Voltage Drop Calculator

Understanding Voltage Drop

When electrical current moves through a wire, it is pushed by electrical potential (voltage) and it needs to surpass a certain level of contrary pressure caused by the wire. The voltage drop is the amount of electrical potential (voltage) loss caused by the contrary pressure of the wire. If the current is alternating, such contrary pressure is called impedance. Impedance is a vector, or two-dimensional quantity, consisting of resistance and reactance (reaction of a built-up electric field to a change of current). If the current is direct, the contrary pressure is called resistance.

Excessive voltage drop in a circuit can cause lights to flicker or burn dimly, heaters to heat poorly, and motors to run hotter than normal and burn out. It is recommended that the voltage drop should be less than 5% under a fully loaded condition. This can be achieved by selecting the right wire, and by taking care in the use of extension cords and similar devices.

Four Major Causes of Voltage Drop

• Wire Material: Silver, copper, gold, and aluminum are among the metals with the best electrical conductivity. Copper and aluminum are the most common materials used. Copper is a better conductor than aluminum and will have less voltage drop for a given length and wire size.
• Wire Size: Larger wire sizes (those with a greater diameter) will have less voltage drop than smaller wire sizes of the same length. In the American Wire Gauge (AWG) system, every 6-gauge decrease doubles the wire diameter, and every 3-gauge decrease doubles the cross-sectional area.
• Wire Length: Shorter wires will have less voltage drop than longer wires. Voltage drop becomes a serious issue when running wire over long distances, such as to an outbuilding or well pump.
• Current (Ampacity): An increase in current through a wire results in an increased voltage drop. The ampacity of a wire depends on the material, temperature, and whether cables are bundled together (which generates heat).

Voltage Drop Calculation Formula

Ohm's Law is the basic foundation for calculating voltage drop:

Resistance is often measured as length-specific resistance (e.g., ohms per kilometer). Also, because electricity must travel to the load and back, the wire length is round-tripped. Therefore, the formula for a single-phase or direct current (DC) circuit becomes:

The formula for a three-phase circuit becomes:

Typical AWG Wire Sizes & Resistance Chart

American Wire Gauge (AWG) is a wire gauge system used predominantly in North America for the diameters of round, solid, non-ferrous, electrically conducting wire. The following is a list of typical AWG wires, their sizes, and their copper resistance.