Wiring up my 250ex

[kenwhite]

I have and its was clear as mud for me

The green wire is used as the
ground and the black and white wires are
“hot” 110 V wires. To make a 220V circuit
work, two “hot” wires must be used from
separate phase 110V wires from the panel.
Do not attempt to wire the unit from the
same phase 110 V leg of the panel. Identify
your green ground wire first and properly
attach the wire to the ground prong.
The remaining black and white wires are
your 110V “hot” wires. Attach the two remaining
wires to the remaining two
prongs

What that paragraph is really trying to say is that 2 hot wires from the same phase would measure 0 VAC across them, so your box wouldn't work. Two hot wires from the opposing phases would generate 220/240 VAC so everything will work, and any hot measured to either neutral/return/ground would be 110/120 VAC...