Can I mix smoke alarm brands on one interconnect?

No. Alarms must be listed as compatible for interconnection, and manufacturers list only their own compatible models. Mixing brands violates NEC 110.3(B).

Does the smoke alarm circuit need AFCI protection?

Yes, where the circuit supplies outlets or devices in the areas listed in NEC 210.12(A). There is no smoke alarm exception.

How many smoke alarms can be interconnected?

The instructions set the limit. A common cap is 12 smoke alarms and 18 total devices on one interconnect, with a maximum total conductor length.

Do all smoke alarms in a house have to be the same brand by code?

The NEC does not contain a rule that says every smoke alarm must be the same brand. What forces the answer in practice is the product listing. NFPA 72, the National Fire Alarm and Signaling Code, requires interconnected alarms to be listed for interconnection with each other, and manufacturers list their alarms only with their own compatible models. Mixing brands on a single interconnect loop violates 110.3(B), which requires listed equipment to be installed in accordance with the instructions included in the listing.

Combination smoke and carbon monoxide units

The same rule applies when you add a combination smoke and carbon monoxide alarm to an existing loop. A combination unit from the same manufacturer and the same signal family will usually interconnect with the smoke only units in that family, but not always. Some manufacturers use a different signaling protocol between their older and newer lines, so an alarm from the same brand but a different generation may sound its own horn without triggering the loop.

Read the installation sheet. It lists the specific model numbers the alarm is compatible with. If your replacement is not on that list, replace the whole set. Manufacturers also cap the number of devices on one interconnect, commonly 12 smoke alarms with up to 18 total devices, and cap the total interconnect wire length.

What the NEC does say

Section 210.12(A) requires AFCI protection on 120 volt single phase 15 and 20 ampere branch circuits supplying outlets or devices in dwelling bedrooms, hallways, and the other listed areas. There is no exception for smoke alarm circuits, so the smoke alarm homerun in those areas gets a combination type AFCI.

Section 760.41 and Article 760 apply to fire alarm systems, not to single station and multiple station smoke alarms in dwellings, which are covered by NFPA 72 Chapter 29 and by the product listing. For the interconnect conductor, use the cable type called for in the alarm instructions. Most manufacturers specify 14 AWG or 12 AWG three conductor NM cable, with the red conductor as the interconnect.

Where the alarms go

NFPA 72 and the model residential codes require a smoke alarm inside each sleeping room, outside each separate sleeping area in the immediate vicinity of the bedrooms, and on each additional story including basements. Alarms must not be installed within 900 mm, or 36 inches, of the tip of a ceiling fan blade or of a supply register, and NFPA 72 sets a minimum separation from cooking appliances, generally 6 feet for ionization and 6 feet for photoelectric with allowances down to 10 feet for the stricter case.

Carbon monoxide alarms follow their own placement rules, usually outside each sleeping area and on each level where fuel burning appliances or an attached garage exist.

Field takeaway

Nothing in the NEC names a brand, but the listing does the job for it. Keep every interconnected alarm in one manufacturer family and confirm the model compatibility list on the instruction sheet. Put the alarm circuit on a combination type AFCI where 210.12(A) applies, and honor the device count and wire length limits in the instructions.