Since most manufacturers of single station smoke alarms are limiting you to between 10 and 12 units on a circuit, what does one do when you need more than 12 units for a home?

I suppose you will need to find a manufacturer that doesn't have the restriction or find one with a "repeater" type adapter to link 2 loops.
In real life I would expect a home that big to have a real alarm system. Even my wife's micro mansions (2000-4000 sq/ft) had a system that picked up the smokes, CO, pool alarm and intrusion alarm.

Greg some areas will not allow a fire alarm system to take the place of the required smoke alarms.

I don't know the solution to the question.

Bob that seems strange to me but I never underestimate the absurdity of government.

Off the cuff, I don't see a problem.

The limit seems to be based upon the current they need when in alarm. This has nothing to do with interconnection.

I would think that you could continue to link them all together with that third wire, even if the detectors are powered by different circuits.

I just inspected a home that had a need for 31 yes THIRTY ONE, smoke alarms and while it appears that it would be possible to interlock them electrically with some sort of relay system- the manufacturers tell me that that's not allowed due to the listing. As for a Fire alarm system- The State of Michigan will recognize it as satifying the requirements as called out in the Michigan Residential Code (same as the IRC only with the Michigan spin).

I am wrong about the alarm system. My wife says no, they use stand alone smokes.
Everything else is on the central system.

Give me a break, I have already been punished

Reno, what exactly does go down that 3d wire and would the smokes being on opposite phases change anything?
Do all brands work the same way?
I have never really looked into this.

As I understand it, that third wire only links some contacts together ... it there's continuity in one detector, they all sound off.

Yes that true John but if you have third wire interconneted and also the netural line as well as common return [ i have little trouble to grasp this idea how it work ]

but for hot's line it can be on common or diffrent phase or leg but i am sure but not excat answer for this one but i am thinking out loud that both red [ interconnection ] and white wires to actavie it

Merci , Marc

If they are saying when the white wire and the red wire are at the same (or close to the same) potential the alarm goes off but that implies it floats at 120v normally. If you tagged both sides of the panel with different detectors wouldn't that be 240v?
I tried to find out how my Kidde detectrors worked because I wanted a heat detector in the garage but they seemed very close mouthed about details.

G, I really don't know the internal details.

Nothing inside the detectors operates at 120. There's a battery, remember? You'd have to test one to find out for sure how they operate.

I guess I will look into it. You are right, this has to be 9v or so, which brings us back to where the horn drive comes from. You can use the neutral for a DC common but that puts battery voltage on the signal wire to blow the horn. If this is not buffered by the remote detector (relay or solid state) it still depends on the supply coming from the signalling detector. It even takes a bit of current to pick a relay. Probably more than the horn.

Now, that is one detail I can speak to ... in my testing of smoke alarms (lab setting), it was the horn that gobbled up the power. This, indeed, was the reason the first smokes had to use special batteris (F cells). The entire reason for the change to the higher-pitched electronic horns was that the mechanical ones simply used too much power.

If you need 24 or less devices try these out from Kidde.

http://www.kidde.com/utcfs/Templates/Pages/Template-53/0,8062,pageId%3D4363%26siteId%3D384,00.html

I saw that they say all the alarms must be connected to the same branch circuit "single continous (non-switch) power line, which is not protected by a GFCI". I guess AFCI is fine. It also says 18 ga wire up to 1000' is OK.

Greg- I'm still not comfortable with what I'm reading and I don't see anyone wiring smoke alarms (single station) with #18 wire. I think what was discussed in this thread has to do with a fire alarm system. Set me straight!

I am guessing they are saying the signal wire can be 18ga. That might be an issue with wire in pipe. I haven't seen any (2)14-(1)18/3-wg Romex

The link Rob posted is for the regular residential interconnected single station smoke alarm and it does say "up to 24" on the loop

Greg- I went back to that link and read the data sheets and it's talking about "devices" and includes mainly initiating devices (heat sensors, pull stations etc) and when you read all the data, you are still looking at 12 Smoke Alarm enunciating devices.

The 12 alarm, 18 enunciator limit seems to be from NFPA72 not the hardware limitation of the unit. They seem to be saying the unit itself will run 24 but NFPA is limiting them.
It still seems to be a single station smoke alarm, with optional interconnected devices, not a centrally controlled alarm system.
I suppose the final answer will be how it is listed.
For the original question, it does seem that NFPA limits you to 12 alarms in a house if they are all going to be interconnected. I suppose you need to take your question to them if you need more than 12. I am sure they have an answer

Why would your fire code limit the number of interconnected smokes? Why wouldn't they allow manufacturers to set the limits of their devices and then get them listed for that?

NFPA is out of control.