Copper is used for most electrical wiring because of its low cost and excellent heat transfer and conduction properties.  Traditional AC fixtures are commonly wired in parallel at 120VAC.  LED fixtures usually run on DC current; they commonly use a driver to convert 120VAC to 12VDC.

LEDs can be wired in parallel or series.  When LEDs are wired in parallel, the positive terminal of each fixture connects to the positive side of the system, and each negative terminal connects to the negative side of the system.  In such a parallel circuit, each 12VDC light added on adds to the total current in the system.  The main advantage of wiring in parallel is plug and play, meaning the ability to easily add new fixtures to the system without reconfiguration.  LED fixtures can also be wired using series DC wiring.  In a series DC circuit the negative terminal of each LED fixture connects to the positive terminal of the next LED fixture.  Wiring in series has the advantage of a lower total current in the circuit.  Compared with parallel wiring, this will significantly reduce the power consumption of the wiring and thus has the advantage that the LED power supply may be installed farther away from the LED fixtures.

If five identical light fixtures are wired as either a series or parallel circuit, the power wasted through the wiring (between driver or power source and the fixtures) in the parallel circuit will be 25 times greater than that in the series circuit.  The voltage drop across the wiring in the parallel circuit will be five times greater.  For example, in a 33W system with five 12VDC lights (each at 700mA), the wattage wasted on 100ft of 18Gauge wire is ~7.8W in a parallel circuit and only ~0.3W in a series circuit.