Q:

Why does my GFCI receptacle trip when I connect the load wires?

I was trying to install a GFCI outlet and when I install it with the five wire setup with the line on top and load on bottom, the outlet continually trips. I made sure which wire was the hot-line and confirmed it by successfully wiring the GFCI with only the line wires in a three wire setup. I connected the load wires again and it tripped again? I even tried a second GFCI!
Ideas? The outlet works fine when a standard outlet is there, so it's got to be something simple. 

A:

It sure sounds like you have a ground fault on one of the down stream outlets or devices. Check the load side devices for proper wiring etc. Hook the circuit up without the GFCI and use a plug in tester to determine if any devices have reversed hot, open neutral, etc. Also check to see if there are any motor loads on the load side circuits, motors can cause nuisance tripping. 

A:

In addition to Shirlock Homes excellent answer, you should also check down stream of the circuit load if there exist any GFCI outlets ahead of this.  I have a GFCI outlet on the Load of another GFCI outlet then they can cause each other to trip.  If this is the case, you can simply replace the downstream GFCI outlet with a standard outlet as it will already be GFCI protected anyway.
Again things to check:

Short or ground fault down the circuit
Reversed polarity on receptacles downstream
GFCI outlets down the circuit.

