Estimate the voltage drop across a wire run from its resistance, length, load current, and phase.
Current has to travel down the wire and back, so the effective resistance is doubled for DC and single-phase AC circuits: Voltage Drop = 2 × I × R × D. Three-phase circuits use √3 instead of 2, since the phases share the return path differently. R is the wire's resistance per unit length, and D is the one-way run distance.
A voltage drop under 5% of the source voltage is the generally recommended limit — above that, lights can flicker or dim, heaters underperform, and motors run hotter and wear out faster. Running thicker wire (lower resistance) or shortening the run both reduce drop; adding parallel conductor sets per phase splits the current and reduces the effective resistance the same way.
Manufacturer datasheets list it, often in ohms per 1,000 feet or ohms per kilometer. NEC Chapter 9, Table 8 also publishes standard DC resistance values for common copper and aluminum wire gauges.
No — this uses resistance only, which is accurate for DC and a reasonable approximation for AC at typical power-line frequencies over shorter runs. For precise AC work with reactance, use impedance data from your wire's specification.
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