are connected across the two line conductors. This means that (for the supply of the same amount of power) the current is halved. Hence, smaller conductors may be used than would be needed if the appliances were designed to be supplied by the lower voltage. If the load were guaranteed to be balanced, then the neutral conductor would not carry any current and the system would be equivalent to a single-ended system of twice the voltage with the line wires taking half the current. This would not need a neutral conductor at all, but would be wildly impractical for varying loads; just connecting the groups in series would result in excessive voltage and brightness variation as lamps are switched on and off. By connecting the two lamp groups to a neutral, intermediate in potential between the two live legs, any imbalance of the load will be supplied by a current in the neutral, giving substantially constant voltage across both groups. The total current carried in all three wires (including the neutral) will always be twice the supply current of the most heavily loaded half. For short wiring runs limited by conductor ampacity, this allows three half-sized conductors to be substituted for two full-sized ones, using 75% of the copper of an equivalent single-phase system. Longer wiring runs are more limited by voltage drop in the conductors. Because the supply voltage is doubled, a balanced load can tolerate double the voltage drop, allowing quarter-sized conductors to be used; this uses 3/8 the copper of an equivalent single-phase system. In practice, some intermediate value is chosen. For example, if the imbalance is limited to 25% of the total load (half of one half) rather than the absolute worst-case 50%, then conductors 3/8 of the single-phase size will guarantee the same maximum voltage drop, totalling 9/8 of one single-phase conductor, 56% of the copper of the two single-phase conductors. Balanced power In a so-called balanced power system, sometimes called "technical power", an isolation transformer with a center tap is used to create a separate supply with conductors at balanced voltages with respect to ground. The purpose of a balanced power system is to minimize the noise coupled into sensitive equipment from the power supply. Unlike a three-wire distribution system, the grounded neutral is not distributed to the loads; only line-to-line connections at 120 V are used. A balanced power system is used only