Valcom designed their system for phone guys using the wire they had- two pair 24ga CAT3. One pair for audio and one pair for power. The audio pair doesn't have really any restriction in length but the power does. If you go to the Valcom website they have a table of various gauges and permissible lengths and number of speakers. But since you say they are using multiple power supplies I don't think that distance will be a factor. If you do decide to use 18ga cable, no problem. Just make sure that it is two pair twisted.

-Hal


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