My old setup:
 
6mm2 cable from under dash main terminal. 5 m cable
 Voltage on the battery 14.25v
 V under dash which is OEM connected to the battery with 10mm2 cable 14.25v
 Everything that uses current was shut off, the car was ideling.
 Voltage on the relay at the pump showed 13.8 v
 That was .45 loss.

 Then I turned on lights, clima, stereo... Car was still ideling.
 V on the battery 14v
 V under dash 14v
 V at the pump 13.4


 Then I rewired with 10mm2 cable straight from battery to relay. 5.5 m cable.
 Everything else was the same.

 With car ideling and everything else shut off
 V at battery 14.25
 V under dash 14.25
 V at pump 14v

 With everything turned on
 V at battery 14v
 V under dash 14v
 V at pump 13.8v

 This shows that being DC current needs very fat gauge cable to minimize voltage loss.
 Remember all of this is tested with car ideling. Up in full rev and power demand losses would be even greater.
 
So- use really fat wire from under dash of straight from battery if you want too keep your voltage losses minimal, to be able to get most of your fuel setup.

30Amps was placed back at the pump, and the ground Cable kept as short as possible