Okay, let's break down the NEC 2017 requirements for your situation with the fuel cell system in the hospital. Here's how Articles 692 and 705 would apply, and where to find the relevant sections

Regarding the Cable Replacement (Between Fuel Cell and 500A Breaker)
Article 692: Fuel Cell Systems This is the primary article governing the installation of fuel cell systems. 692.8 Wiring Methods

This general section dictates acceptable wiring methods for fuel cell circuits, referring to other NEC chapters for specific requirements (e.g., Chapter 3 for conductor types, raceways, etc.).

692.9 Circuit Sizing and Current

This section is most crucial for your cable sizing. It directs you to calculate the maximum current the fuel cell system can produce. Then, you'll need to size the conductors appropriately, taking into account factors like ambient temperature, conduit fill, and termination temperature ratings.

692.8(A) Ampacity of Conductors The ampacity of the circuit conductors shall be not less than 125 percent of the fuel cell system’s rated output current.

Regarding the 500A Breaker in the Main Service Switchboard Article 692: Fuel Cell Systems 692.8 Wiring Methods

This general section dictates acceptable wiring methods for fuel cell circuits, referring to other NEC chapters for specific requirements (e.g., Chapter 3 for conductor types, raceways, etc.).

692.9 Circuit Sizing and Current

This section is most crucial for your cable sizing. It directs you to calculate the maximum current the fuel cell system can produce. Then, you'll need to size the conductors appropriately, taking into account factors like ambient temperature, conduit fill, and termination temperature ratings.

692.8(B) Overcurrent Protection Overcurrent protection shall be provided in each ungrounded conductor of a fuel cell system output circuit. The overcurrent protection shall be sized to protect the conductors in accordance with 240.4. Article 705: Interconnected Electric Power Production Sources This article is essential because you have a utility-interactive fuel cell system. This means the fuel cell can send power back to the utility grid. Article 705 deals with the safe interconnection of these systems. 705.12 Point of Connection. This section is where you'll find the specific requirements for how the fuel cell connects to the main service switchboard. It often directs you to consider the sum of the power source outputs feeding into that breaker and the busbar rating. 705.12(B) Supply Side Interconnection. This section typically applies when the fuel cell connects on the supply side of the service disconnecting means (the main breaker). This might be your case if the 500A breaker is the main service disconnect. Read this section carefully. It will outline requirements related to conductor sizing, breaker rating, and potential backfeed into the grid. 705.12(D) Ampere Rating of Disconnect. The overcurrent device disconnecting the generator output shall be rated in accordance with 705.12(D). 705.12(D)(2) Other Installations. For other installations, the sum of the overcurrent devices supplying power to a busbar or conductor shall not exceed the ampacity of the busbar or conductor.

Key Considerations and Steps


Fuel Cell Output

Obtain the rated output current of the fuel cell system from the manufacturer's documentation. This is critical for all calculations.2.

Conductor Sizing (Article 692 & 705)

Size the conductors between the fuel cell and the 500A breaker to at least 125% of the fuel cell's rated output current. Consider voltage drop, ambient temperature, conduit fill, and termination temperature ratings when selecting the conductor size.3.

Breaker Size (Article 692 & 705)

The breaker protecting the fuel cell output circuit must be sized to protect the conductors (following 240.4). Crucially, Article 705.12(D)(2) dictates that the sum of all overcurrent devices feeding the busbar (including the fuel cell breaker and any other feeders) cannot exceed the busbar rating of the switchboard. This is paramount for safety.

Important Notes



Coordination Study

For complex installations like this, especially in a hospital setting, a coordination study is highly recommended (and often required by code). This study analyzes the overcurrent protective devices (breakers and fuses) in the system to ensure proper selective coordination. This means that a fault close to the fuel cell should cause only the closest breaker to trip, rather than tripping a main breaker and shutting down a larger portion of the hospital's power.

Engineering Expertise

Given the complexity and the life-safety implications of electrical systems in hospitals, it is strongly advised that a qualified and licensed electrical engineer be involved in the design and approval of these modifications. They can perform the necessary calculations, ensure code compliance, and provide proper documentation.*

Local AHJ

Always consult with the local Authority Having Jurisdiction (AHJ) to clarify any specific requirements or interpretations they may have.

In summary: Your cable replacement will be heavily influenced by Article 692.9, and the 500A breaker will be primarily governed by Article 705.
12. Ensure the conductor ampacity and overcurrent protection are appropriately sized and coordinated according to the fuel cell's output, conductor ampacity, and the switchboard busbar rating. Flag for review