You're right to be questioning this! Let's break down 7.9.
2.3 and how it applies to emergency lighting design.

Understanding 7.9.
2.3

This section of the Life Safety Code (NFPA 101) aims to ensure reliable emergency lighting under various failure scenarios. Let's look at each point individually: (1) Failure of a public utility or other outside electrical power supply

This is the most common scenario. A power outage on the grid or from your primary generator triggers the emergency lighting system to activate.

(2) Opening of a circuit breaker or fuse

This is the key point you're asking about. This section requires that the emergency lighting system activates if a circuit breaker feeding the normal lighting trips, or a fuse blows feeding the normal lighting. It's not talking about a breaker on the emergency panel itself.

(3) Manual act(s), including accidental opening of a switch controlling normal lighting facilities

This is less frequently the issue in modern construction but would be the case when a manual switch is in place to shut off power to the normal lighting.

Why One Emergency Circuit Might Not Be Enough

Your question highlights the problem: If you only have one emergency circuit for a space, you might not fully comply with 7.9.
2.3(2). Here's why:1. The Intent

The core idea is that a single point of failure in the normal lighting system shouldn't also kill the emergency lighting.2.

Scenario

Imagine this: you have a room with normal lighting powered by a breaker on panel 'A'. You also have emergency lighting in that same room, powered by a different emergency panel and different circuit. If the breaker for the normal lighting (on panel A) trips, the normal lights go out, and the emergency lights must come on.3.

Your Single-Circuit Case

If your only lighting in the room is on a single emergency circuit, and that circuit breaker trips (perhaps due to a fault), all the lighting is lost. You haven't provided a backup for the failure of the normal lighting because your emergency lighting is the only lighting.

How to Incorporate 7.9.
2.3(2) into Your Design

Here's how to properly address this requirement and incorporate it into your design:1. Separate Circuits for Normal and Emergency Lighting

This is the most fundamental approach. Provide separate circuits for your normal lighting and your emergency lighting. The emergency circuit must come from an emergency power source (generator, UPS, or battery-backed system). This allows the emergency lighting to still function if the breaker to the normal circuit trips.2.

Emergency Lighting as a Supplement, Not the Only Light Source

Design the emergency lighting to supplement the normal lighting, not to replace it entirely during normal operation.3.

Consider Dual-Head Emergency Lights (Where Allowed)

Some emergency lighting fixtures have two heads: One head is normally powered by the normal lighting circuit. The other head is powered by the emergency circuit. If the normal lighting circuit fails, the second head (powered by the emergency circuit) automatically illuminates. These are becoming less common.4.

Automatic Transfer Switches (ATS)

In some cases, an automatic transfer switch (ATS) is used. This can switch a lighting fixture between the normal power source and the emergency power source if the normal source fails.5.

Battery Packs in Fixtures

Emergency fixtures will have a built in battery pack, this will need to be tested monthly to ensure proper operation of the batteries and the emergency lighting fixture.

Important Considerations Authority Having Jurisdiction (AHJ)

Always consult with your local AHJ (building inspector, fire marshal, etc.) to confirm that your design meets their interpretation of the Life Safety Code. Interpretations can vary.

Specific Occupancy

The Life Safety Code has different requirements based on the occupancy classification of the building (e.g., assembly, business, healthcare). Make sure you are following the requirements for the specific occupancy.

Load Calculations

Carefully calculate the load on your emergency panel to ensure you have enough capacity for all the required emergency lighting and other essential emergency systems.

In Summary

Providing only one circuit to a space, even if it's from an emergency panel, is generally not enough to comply with 7.9.
2.3(2) of the Life Safety Code. You need to design your lighting system such that a failure in the normal lighting circuit (like a tripped breaker) does not also disable your emergency lighting. Separate circuits for normal and emergency lighting are the key. Flag for review