over the years I have seen a few problems with the interconnect between smokes, while most of the time the neutral will not be long enough to cause a problem but remember that the interconnect is using the neutral as part of the 9 volt DC path to trip the other alarms, in most of the problems I have seen was where someone wired the smokes to each rooms power, in many cases this was putting the smokes on different circuits.
while in most cases this might not cause a problem but if the run is very long I.E. from that room to the panel back up to another smoke can cause problems with the interconnect having enough voltage to signal the other smokes, remember we are only talking about 9 volts, and a very limited current to begin with, the resistance of the neutral can further limit the amount of current the other smokes will receive and may not allow them to alarm.
Now for the second problem I have run into, think about what would happen if you lost the neutral to one of the smoke circuits, you would now have 120 volts on that neutral, would the interconnect care? It could since the other smokes are still referencing the neutral on their circuit, this would put the interconnect circuit in series with this 120 volts by going through the lost neutral smoke and back feeding the other smokes from the red wire? will this cause failures? well in one case I had 3 out of 7 smokes that had failed interconnects, re-fed them on one circuit (Hall) which put them on one circuit with one common neutral and the interconnect problem went away.
Wiring them on only one circuit that has the same neutral will not do the above as loosing the neutral will just put both the interconnect and the neutral at the same potential, of course loosing a neutral between two smoke would not be good.
I have never just wired smokes on different circuits, I usually put them on a single circuit that is used allot like a kitchen lighting or hall or bedroom lighting, so if it is out they at least have it fixed, for this reason I also never put them on their own circuit.
The simple workings of how they cause the other smokes to sound is nothing but a 9 volt DC voltage sent from the tripped smoke to the others, they started using this voltage so the interconnect would still work even if the smoke was being powered by just the battery.
So with the above known you should see that testing the interconnect is doing nothing more then touching a 9 volt battery negative to the neutral and the positive to the red interconnect wire should cause the smoke to sound, there is a dropping resistor on the output of each smoke to protect it from over voltage and interconnect shorts but when hit with 120 volts it well e'rrr lets say smokes, then the interconnect is gone, I'm surprised Gar hasn't posted any reverse engineering on smokes as he has on other devices, I did this to find a way to test the interconnects after having to just replace a whole house full of smokes, after seeing we can test them we saved allot of money by only replacing the ones that were bad.
So even if a single wire is allowed by code, my vote would be no don't do it because of the problems of putting the smokes on different circuits can cause you many problems.
