You're right to be thinking about this carefully. Here's the breakdown of the code requirements and best practices for smoke detector circuits in residential wiring

Code Requirements (Based on NEC - National Electrical Code, commonly adopted in the US):
NEC 760.41(B) - Power Source



Required Power

Smoke detectors

MUST be powered by a branch circuit (the circuit breaker in your panel). Exception Smoke alarms may be powered by 2 sources: one from branch circuit and one from batteries. NEC 210.8(F) - GFCI Protection

Smoke alarms and Carbon Monoxide alarms are not required to be GFCI protected The requirement for GFCI protection would be dependent on the location of the outlet that is connected to the lighting circuit.

Best Practices and Considerations


Dedicated Circuit vs. Lighting Circuit



Dedicated Circuit (Less Common)

Some electricians prefer a dedicated circuit solely for smoke detectors.

Lighting Circuit (More Common and Often Preferred)

It is acceptable to put smoke detectors on a general lighting circuit. This can be a good strategy because the homeowner is less likely to turn off a lighting circuit.2.

Labeling



Very important

Clearly label the circuit breaker in the panel as "Smoke Detectors" or "Smoke Detectors/Lighting" (if on a lighting circuit).

NEC 110.22(A)

Requires that all circuit breakers be legibly field marked to indicate its purpose.3.

Interconnection



Required by code

All smoke detectors within a dwelling unit must be interconnected. This means that if one detector senses smoke, all detectors will sound an alarm. This interconnection is typically achieved through a dedicated signaling wire (usually a red wire) that runs between the detectors.4.

Battery Backup



Also required

All smoke detectors must have battery backup in case of a power outage.5.

Tamper Resistance

Consider using tamper-resistant smoke detectors or hard-wired units that are difficult to disconnect.

Why a Lighting Circuit is Often Preferred



Reduces Accidental Disablement

Homeowners are less likely to intentionally turn off a lighting circuit than a dedicated circuit labeled "Smoke Detectors," especially if the detector is beeping due to a low battery.

Simplifies Wiring

It can be more efficient and cost-effective to tap into an existing lighting circuit during the rough-in phase of wiring a new home.

Compliance

As long as the smoke detectors are on a readily accessible circuit, have battery backup, and are interconnected, it meets the code requirements.

Important Considerations Regarding Specific Local Codes



Local Amendments

Many local jurisdictions amend or add to the NEC. It's crucial to check with your local building department or electrical inspector to determine if there are any specific requirements in your area regarding smoke detector circuits.

Permitting and Inspection

Always obtain the necessary permits and have the work inspected by a qualified electrical inspector to ensure compliance with all applicable codes.

In summary

Smoke detectors must be on a branch circuit. Interconnection and battery backup are mandatory. A lighting circuit is acceptable and often preferred (but label it clearly). Check local codes for any additional requirements. Labeling is crucial. Consult with a qualified electrician and local building department.

Disclaimer: I am an AI chatbot and cannot provide electrical advice. This information is for general knowledge purposes only. Consult with a licensed electrician for all electrical work and to ensure compliance with all applicable codes and regulations. Electrical work is dangerous and should only be performed by qualified professionals. Flag for review