Voltage drop is the decrease in electrical potential along the path of a current flowing in an electrical circuit. High voltage drop can cause lights to flicker, motors to run hot, and electronic equipment to malfunction.
The Mathematical Formula
The standard calculation for voltage drop uses the formula:
Vdrop = (K × I × L × Factor) / Area
I: Current in Amperes.
L: One-way distance of the conductor in feet.
Factor: 2 for Single Phase (return path), 1.732 (√3) for Three Phase.
K: Resistivity constant (approx. 12.9 for Copper, 21.2 for Aluminum).
NEC Recommendations
The National Electrical Code (NEC) recommends a maximum voltage drop of 3% for branch circuits and 5% for the combined feeder and branch circuit to ensure efficiency and equipment safety.
Practical Example
Suppose you have a 120V circuit, using 12 AWG copper wire, carrying 15 Amps over 100 feet.
Resistance of 12 AWG Copper is roughly 1.98 Ohms per 1000ft.
Voltage Drop = (2 * 100ft * 1.98 Ω * 15A) / 1000 = 5.94V.
Percentage = (5.94 / 120) * 100 = 4.95%.
In this case, the drop exceeds the recommended 3%, and you might consider upgrading to a 10 AWG wire.
function calculateVoltageDrop() {
// AWG Resistance values (Ohms per 1000ft at 75C)
// Ref: NEC Chapter 9, Table 8
var resistanceData = {
"14": { copper: 3.07, aluminum: 5.04 },
"12": { copper: 1.93, aluminum: 3.17 },
"10": { copper: 1.21, aluminum: 1.99 },
"8": { copper: 0.764, aluminum: 1.26 },
"6": { copper: 0.481, aluminum: 0.79 },
"4": { copper: 0.302, aluminum: 0.497 },
"2": { copper: 0.190, aluminum: 0.312 },
"1": { copper: 0.151, aluminum: 0.247 },
"1/0": { copper: 0.119, aluminum: 0.196 },
"2/0": { copper: 0.0948, aluminum: 0.156 },
"3/0": { copper: 0.0751, aluminum: 0.123 },
"4/0": { copper: 0.0596, aluminum: 0.098 }
};
var phaseFactor = parseFloat(document.getElementById("vd_phase").value);
var material = document.getElementById("vd_material").value;
var voltage = parseFloat(document.getElementById("vd_voltage").value);
var current = parseFloat(document.getElementById("vd_current").value);
var distance = parseFloat(document.getElementById("vd_distance").value);
var wireSize = document.getElementById("vd_wire_size").value;
if (isNaN(voltage) || isNaN(current) || isNaN(distance) || voltage <= 0 || current <= 0 || distance 3) {
warningEl.innerText = "Warning: Voltage drop exceeds the NEC recommended 3% limit!";
} else {
warningEl.innerText = "Efficiency Tip: This configuration is within recommended limits.";
warningEl.style.color = "#27ae60";
}
document.getElementById("vd-results").style.display = "block";
}