The lights on the first floor, upstairs bathroom and upstairs Laundry room won't work. The breakers did NOT pop The air conditioning was running this morning but is not working now. When the thermostat turns the air off those lights turn off, but when the Tstat turns the air on, the lights come back on but they are slightly dim. I have to let the Tstat think the air is on and let it run the furnace fan constantly. If I turn the A/c off, either at the breaker or the main line at the A/C unit, all of the lights downstairs turn off...no the breakers do not pop but some of the GFI's pop. The compressor on the A/C is not working (no real air conditioning) I am in the USA. The The A/C is in the upper left of the panel, and most of the affected rooms are on the right side of the panel. The problem affects all but the last couple of breakers on the right side. The dryer, in the laundry room sounds like it isnt getting enough power and wont completely start. When I run the garbage disposal, all of the affected lights dim considerably.
Which country, Michael? USA, by any chance, judging from your jargon? If so... I think you may have a grounding issue at the panel or at the line coming in from the pole. I've seen similar issues, and issues with lights burning EXTREMELY brightly for a short while before they burn out, from just such grounding issues.
Especially if the AC and the 1st-floor lights are served by breakers near the bottom of the panel, or from a separate subpanel... in the former case, the grounding problem may be inside the panel itself. In the latter case, it may be in the subpanel or the wiring leading to it.
I sounds like the problem might be that the right side of your service panel either isn't connected to the main at all or has a really bad connection. I'm guessing what is happening is that when the A/C is "on", it is back feeding the right side of your panel through the breaker. This could also explain why your A/C compressor isn't working - it would be getting a 120v feed instead of a 240v feed.
This is something that would require testing the voltage at the mains in the box, and as @Tester101 pointed out in his first comment, fixing this isn't a DIY job. Cut the breaker to the A/C in the main service panel to keep this from doing any damage (I'm thinking electronics around the house), and get an electrician out there before you turn it back on.
Well, you guys are absolutely brilliant. The problem is nearly fixed. I had comEd come out do just what you said. It turns out that one of the phases coming into the house is bad! So everyone was spot on...the AC was operating at 120 and feeding the rest of the house when it was "on". Half the panel was out because of the bad phase.
The "boys in red" at ComEd put a jumper at the service box as a band aid. This solved all of the issues. They are coming back out to rip up the yard and replace the main feed to the house.
I couldn't be happier.
Thanks for your comments and feedback,