Selecting the correct wire size for a Direct Current (DC) electrical system is crucial for safety, efficiency, and reliability. An undersized wire can lead to overheating, fire hazards, increased energy loss due to voltage drop, and reduced performance of connected devices. An oversized wire is simply more expensive and harder to work with.
This calculator helps determine the appropriate American Wire Gauge (AWG) size needed for a specific DC circuit based on several key parameters:
Key Factors and Their Importance:
Current (Amps): The amount of electrical current the wire will carry. Higher current requires a larger diameter wire to prevent overheating.
Voltage (Volts): While not directly used in the AWG lookup for a given current, it's essential for calculating the acceptable voltage drop in absolute volts (Voltage Drop = Current x Resistance).
Wire Length (Feet): Longer wires have higher resistance, leading to greater voltage drop and heat generation. The calculator considers the total length of the wire run (positive and negative conductors combined).
Maximum Allowable Voltage Drop (%): This is the percentage of the source voltage that can be lost across the wire. Sensitive electronics may require a low voltage drop (e.g., 1-3%), while general power transmission might tolerate more (e.g., 5-10%). A lower allowable drop necessitates a larger wire.
Wire Material: Copper is the most common and efficient conductor, offering lower resistance than aluminum for the same gauge. Aluminum is lighter and cheaper but requires a larger gauge for equivalent conductivity.
Ambient Temperature: Wire ampacity (current-carrying capacity) decreases as temperature increases. This calculator adjusts for temperature based on standard NEC (National Electrical Code) tables for common insulation types.
The Calculation Logic:
Calculate Total Resistance Needed:
First, determine the maximum allowable voltage drop in volts: Max Voltage Drop (V) = Voltage (V) * (Max Voltage Drop (%) / 100)
Then, using Ohm's Law (R = V/I), calculate the maximum resistance the wire can have: Max Resistance (Ohms) = Max Voltage Drop (V) / Current (A)
The total wire resistance is also calculated as: Wire Resistance (Ohms) = (Resistance per foot * Total Wire Length (ft)). We need to find the wire gauge where (Resistance per foot * Total Wire Length (ft)) <= Max Resistance (Ohms).
Determine Resistance Per Foot for Each AWG: Standard tables provide the DC resistance of each AWG size for copper and aluminum at a reference temperature (e.g., 20°C for copper).
Temperature Correction: The resistance of conductors increases with temperature. A correction factor is applied to the resistance per foot based on the ambient temperature. For copper: R_T = R_ref * [1 + α * (T - T_ref)], where α (temperature coefficient) is approx 0.00393/°C for copper and T_ref is 20°C. For aluminum, a similar formula applies with a different α.
Iterate and Find Suitable AWG: The calculator iterates through AWG sizes, starting from smaller gauges, calculating the total resistance for the given length and temperature. It finds the smallest AWG number (which corresponds to the largest wire diameter) where the total calculated resistance is less than or equal to the maximum allowable resistance derived from the voltage drop.
Ampacity Check (Simplified): While this calculator prioritizes voltage drop, a full calculation also considers ampacity (the maximum current a wire can safely carry without exceeding its temperature rating). This calculator uses typical ampacity guidelines as a secondary check, especially for higher currents where overheating is a primary concern. The NEC provides detailed tables for ampacity based on wire size, material, insulation type, and installation conditions (e.g., in conduit, free air).
Example Usage:
Consider a solar power system:
You have a load drawing 25 Amps.
The nominal system voltage is 12 Volts.
The wires need to run 75 feet from the battery bank to the inverter.
You want to limit the voltage drop to a maximum of 3% to ensure optimal inverter performance.
You are using Copper wire.
The expected ambient temperature is 40°C.
Inputting these values into the calculator will yield a recommended AWG size, ensuring sufficient conductivity and minimizing energy loss.
Disclaimer:
This calculator provides an estimate for common applications. Always consult with a qualified electrician and refer to local building codes and the National Electrical Code (NEC) for specific installations, especially for critical applications or where safety is paramount. Factors like wire insulation type, installation method (conduit, free air, bundling), and specific device requirements can influence the final choice.
