Without more information I'm guessing a bit, but I'll bet that you're connecting your generator's 220VAC across your house's two 120VAC phases, with no neutral connection. That will work fine, IF you are drawing equal amounts of power from the two phases.
But what if you aren't? Let's say you plug a 1500W hair drier into one 120VAC outlet, and a 100W lightbulb into another 120VAC outlet, and they're on opposite phases. The only path for the current from the generator goes as follows:
- From one generator output into the hair drier
- Out of the hair drier into the house neutral
- Out of the house neutral into the lightbulb
- Out of the lightbulb back into the generator
So, you've basically connected the hair drier and lightbulb in series across the generator. But, since the hair drier's impedance is so much lower than the lightbulb, the lightbulb will bear the brunt of the 220VAC generator output, and will blow before the hair drier even starts to get warm.
(This is a general principle; if you have two devices in electrical series, the one with the greatest resistance will dissipate the greatest amount of power.)
So, as you flick the circuit breakers on, you're inevitably adding unbalanced loads to the two sides of your breaker panel. Bingo: the less-loaded side will see higher voltage, perhaps even a drastically higher voltage.
(Note that even if your generator has a neutral, and you're using it, there's no guarantee that drawing a lot of current from one of the two phases won't mess with the voltage on the other phase.)
For way too much detail, here's a simplistic calculation, based on the standard voltage, current, resistance and wattage calculations:
- Current (amps) = Voltage (volts) / Resistance (ohms)
- Power (watts) = Voltage * Current = Voltage^2 / Resistance
Based on its wattage, the resistance of the lightbulb is:
- 100 Watts = 110V^2 / Resistance
- 100 Watts = 12100 / Resistance
- Resistance = 12100 / 100 = 121 ohms
Based on its wattage, the resistance of the hair drier is:
- 1500 Watts = 110V^2 / Resistance
- 1500 Watts = 12100 / Resistance
- Resistance = 12100 / 1500 = 8 ohms
The total current of the circuit is:
- Current = Voltage / Resistance
- Current = 220 / (121 + 8) Ohms
- Current = 1.7 amps
Voltage across the hair drier is:
- Voltage = 1.7 amps * 8 ohms = 13.6 volts
Probably not enough to even spin the blower. But, what's happening to our poor little 100W lightbulb? Let's see:
- Voltage = 1.7 amps * 121 ohms = 205 volts.
- Power = Voltage * Current = 205 * 1.7 = 350 Watts. (Pop!)
(To check the calculations: if you add the two calculated voltages, you get 205V + 13.6V = 218.6V, which is close enough to 220V for our purposes.)