Yes, something definitely seems amiss. Here's a breakdown of the issues and what needs to be considered

Problems with the Setup:

1. Parallel Conductors and Overcurrent Protection

Parallel conductors are generally permitted by the NEC (typically starting at 1/0 AWG and larger), but they MUST be the same length, material, installed in the same raceway, and terminated in the same manner. Crucially, each parallel conductor must have its own overcurrent protection sized appropriately for the conductor. Your setup violates this core principle. You have two different size breakers feeding a common point; the 100A breaker would trip well before the 200A under significant load, leading to unequal load sharing and potentially overloading the 200A breaker's conductors.2.

Backfeeding a Generator Breaker

The ATS is designed to switch the load between the utility source and the generator source. When the ATS switches to generator power, you are essentially backfeeding power into the generator's breakers. This is generally not a problem if the generator is designed for it. However, most smaller generators aren't designed for parallel operation or backfeeding in this manner.3.

Generator Capacity vs. Load

A 400A ATS suggests a significant load. Is the generator capable of supplying anywhere near that amount of power? If the generator is sized significantly smaller than the ATS capacity, the ATS is overkill, and the generator will likely be overwhelmed. Using multiple breakers doesn't magically increase the generator's capacity.4.

Potential for Damage and Fire

This setup is dangerous. Unequal load sharing can lead to overloading conductors. The failure of a breaker or a conductor could result in significant damage to the generator, the ATS, and potentially even cause a fire.

Why This Doesn't Work (Simplified)

Imagine you have two water hoses filling a bucket. One hose is thin (100A breaker equivalent), and the other is thick (200A breaker equivalent). If you try to fill the bucket quickly, the thin hose will be at its maximum capacity long before the thick hose even gets close to its. If the bucket continues to fill, all the extra water will be forced through the thick hose, potentially bursting it.

What Needs to Happen


Verify Generator Capability

The first step is to determine the actual rated output of the generator (kW or kVA). This is absolutely critical.2.

Consult the Generator Manufacturer/Manual

The generator manual will specify how (or if) the generator is intended to be connected to an ATS. It might allow parallel operation in a specific manner (usually with a specialized synchronizing panel) but will almost certainly not allow this configuration.3.

Determine the Actual Load Requirements

While a 400A service suggests a large load, the actual load demand at any given time might be much less. A load calculation, following NEC guidelines (Article 220), is necessary to determine the actual generator sizing needed.4.

Correct Wiring Configuration



If the generator output is significantly less than the 400A service

You need to use a smaller ATS and a single breaker at the generator to match its output capacity. For example, if the generator is rated for 50A, install a 60amp breaker at the generator, and use a 100A ATS (the smallest generally available)

If the generator is designed for parallel operation

Follow the manufacturer's instructions precisely. This will almost certainly involve a more complex system with synchronizing controls.*

Consider a different ATS

Depending on the generator's output and desired functionality, you might want to explore smaller ATS options designed for smaller generators.5.

Proper Overcurrent Protection

Ensure that the overcurrent protection (circuit breakers) are correctly sized for the conductors and the equipment they are protecting. Use the correct wire gauge for the amperage rating of each breaker and run type THHN.

Your Recommendation to the Customer

Tell the customer, in the strongest possible terms, that the proposed wiring configuration is unsafe and non-compliant with the electrical code. Explain the potential hazards and the need for a proper assessment and redesign. Recommend that they consult with the generator manufacturer or a qualified electrical engineer. Stress that you cannot proceed with the wiring until the issues are resolved.

In summary: Do not wire this up as described. It's a recipe for disaster. Get more information about the generator's specifications and consult with experts before proceeding. Flag for review