Anyone know where I can find a voltage drop calculator? I wanna figure out the wire size n fuses etc I should use to wire the leisure battery in for split charging? It’s a 420a leisure battery 
Nevermind, found what I need
Drop in volts = (L x I x 0.017) divided by A.
L = cable length in metres, I = current (in amps), A - cross sectional area of cable in mm.sq.