Voltage drops when turn on electric fan

Conclusion, I had the alt checked today and it is fine even with load. So the winning guess is number 16 - 67Dart273 with belt slippage. All weekend I kept getting belt chips all over the engine bay. I put a new belt on today and went to tighten the lower bolt through the slot bracket and it kept spinning, so I had to put a nut on back side and now I have 14.5 at idle and 14 with fan and lights on, rev up and it goes back up to 14.5. So I guess with the bolt not holding the alt tight I was getting slippage. Oh and as I mentioned in my #19 reply I had 2 shows to go to this past weekend, at the show sat. I got a trophy for pro street tubbed. So thanks for all the responses.