Strictly speaking there is no maximum length of EV charging cable. However, UK regulations for charging cables do specify that the minimum voltage drop along the length of the cable is 5%. The longer the cable, the more electrical resistance there is, which causes the voltage to drop. EV cable supplier Viper state that with a 7.4kW home charger, the maximum cable length you can use without exceeding the 5% voltage drop is 45 metres - you can view their products here - viper-ev.com/products/ev-charging-cables/