The National Grid is a network of transformers and cable which supplies most of the UK with electricity from power stations.Long-distance transmission of electricity is more efficient at high voltage so the voltage is stepped up to 275,000V or 400,000V a.c..Factories are supplied with all three phases at 33kV or 11kv, houses are supplied a single phase from a local sub-station at 230V a.c.If the current is high, the power losses due to the heating effect of the current I2R are high so for long-distance transmission the voltage is stepped up so the current is low.