible that Vendor B is using different design parameters, or that their UL listing allows for closer breaker spacing under certain conditions. They may have newer testing data.3.

Listing Limitations

Some manufacturer listings have limitations or restrictions depending on the specific components used (breaker models, busbar configurations, etc.). One vendor might have a listing that allows closer spacing than another.4.

Communication/Misunderstanding

It's always possible that there's been a miscommunication about the exact breaker configuration or the vendors' interpretation of the standard.

How to Resolve This

Here's a structured approach to getting the right answer:1.

Get Specific UL Documentation from Both Vendors

Ask both vendors to provide the specific UL 891 listing documents or reports that support their designs. This will show you exactly what configurations and ratings are covered by their listings. Ask them to highlight the areas applicable to your design. Don't accept vague answers. Get the document!2.

Ask Both Vendors to Justify Their Designs Relative to UL 891

Ask Vendor A to clearly explain why they believe stacked breakers are no longer permissible in your specific application, citing specific clauses within UL 891 or their listing. Ask Vendor B to explain how their design meets UL 891, especially considering the potential thermal concerns of high-amperage, closely spaced breakers.3.

Consult with a Qualified Testing Lab (e.g., UL, ETL)

If you're still unsure, consider consulting directly with a recognized testing laboratory like UL or ETL. They can provide definitive interpretations of UL 891 requirements and can review the vendors' proposed designs. This might cost some money, but it can save you a lot of headaches later.4.

Review the Short Circuit Current Rating (SCCR) Requirements

Ensure both designs meet the required SCCR for your application. Close proximity of high-amperage breakers can impact the overall SCCR rating of the switchboard. The testing lab or a qualified engineer should be able to confirm this.5.

Consider the Breaker Type and Application

The type of breaker (e.g., insulated case, power circuit breaker) and its intended application (e.g., main breaker, feeder breaker) can influence the permissible spacing and thermal performance.6.

Thermal Modeling/Simulation

If thermal concerns are significant, consider requesting thermal modeling or simulation from the vendors. This can provide a more detailed assessment of the temperature rise within the switchboard.

Key Questions to Ask

"Can you provide the specific UL 891 listing document that supports your design with these breaker configurations?" "How does your design address potential thermal issues associated with closely spaced high-amperage breakers?" "Can you provide test data or calculations to demonstrate that your design meets UL 891 temperature rise requirements?" "What is the SCCR rating of the proposed switchboard, and how does it meet the requirements of our application?"

In summary: There likely isn't a complete* ban on stacked breakers, but the UL 891 standard and its testing requirements have become more stringent regarding thermal management, which can limit how closely high-amperage breakers can be placed. You need to dig into the specific designs, vendor listings, and test data to determine which vendor's solution truly meets the requirements of UL 891 and your application. Good luck! Flag for review