You're right, it's generally permissible to use separate generators for life safety and optional standby loads, but there are definitely some code considerations to keep in mind.Here's a breakdown of my thoughts and where to look in the NEC

Key Points and Considerations:
NEC Compliance is Paramount

The National Electrical Code (NEC) is your primary guide. You must adhere to all applicable sections.

Separation of Systems

The whole point is to keep the life safety system completely separate and independent from the optional standby system. This separation must be maintained throughout the entire system.

Transfer Switches

Each generator must have its own automatic transfer switch (ATS). The ATS for the life safety generator is critical for reliability. You cannot have a single ATS switching between two generators for life safety.

Load Calculations

Verify that each generator is sized appropriately for the load it will serve. Oversizing is better than undersizing, especially for life safety. Consider future growth.

Coordination Studies

A coordination study may be required to ensure proper selective coordination between the generator breakers, ATS breakers, and downstream overcurrent protective devices.

Testing and Maintenance

Rigorous testing and maintenance schedules are essential for both generators, especially the life safety one.

Local Amendments

Always check for local amendments to the NEC. Your jurisdiction may have stricter requirements.

Documentation

Thoroughly document the entire system, including wiring diagrams, load calculations, ATS settings, and testing procedures.

Relevant NEC Sections to Investigate



Article 700 (Emergency Systems)

This is the big one for the life safety generator. It covers requirements for capacity, transfer switches, overcurrent protection, wiring methods, testing, and maintenance. Pay close attention to: 700.4 - Tests and Maintenance 700.5 - Capacity 700.12 - General Requirements (for the wiring methods) 700.28 - Coordination

Article 701 (Legally Required Standby Systems)

This applies if the building has loads that are legally required to have standby power (e.g., some fire pumps, certain types of ventilation).

Article 702 (Optional Standby Systems)

This covers the generator that supplies the optional loads. It's generally less stringent than Article 700.

Article 705 (Interconnected Electric Power Production Sources)

This article is generally not applicable in your scenario unless the generators are intended to be operated in parallel with each other or the utility. If they are, it adds a significant layer of complexity.

Article 445 (Generators)

This covers general requirements for generators, including overcurrent protection, marking, and location.

Article 220 (Load Calculations)

Ensures you calculate the load correctly for both the emergency and optional standby systems.

Article 230 (Services)

While not directly about the generators, this article is relevant if the generator connects to the building's service equipment.

Article 250 (Grounding and Bonding)

Proper grounding is critical for safety and to ensure proper operation of overcurrent protective devices.

Common Problems/Things to Watch Out For



Neutral Grounding

Ensure proper neutral grounding for each generator system. Do not have multiple neutral-to-ground bonds in the same system (this creates circulating currents). The bond is typically at the generator itself.

Wiring Separation

Keep the wiring for the life safety system physically separate from the wiring for the optional standby system. This helps prevent a fault in one system from affecting the other.

ATS Reliability

Use high-quality, reliable automatic transfer switches, especially for the life safety generator. Consider features like bypass isolation switches for maintenance.

Testing

Regularly test both generators and ATSs under load. Document the results.

Maintenance

Follow the manufacturer's recommended maintenance schedule for both generators.

Fuel Supply

Ensure an adequate and reliable fuel supply for both generators. Consider fuel storage capacity, fuel quality, and fuel delivery arrangements.

AHJ Approvals

Obtain approval from the local AHJ (Authority Having Jurisdiction) before starting the installation. They may have specific requirements or interpretations of the code.

Example Scenario and Code Implications

Let's say the life safety generator powers emergency lighting and fire alarm system and the other powers HVAC.

Life Safety (NEC 700)

The generator must be sized to handle the full load of the emergency lighting and fire alarm system. The ATS must be automatic and should have a manual bypass. Wiring must be kept separate and independent. Conduit is often used. Regular testing and maintenance are mandatory.

Optional Standby (NEC 702)

Sizing is less critical (but still important). ATS can be manually operated. Less stringent wiring requirements.

In Summary:

Using two separate generators is perfectly acceptable and often a good solution, provided you meticulously follow the NEC and any local amendments, and maintain proper separation and reliability between the life safety and optional standby systems. Get a licensed electrical engineer involved if you're not 100% confident in your ability to comply with all applicable codes and standards. I hope this helps you out Flag for review