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I read somewhere about plugging lights into different phases. If I remember correctly your electrical box has 2 phases, the right side and left side. That being said since I have 2 30 amp controllers should I be sure to plug 1 controller into 2 outlets on the right side and the other controller into 2 outlets from the left side? Or does that even matter? I have 100 amp service btw.
On split phase (residential) power, the inlet cords do not need to be on the same end of the split phase.. They can be the same or opposite with no issues.. If they are on the same end, the worst voltage differential in your controller will be 120V. If they are on opposite ends, you could in theory get 240V from touching the wrong combination of points..
Generally to get best utilization of the power available to your house, you do want to draw approximately the same amount of power from both sides of the split phase. Otherwise you will get to the labeled trip current on your main breaker faster than you would otherwise.
However, I have never encountered any panel where the split phase was separated right to left. Normally, it alternating rows of breakers are alternating ends of the split phase, so two adjacent spaces provide 240V power. So in a typical numbering scheme where the breakers are numbered top to bottom along the left, and again down the right, typically all the even numbered spaces are on one end of the split phase, and the odd numbered spaces are on the opposite side of the split phase.
Technically it is not left and right. It is every other row. Think of how you fold your hands together for prayer. The fingers (not thumbs) alternate between left and right hands. Similar concept on how the bus bars inside your breaker box are set up.
If your breakers in the left side are numbered sequentially and then the right column starts at 11 (for a 20 space) or 21 (for a 40 space), then it is actually odd and even. If the breaker numbers are odd on the left column and even on the right, I'm not sure of a good logic to use.
____________________ -Dave
It's "Merry Christmas" not "Happy Holidays"
I'm not an electrician. I have a lot of electronics training, but wiring and power for my house was kind of a mystery. I knew that I had 220V coming into the house and somehow I also had 110V. Then I finally saw a picture of a "center tapped transformer" that is actually what supplies my house (and probably yours!) with power. A center tapped transformer, as shown in the picture I ripped from Wikipedia, finally explained why the two phases were EXACTLY 180 degrees out of phase and had a common ground.
The side labeled V1 would be Phase A and V2 would be Phase B and these connect to your breaker panel. As earlier posters said, the phases alternate down the panel so that breakers on the same side go A B A B A B A B and on the other side go B A B A B A B A. A two pole (220V) breaker just connects to an A and a B that are next to each other.
The center tapped transformer drawing is exactly how houses are fed.
If the left and right sides both start on the same row, (as normal) they would both start with A... Hopefully this photo of the buss that the breakers plugs into will help.
You can see how the buses run down the left and right sides, but they take turns feeding the tab that the breaker plugs into, and the tab accepts one breaker on the left, and one on the right...
(No, this is not any panel I have any association with, just a photo I found on photobucket)
- Kevin
of AC and 
of DC from D-Light