Why does my A/C drain line overflow?

The drain line or drain pan gets clogged up. This is caused by dirt that has accumulated on the evaporator coil and has washed down into the drain pan and drain line. Also if water is sitting in the pan or drain line and does not drain out completely it can cause algae to grow causing the system to overflow. If the system pulls air through the evaporator coil the drain line must have a P-Trap installed in it, this prevents the blower from pulling air up through the drain line causing a negative pressure on the line which will allow the condensation to drain.