Voltage Drop Calculation Formulas Explained

The Foundation: Ohm's Law

All voltage drop calculations are built upon Ohm's Law, one of the most fundamental relationships in electrical engineering. Discovered by Georg Ohm in 1827, this law establishes the direct relationship between voltage, current, and resistance in an electrical circuit. Understanding this foundation is essential before diving into more complex voltage drop formulas used in professional electrical design.

In the context of voltage drop, we apply this law to the conductor itself. The conductor acts as a resistor (albeit a small one), and the current flowing through it creates a voltage drop across its length. This is why we can calculate voltage drop by multiplying the current by the conductor's total resistance.

Single-Phase and DC Voltage Drop Formula

For single-phase AC circuits and DC circuits, the voltage drop formula accounts for current flowing through both the outgoing and return conductors. This is why we multiply by 2—the total circuit length includes both conductors in the circuit.

The factor of 2 represents the round-trip current path. Current flows from the source through one conductor to the load, then returns through the other conductor. Both conductors contribute to the total voltage drop. The division by 1000 converts the resistance units from ohms per 1000 feet to match our length in feet.

Three-Phase Voltage Drop Formula

Three-phase systems use a different multiplier because of the 120° phase relationship between the conductors. Instead of multiplying by 2, we use √3 (approximately 1.732). This factor accounts for the vector relationship between phases in a balanced three-phase system.

In a balanced three-phase system, the currents in the three conductors are equal in magnitude but displaced by 120° in phase. The √3 factor emerges from the vector addition of voltages in the three-phase configuration. For line-to-line voltage calculations, this factor properly accounts for the relationship between phase and line quantities.

Percentage Voltage Drop

While absolute voltage drop in volts is useful, percentage voltage drop provides a standardized way to evaluate circuit performance regardless of system voltage. NEC recommendations are expressed as percentages, making this the most practical form for compliance checking.

A 3% voltage drop at 120V equals 3.6 volts, while 3% at 480V equals 14.4 volts. The percentage representation allows direct comparison and compliance verification regardless of the voltage level involved.

Resistance Values: NEC Chapter 9 Table 8

Conductor resistance values used in voltage drop calculations come from NEC Chapter 9, Table 8. This table provides DC resistance values for both copper and aluminum conductors at 75°C. For most practical voltage drop calculations using the simplified formula, these DC resistance values provide acceptable accuracy.

Try the Calculator

Now that you understand the formulas, put them into practice with our professional voltage drop calculator. Input your circuit parameters and get instant, accurate results with NEC compliance checking.