You've identified a valid concern, and it hinges on the NEC's (National Electrical Code) rules regarding service equipment and main overcurrent protection. Let's break down the issues and the potential code implications

The Situation:
Original Setup

480V, 1600A GE Spectra gear in two sections, connected by a bus. One section contains the service disconnect (the 1600A main breaker). The other section is a distribution panel (MDP) with multiple breakers.

Proposed Change

Remove the bus connecting the two sections. Run conductors from the 1600A main breaker to an Automatic Transfer Switch (ATS). Run conductors from the ATS back to the second section of gear (the MDP). ATS is "in sight" of the service disconnect.

The Core Question: Does the re-fed MDP now require a main breaker?The Likely Answer: YES, the re-fed MDP likely REQUIRES a main breaker.

Here's why:1. Definition of Service Equipment and Service Disconnect

The 1600A breaker is, by definition, the service disconnect. The conductors downstream of that breaker are no longer considered "service conductors".2.

ATS as a Source, Not Just a Switch

While the ATS is "in sight" of the service disconnect, it fundamentally alters the source of power for the second section of gear. It's no longer directly fed from the service disconnect; it's fed from a source (the ATS) that is, in turn, fed by the service disconnect. The ATS effectively isolates that second section from being directly considered part of the service entrance.3.

NEC Article 230 (Services) Implications



NEC 230.70(A)(1) (Location)

The service disconnecting means must be readily accessible and located either outside of a building or structure or inside nearest the point of entrance of the service conductors. This is not directly applicable here because the service disconnect (1600A breaker) is staying in its original location.



NEC 230.71 (Maximum Number of Disconnects)

Generally allows a maximum of six service disconnecting means for each service. Again, not directly applicable unless there are already multiple service disconnects on the property.4.

NEC Article 408 (Switchboards, Switchgear, and Panelboards)



NEC 408.36 (Overcurrent Protection)

This is the crucial section. Unless specific exceptions apply, panelboards (which the MDP is considered) must be protected by overcurrent devices with a rating not greater than the panelboard's rating. Specifically:

(A) Panelboards Supplied Through a Transformer

If the panelboard is supplied through a transformer, the overcurrent protection for the panelboard must comply with 240.21. This isn't relevant here.

408.36(B) Panelboards Supplied From Another Panelboard

If the panelboard is supplied from another panelboard, the overcurrent protection for the panelboard must comply with 408.36(B)(1) through (B)(3). Typically this requires a main breaker.
5.

Why the ATS Changes Things

The ATS is the source for the re-fed MDP. The ATS changes the fundamental relationship between the service disconnect and the downstream panelboard.

Exceptions and Considerations



Engineering Judgement and Code Interpretation

The engineer could argue that the intent of the code is met because of the proximity of the ATS to the service disconnect ("in sight"). However, this is a gray area, and the AHJ (Authority Having Jurisdiction - the electrical inspector) has the final say. It's a weak argument, in my opinion.

Specific Load Calculations

If the engineer has performed a thorough load calculation for the MDP, and it can be definitively shown that the total connected load is significantly less than the rating of the conductors and the panelboard itself, perhaps the AHJ might allow it. However, this is unlikely without a main breaker.

Local Amendments

Always check for local amendments to the NEC. Some jurisdictions have stricter requirements regarding overcurrent protection.

Best Practice and Recommendation



Install a Main Breaker

The safest, most code-compliant, and likely required approach is to install a main breaker in the re-fed MDP. This breaker's ampacity should be properly sized for the load served by the MDP and the ampacity of the conductors feeding it from the ATS.

Consult with the AHJ

Before starting any work, discuss the plan with the local electrical inspector. Get their opinion in writing if possible. This will prevent costly rework later.

Comprehensive Load Calculation

The engineer must provide a thorough load calculation to demonstrate the expected load on the re-fed MDP. This will be necessary whether or not a main breaker is installed.

In summary, while it might be technically possible to argue against a main breaker in the re-fed MDP, the most prudent and defensible approach is to install one. This ensures code compliance, enhances safety, and avoids potential issues with the electrical inspector.* Flag for review