Okay, let's break this down

Understanding Service Rated ATS (SRATS) and Non-Service Rated ATS
Service Rated ATS

These are designed and listed to be the first point of disconnect from the utility service. They typically include overcurrent protection (a breaker or fuses) and often have features specific to managing the transfer of power from the utility. They are suitable for use as service equipment.

Non-Service Rated ATS

These are not designed or listed to be the first point of disconnect. They are intended to be fed downstream from a service disconnect. They don't inherently have the same overcurrent protection or ground fault protection requirements as a SRATS.

Can a SRATS be fed from a Main Breaker in a Switchboard? Technically, yes, but... It depends on how it's installed and what your goals are. If your SRATS is intended to replace the service disconnect

Then it can be fed from a sub-panel or a main breaker in a switchboard, assuming the overcurrent protection is adequate and the entire installation is code-compliant.

If you don't need a service rated ATS, and you just need to be able to switch sources, it's generally simpler and potentially cheaper to use a non-service rated ATS.Potential Issues When Feeding a SRATS from a Switchboard Breaker


Coordination Issues (Tripping)

This is your biggest concern. You need to ensure selective coordination between the switchboard breaker and the SRATS main breaker (or fuses).

Selective Coordination

The goal is that only the breaker closest to the fault trips. If a fault occurs downstream of the SRATS, you want the SRATS breaker to trip, not the upstream switchboard breaker. If the switchboard breaker trips first, you lose power to a much larger portion of your facility.

Coordination Study

Achieving selective coordination requires a detailed short-circuit and coordination study. This analysis will determine the time-current characteristics (TCC) of each breaker and ensure they are properly coordinated. It's typically done by a qualified electrical engineer. The size and settings of each breaker may need adjusting to ensure they coordinate properly.2.

Redundancy of Overcurrent Protection

The main breaker in your switchboard provides overcurrent protection. The SRATS will also have overcurrent protection. This isn't necessarily wrong, but it might be redundant.3.

Ground Fault Protection

If the switchboard has ground fault protection, you need to ensure the SRATS doesn't interfere with it.4.

Code Compliance

Ensure the entire installation, including the SRATS and its connections, complies with the National Electrical Code (NEC) and any local amendments. Pay close attention to grounding, bonding, and overcurrent protection requirements.5.

Listing and Labeling

The SRATS and all upstream components must be properly listed and labeled for their intended use.

Easier to Change to a Non-Service Rated ATS? In many cases, yes. If your primary goal is to have an ATS and you don't require the SRATS functionality (e.g., as the first point of disconnect from the utility), then changing to a non-service rated ATS is often simpler and less expensive. Here's why: No Need for Coordination Study (Potentially)

Because the Non-SRATS is not your service disconnect, and assuming the upstream breaker in the switchboard is already appropriately sized, you may be able to install the Non-SRATS downstream without needing a full coordination study. However, it's always best practice to check with an electrical engineer.

Simpler Installation

The installation of a non-service rated ATS is generally less complex.

Lower Cost

Non-service rated ATS units are often less expensive.

Recommendation


Evaluate Your Needs

Determine why you need an ATS. Is it purely for backup power, or does it need to serve as the service disconnect?2.

Consult with an Electrical Engineer

An engineer can perform a load analysis, short-circuit study, and coordination study to determine the best solution for your specific application. They can advise you on the correct size and type of ATS and ensure proper coordination.3.

Consider the Cost

Compare the costs of: Keeping the SRATS and performing a coordination study, adjusting breaker sizes, etc. Replacing the SRATS with a non-service rated ATS.

In summary, you can* feed a service-rated ATS from a main breaker in a switchboard, but you need to address potential coordination issues. Changing to a non-service rated ATS might be simpler and more cost-effective if the service-rated functionality isn't essential. Always involve a qualified electrical engineer to ensure a safe and compliant installation. Flag for review