Weird Voltage Differential

Hello everyone,

I have been having a problem and the solution has been eluding me. Any information you could provide would be much appreciated. Here's the issue:

I am working on a site which has incoming 240VAC (single phase). Once the power comes in from the utility it runs into the main service panel. There are approximately 25 or so circuits ranging from 100 Amps to 15 amps.

With the main breaker in the off position (open circuit), each leg of the 240 reads exactly 120 VAC. When the main panel is energized, the voltage drops slightly on each legs to about 115VAC. Now, here's the really weird stuff:

Once the entire panel is energized, there's a voltage differential between each leg. One leg is 115 VAC and the other leg is 130 VAC. There is minor fluctuation as well. Now, when any of the circuits are put under a load, the voltage jumps up to approximately 130-138 VAC on its respective leg, and the other leg drops voltage approximately 5 to 15 volts (it seems to depend as to how much current is being drawn at any one particular time).

It's as if there is another power generator within the main service panel. But, the only incoming generator is the utility.

Bottom line, with the main service panel powered off, the incoming voltage from the utility is good (albeit there's no load on it). Once the entire panel is energized, there are voltage differential.

If anyone has any suggestions, I would appreciate your information. Thank you!