I was working in an adult care home last week when I had to remove a smoke alarm that was in my way. When I pulled it off the mount, I discovered it was wired to line voltage with 4 conductor telephone wire. Upon investigation I found the entire system was wired and interconnected with the same phone wire, plus no boxes, just flying splices tucked away in the walls behind each unit!
Before I submit a bid for rewiring the entire system, which would require a lot of cutting into walls (this is a residential structure) I thought there might be a low voltage interconnected system that I could use with the existing low voltage 4 conductor wire.
Anyone know of such a system?

Whatever hack installed this one in the first place probably took a lot of other shortcuts as well. Hidden ones. Before you take on the liability of assuming his wiring is good for a low voltage system, think it through. I would bid this to run new wiring. Then you know it is well done and will work when it needs to.

I'm currently doing a fire alarm job at a children's psychiatric center. All of our smoke detectors and duct detectors are low voltage where we either use 1-pair 16ga. shielded cable or a 2-pair 16ga. shielded cable, depending on what circuit we're working on. But yes, everything we do is on 24v. The only thing we have done on 120v, in this fire alarm install, is for a couple of duct damper switches. Other than that, low-voltage.