?? How would you have in, or out of phase choices to worry about on a single phase 240v system?
I am ready to learn something new today.
Short answer: You wouldn't if all you had was 240V, but this is a 120/240V split phase system.
Long answer:
Consider a single phase transformer that produces 240V across the wires that come off the end of secondary winding. Call those wires L1 & L2.
Consider if you then tap that winding at it's half-way mark (center tap). You will then have 120V from the center tap across 1/2 the winding to each of the end wires. (Note typically the center tap is referenced to (earth) ground, and we establish this as neutral 0V.)
So voltages are
L1-L2: 240V (with respect to each other)
L1-N (or ground): 120V
L2-N: 120V
From above we know we have 120V (to center tap) +120V (from center tap) = 240V across total winding.
But L1 and L2 are from opposite ends of the winding, so the (2) 120VAC waveforms are 180 degrees out of phase with each other.
There's better ways to explain or illustrate this...but consider if they were in phase with each other then L1-L2 would be zero volts (as they cancel each other) volts instead of adding to each other to be 240V.
...so in a 120V/240V panel board where the breakers are one side are alternately connected to L1 & L2 (Example: Left side breakers (poles) are odd numbered 1,3,5,7,9,11 etc....; then poles (breakers) 1, 5, 9,13, etc..are L1. Poles 3, 7, 11, 15, etc..are L2.
You can only get 240V from a double pole breaker that spans across two poles where one is L1 and the other is L2.
Now to answer your question: If someone tried to bring 240V to a external location but wired from two poles in the panel that were both "L1", both wires would be in phase with each other and there would be zero volts between the two wires instead of 240V. (although each wire would still be 120V compared to ground (and the center tap).
