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.