The Halacha establishes that rain water is not Mukse. Since it was already present in the clouds, there is no issue of Nolad- an object that came into being on Shabbat. However, Hacham Ovadia rules that condensation formed from an air conditioner is Mukse. Since it was not present in the air conditioner before Shabbat it constitutes a problem of Nolad. Therefore, it is prohibited to place a vessel underneath the air conditioner to catch the drip. Doing so would violate the prohibition of "En Mevatlin Keli MeHechano"-neutralizing a vessel from use on Shabbat by allowing Mukse to rest in it. The only solution is to place the vessel to catch the water before Shabbat.
SUMMARY
Water condensation from an air conditioner is Mukse. Therefore, one may not place a vessel underneath the air conditioner to catch the drip.
