Okay, let's break this down. You're trying to understand if you can install a 200A disconnect downstream of your 80kW generator's 300A breaker to protect your 200A ATS and electrical service.

The Short Answer

Yes, generally speaking, you can install a 200A fusible disconnect downstream of the 300A breaker on the generator, before the ATS. However, it's CRUCIAL to understand the reasons why you might do this and how it impacts the overall system. It's also imperative to have a qualified electrician make the final determination based on a complete evaluation of your specific setup.

Here's a more detailed explanation



Overcurrent Protection Philosophy

The primary goal of overcurrent protection (breakers and fuses) is to protect conductors and equipment from damage caused by excessive current flow (overloads or short circuits). You are essentially trying to protect the downstream ATS and service conductors.

Why the 300A Breaker on the Generator? Generator manufacturers often size the output breaker generously for several reasons: Motor Starting

Generators must be able to handle the inrush current of motors (like those in HVAC systems, pumps, etc.) without tripping the breaker. This inrush can be significantly higher than the continuous running current.

Harmonic Loads

Non-linear loads (e.g., electronic devices) can generate harmonic currents, which increase the overall current drawn from the generator.

Generator Design

The generator's internal design and cooling system may dictate a higher breaker rating than the continuous output rating would suggest.

Voltage Dip

The generator manufacturer wants to prevent the breaker from tripping during temporary voltage dips during motor starting.

Protecting the ATS and Service

Your concern about protecting the 200A ATS and service is valid. The 300A breaker on the generator alone does not adequately protect the downstream 200A-rated components and conductors from overcurrents below 300A.

Here's the Reasoning for Installing a 200A Disconnect


Selective Coordination

You want the 200A disconnect's fuses to blow before the 300A breaker on the generator trips in the event of an overload or fault on the load side of the ATS. This ensures that only the affected circuit is de-energized, rather than the entire generator output. This is a key principle of selective coordination.2.

Protection of 200A Rated Equipment

The 200A disconnect, with correctly sized fuses, will protect the 200A ATS and the conductors feeding your electrical service from overcurrents exceeding their rating.

Important Considerations and Code References


NEC Article 445 (Generators)

This article covers the installation and wiring of generators. It doesn't specifically prohibit installing a disconnect downstream of the generator's output breaker, but it does emphasize the need for proper overcurrent protection and coordination.

445.12 Overcurrent Protection

"Generators shall be protected from overcurrent." This section covers the overcurrent protection at the generator itself. Your 300A breaker fulfills this requirement.

445.13 Ampacity of Conductors

The conductors from the generator to your 200A disconnect must be sized to carry the load calculated according to Article 220 and be protected per 240.4

445.18 Disconnecting Means

"Generators shall have a disconnecting means by which the generator and all associated equipment as applicable, can be disconnected from the premises wiring..." This section primarily discusses the disconnecting means at or very near the generator.2.

NEC Article 240 (Overcurrent Protection)

This is the core article for overcurrent protection.

240.4 Protection of Conductors

This section is critical. It states that conductors must be protected against overcurrent in accordance with their ampacities. The conductors from the 300A breaker to the 200A disconnect must be sized appropriately for the 300A rating. The conductors from the 200A disconnect to the ATS need to be rated for at least 200 amps.

240.21 Location in Circuit

Discusses when overcurrent protection is required for feeder taps. It probably does not apply in your case as you're trying to protect your ATS and service equipment not conductors.3.

NEC Article 700 (Emergency Systems) or Article 701 (Legally Required Standby Systems)

If your generator is part of an emergency or legally required standby system, these articles may impose additional requirements for reliability and coordination. Review the selective coordination requirements if the generator serves emergency or legally required loads.4.

Series Ratings

Don't confuse your situation with "series ratings." Series ratings are when you use a breaker with a lower interrupting rating because it's protected by a breaker with a higher interrupting rating upstream. That's not what you're doing. You are attempting to achieve selective coordination.5.

Coordination Study

For critical applications (hospitals, data centers, etc.), a coordination study is highly recommended. This study analyzes the time-current characteristics of all the overcurrent protective devices in the system to ensure proper selective coordination. A coordination study plots the time-current curves of each overcurrent device in the system and confirms that the downstream devices trip before the upstream device.

Practical Steps and Recommendations


Consult a Qualified Electrician

This is absolutely essential. Your electrician will perform a load calculation, verify conductor ampacities, and ensure that the entire system is compliant with the NEC and local codes.2.

Fuse Selection

The fuses in the 200A disconnect must be carefully selected to: Be rated for the voltage and interrupting rating required by your system. Coordinate with the 300A breaker on the generator. Provide adequate protection for the 200A ATS and service.3.

Conductor Sizing

Make sure all conductors are sized appropriately for the current they will carry, considering derating factors for ambient temperature, conduit fill, etc.4.

Disconnect Rating

The disconnect switch must be rated for the available fault current at its location.5.

Marking and Labeling

Clearly label all equipment and conductors to identify the source of power and the ratings of the protective devices.

In summary: Installing a 200A fusible disconnect downstream of the 300A generator breaker is often a good practice to protect the ATS and service. It is essential that a licensed electrician performs the installation and verifies that the entire system complies with the NEC and local codes. This electrician will consider selective coordination, load calculations, conductor sizing, and all other relevant factors. Flag for review