I installed a heated floor & was using a 15 amp Gfci but was fine until i energized the floor. So I purchased a 20amp GFCI do I need to use 12-2 wire for that even to test it. The 20 amp keeps tripping after i hook up the themostat. Bottom line do I have to have 12-2 wire when using a 20 GFCI to keep it from tripping.
The amperage would have no bearing on weather or not it will trip. The GFCI is reacting to the imbalance between hot and neutral which could be from the high inrush current when the heated floor is turned on.