on the gfci it has 2 sets of terminals on the back.line and load.line is the power coming and the load is the power going out. 20 amp feed through means that the load can be up to 20 amp circuit meaning if you install the gfci on a 20 amp breaker with #12 wire it will be perfectly legal to do so but the gfci socket itself is rated for 15 amps so you don’t want to plug anything into it that draws more than 15 amps.make sure you don’t connect the incoming power to the load terminals.the gfci will not work properly.
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