Voltage drop is a phenomenon in which the voltage through a circuit is converted to heat within the conductive material due to resistance within that material. Though voltage drop is of little consequence within a home or structure, it often comes into play when devices such as street lamps are powered several meters from a home or building. These large runs of conductive wire provide increased resistance that results in more voltage loss due to heat. Voltage drop can lead to improperly powered equipment or devices and in some cases may provide a safety hazard due to excessive heat within the conductive lines.
Determine the resistance within the conductive material being used to supply the street lamp with power. These resistances are standardised and well known for the commonly used gauges of wire and are listed within the National Electric Code for easy reference. This resistance is given in terms of ohms per foot.
Determine the voltage level to be used by the street lamp.
Multiply the intended length of wire by the ohms-per-foot rating for that wire. This is the resultant overall resistance within the circuit wire.
Multiply this resistance by the current within the wire. The current within the wire is dictated by the circuit to which the wire is connected in the breaker box of the building that connects the street lamp to the electrical grid. The resultant product is the estimated voltage drop within that specific run of cable.
If the voltage drop is excessive a larger, lower gauge wire, may be used to reduce the ohms-per-foot resistance and allow for a lower current to be used.