Voltage Drop Formula & Examples

Voltage drop is the reduction in voltage that occurs while current travels through conductor impedance. Every practical conductor has resistance. As current or conductor length increases, more voltage is lost before the load receives power. A long feeder, a heavily loaded branch circuit, or a conductor that is too small can therefore deliver less voltage than the source provides.

Voltage drop is different from ampacity. Ampacity asks whether a conductor can carry current without exceeding its allowable temperature. Voltage drop asks how much voltage is lost along the route. A final design may need to satisfy both checks, along with overcurrent protection, installation method, terminations, ambient temperature, and local requirements.

For a single-phase or DC circuit using resistance only, a common planning formula is Vdrop = 2 × I × L × R. Here, I is current in amperes, L is the one-way length, and R is conductor resistance per unit length. The factor of 2 represents the outgoing and return conductors. If the resistance is listed in ohms per foot or ohms per meter, keep the length in the same unit.

For a balanced three-phase circuit, a common resistance-only planning relationship is Vdrop = √3 × I × L × R. The circuit factor changes because the three-phase line-to-line relationship is different from a two-wire single-phase path. Real AC circuits may also include reactance and power-factor effects, so manufacturer data or an engineering method may be needed for a detailed design.

Suppose a 120 V single-phase load draws 15 A through a copper conductor with a resistance of 0.0016 ohms per meter. The one-way cable distance is 30 m. The estimated drop is 2 × 15 × 30 × 0.0016, or 1.44 V. The percentage drop is 1.44 ÷ 120 × 100, which is about 1.2 percent.

This result is a voltage-drop estimate, not an automatic approval. The conductor still needs an ampacity check, and the actual installation may have different resistance because of temperature, conductor construction, connections, parallel paths, or AC reactance. Use the example to understand the formula, then enter the project values in the Voltage Drop Calculator.

Metal resistance generally increases as conductor temperature rises. A warm conductor therefore produces more loss than the same conductor at its reference temperature. This is why a planning calculation can show adjusted resistance for a selected temperature. Manufacturer resistance tables remain the preferred source when a final design requires product-specific data.

Three percent is often used as a design target for a branch circuit, while five percent is commonly discussed as a total-circuit planning target. These are design conventions, not a universal replacement for the applicable code, equipment instructions, or project specification. A result below a target does not prove that the wire, breaker, or installation method is correct.

A voltage-drop calculation does not select a final wire size by itself. The Wire Size / Ampacity Calculator can screen load current, continuous-load planning, ambient temperature, and current-carrying conductors. Larger services and motor loads may require additional checks for starting current, phase balance, harmonics, fault current, and equipment ratings.

Electrical work can cause shock, fire, equipment damage, or death when improperly designed or installed. Use this guide and the calculators for preliminary planning only. Have the final design reviewed and the work performed or inspected by a qualified electrical professional under the applicable local requirements.