How does the Enphase D380 know the difference between going single- or three-phase?

rockpaw, 

All Enphase D380 inverters generate 208V or 240V. The trunk cables split the inverter outputs into phases.

The 208V inverters put out 120V on two neutral-hot circuits. The third hot leg is a 120/208 in a three phase system.

Also check this two (208V vs. 240V):

http://www.enphaseenergy.com/downloads/Enphase_Field_Wiring_Diagram_D380_208v.pdf

http://www.enphaseenergy.com/downloads/Enphase_Field_Wiring_Diagram_D380_240v.pdf

You can run;

• a 208V single phase with a 240V trunk cable,
• a 240V single phase with a 240V trunk cable,
• a 208V 3-phase with a 208V trunk cable.