You're right to be confused, and your line of thinking is generally correct. Let's break down the issues and clarify the code requirements.

Understanding 80% vs. 100% Rated Breakers 80% Rated Breakers

These are the standard breakers. They are designed to carry 100% of their rated current for a limited amount of time but are intended to trip if they continuously carry more than 80% of their rated current. Think of it as a safety factor built in to prevent overheating. The NEC (National Electrical Code) generally requires circuits to be sized such that the continuous load does not exceed 80% of the overcurrent device's rating (NEC 210.19(A)(1) and 215.2(A)(1)).

100% Rated Breakers

These are specially designed breakers that can carry 100% of their rated current continuously. They require specific listing and are typically used in industrial applications where continuous high loads are expected. They are also more expensive.

Analyzing Your Specific Scenarios1. 30A Breaker, 30A Nameplate



Possible Explanation #1: 100% Rated Breaker

If the breaker is specifically marked as "100% Rated" and the busway tap box and breaker are specifically listed for use together at 100% of the breaker's rating, then the 30A nameplate is correct. This is less common in typical installations.



Possible Explanation #2: Misunderstanding the Nameplate Purpose

The nameplate is likely indicating the maximum breaker size that the tap box is designed to accept. It doesn't necessarily dictate the continuous current capacity of the entire assembly in all loading configurations.

Most Likely Explanation

The nameplate indicates the maximum current the tap box can deliver to a single circuit. While a 30A 80% rated breaker should not continuously carry more than 24A, the tap box may be built to handle the full 30A (briefly, and for fault conditions).

What You Need to Confirm

Is the breaker 100% rated? Look for markings on the breaker itself. Check the listing information. Is the tap box listed for use with 100% rated breakers? What is the intended load? Is the anticipated load significantly less than 30A continuous? If so, it might be code compliant, though perhaps not best practice.2.

Dual Output 30A Breakers (Two Circuits), 30A Nameplate



This is the most problematic scenario. Your intuition is correct – this sounds wrong. If you have two independent 30A circuits coming from a single tap box, the nameplate should reflect the total current the tap box can safely deliver considering all circuits operating simultaneously.

Likely Problem

The 30A rating likely refers to the maximum current permitted per individual circuit. The manufacturer is assuming that both circuits will not be loaded to their maximum capacity at the same time. This is a common application of diversity.

Diversity/Demand Factor

The manufacturer may be applying a diversity factor. This assumes that not all loads will be at their maximum draw simultaneously. While permissible in some cases (like feeders serving multiple dwellings), it's often not appropriate for tap boxes feeding specific equipment where the load is relatively constant.

Potential Overload

If both 30A circuits are continuously loaded near their maximum capacity, you could easily overload the tap box and potentially the busway itself.

What You Need to Confirm and Do



Check the manufacturer's specifications VERY CAREFULLY. Look for information on diversity factors, total load limitations, and any restrictions on simultaneous loading. Look for the total ampere rating of the tap box.

Verify the busway rating. Make sure the busway itself is rated to handle the combined load of all tap boxes connected to it.

Consider Load Calculations

Perform accurate load calculations for each circuit to determine the actual continuous load you expect. If the combined continuous load exceeds the nameplate rating (or exceeds what is reasonable for the busway), you need to reduce the load or use a different tap box.

Consult an Engineer

If there's any doubt or ambiguity, consult a qualified electrical engineer. This scenario is potentially dangerous if not addressed correctly. They can perform a proper load study and ensure the entire system is safe and code-compliant.

General Best Practices

Err on the Side of Caution

It's always better to overestimate your load and use a larger tap box and busway than to risk overloading the system.

Adhere to NEC Load Calculations

Use the load calculation methods in the NEC (Article 220) to determine the appropriate size of conductors, overcurrent protection, and equipment.

Understand Continuous Loads

Clearly identify which loads are considered "continuous" (operated for 3 hours or more at a time). These loads require special consideration under the 80% rule.

Documentation

Maintain thorough documentation of all calculations, specifications, and decisions made regarding the electrical system.

In summary, a 30A nameplate on a tap box with a single 30A breaker could be acceptable if the breaker is 100% rated and the system is listed accordingly. However, a 30A nameplate on a dual-output tap box with two 30A circuits is highly suspect and requires careful investigation and likely an upgrade. Always prioritize safety and consult with qualified professionals when in doubt. Flag for review