With that 20 amp fuse, I hope you are not using smaller than 12 ga wire, 14 ga at a bare minimum, and the current is not going thru the J-box.
While I do not personally recommend it 14 AWG will support 20 amps for lengths under 18 feet.
Note that wire sizes for lighting is more critical than for other applications — The rated output of a lamp is figured at 13.5 volts, not 12. So with a 1/2 volt drop you are at 13.0 volts. And at 95% of the rated voltage, you are only putting out 80% of the rated luminous intensity - for a 100 watt lamp that’s only 80 watts!! Get what you pay for and figure to the high side when you are sizing wire for lighting.