K-Mart's were notorious for using 25 volts. The bad news was those systems don't take to kindly to employees "adding a speaker". You can usually tell that because you can fry an egg on the amp. They just keep on turning up the volume while the amp works its heart out to compensate for the 8 ohm load across the speaker line.

The only mistake I made was learning the reason you don't put your Mic cable in the same run as the speaker line. Probably the same reason you don't put Valcom amplified speakers in the same run as your speaker lines.... Fortunately I only did it once :-)

Carl