// Constants for Resistance (Ohms per foot at 20°C)
var copperResistancePerFoot = {
"0": 0.0000998, "1": 0.000126, "2": 0.000159, "3": 0.000199, "4": 0.000252,
"5": 0.000318, "6": 0.000401, "7": 0.000505, "8": 0.000636, "9": 0.000802,
"10": 0.00101, "11": 0.00127, "12": 0.00160, "13": 0.00201, "14": 0.00253,
"15": 0.00318, "16": 0.00401, "17": 0.00505, "18": 0.00636, "19": 0.00802,
"20": 0.0101, "21": 0.0127, "22": 0.0160, "23": 0.0201, "24": 0.0253,
"25": 0.0318, "26": 0.0401, "27": 0.0505, "28": 0.0636, "29": 0.0802,
"30": 0.101, "31": 0.127, "32": 0.160, "33": 0.201, "34": 0.253,
"35": 0.318, "36": 0.401, "37": 0.505, "38": 0.636, "39": 0.802,
"40": 1.01
};
var aluminumResistancePerFoot = {
"0": 0.000165, "1": 0.000208, "2": 0.000263, "3": 0.000331, "4": 0.000417,
"5": 0.000526, "6": 0.000663, "7": 0.000835, "8": 0.00105, "9": 0.00132,
"10": 0.00166, "11": 0.00209, "12": 0.00263, "13": 0.00331, "14": 0.00417,
"15": 0.00526, "16": 0.00663, "17": 0.00835, "18": 0.0105, "19": 0.0132,
"20": 0.0166, "21": 0.0209, "22": 0.0263, "23": 0.0331, "24": 0.0417,
"25": 0.0526, "26": 0.0663, "27": 0.0835, "28": 0.105, "29": 0.132,
"30": 0.166, "31": 0.209, "32": 0.263, "33": 0.331, "34": 0.417,
"35": 0.526, "36": 0.663, "37": 0.835, "38": 1.05, "39": 1.32,
"40": 1.66
};
// Temperature Coefficients (per °C)
var tempCoeff = {
"copper": 0.00393,
"aluminum": 0.00390
};
var referenceTemp = 20; // Reference temperature in °C
// Ampacity Table (AWG to Max Amps, approximate NEC values for 75°C copper, 75°C Al)
// This is a simplified lookup and might not cover all insulation types or conditions.
var ampacityTable = {
"18": {"copper": 16, "aluminum": 13},
"17": {"copper": 19, "aluminum": 15},
"16": {"copper": 22, "aluminum": 18},
"15": {"copper": 25, "aluminum": 20},
"14": {"copper": 30, "aluminum": 25},
"13": {"copper": 35, "aluminum": 28},
"12": {"copper": 40, "aluminum": 32},
"11": {"copper": 45, "aluminum": 36},
"10": {"copper": 50, "aluminum": 41},
"9": {"copper": 55, "aluminum": 44},
"8": {"copper": 70, "aluminum": 55},
"7": {"copper": 80, "aluminum": 65},
"6": {"copper": 95, "aluminum": 75},
"5": {"copper": 105, "aluminum": 85},
"4": {"copper": 115, "aluminum": 95},
"3": {"copper": 125, "aluminum": 105},
"2": {"copper": 135, "aluminum": 115},
"1": {"copper": 150, "aluminum": 125},
"0": {"copper": 170, "aluminum": 140},
"2/0": {"copper": 195, "aluminum": 165},
"3/0": {"copper": 225, "aluminum": 190},
"4/0": {"copper": 260, "aluminum": 215}
// Add more if needed for larger gauges
};
function calculateWireSize() {
// Clear previous error messages and results
document.getElementById("errorMessages").textContent = "";
document.getElementById("wireSizeResult").textContent = "–";
document.getElementById("voltageDropResult").textContent = "";
// Get input values
var currentAmps = parseFloat(document.getElementById("currentAmps").value);
var voltageVolts = parseFloat(document.getElementById("voltageVolts").value);
var wireLengthFeet = parseFloat(document.getElementById("wireLengthFeet").value);
var voltageDropPercent = parseFloat(document.getElementById("voltageDropPercent").value);
var wireMaterial = document.getElementById("wireMaterial").value;
var temperatureC = parseFloat(document.getElementById("temperatureC").value);
// Validate inputs
var errors = [];
if (isNaN(currentAmps) || currentAmps <= 0) errors.push("Please enter a valid positive current in Amps.");
if (isNaN(voltageVolts) || voltageVolts <= 0) errors.push("Please enter a valid positive voltage in Volts.");
if (isNaN(wireLengthFeet) || wireLengthFeet <= 0) errors.push("Please enter a valid positive wire length in Feet.");
if (isNaN(voltageDropPercent) || voltageDropPercent 100) errors.push("Please enter a valid voltage drop percentage between 1 and 100.");
if (isNaN(temperatureC)) errors.push("Please enter a valid ambient temperature in Celsius.");
if (errors.length > 0) {
document.getElementById("errorMessages").textContent = errors.join(" ");
return;
}
// Calculate maximum allowable voltage drop in Volts
var maxVoltageDropVolts = voltageVolts * (voltageDropPercent / 100);
// Calculate required maximum resistance per foot
var totalWireResistanceOhms = maxVoltageDropVolts / currentAmps;
var maxResistancePerFoot = totalWireResistanceOhms / wireLengthFeet;
// Determine available resistance data and temperature coefficient
var resistanceData = (wireMaterial === "copper") ? copperResistancePerFoot : aluminumResistancePerFoot;
var tempCoeffValue = tempCoeff[wireMaterial];
// Get sorted AWG keys (numeric sort)
var awgSizes = Object.keys(resistanceData).map(Number).sort(function(a, b) { return a – b; });
var recommendedAWG = null;
var calculatedVoltageDrop = null;
var actualResistancePerFoot = null;
var actualWireSize = null; // To store the AWG string representation
// Iterate through AWG sizes from smallest to largest (highest resistance to lowest)
for (var i = awgSizes.length – 1; i >= 0; i–) {
var awgNum = awgSizes[i];
var refResistancePerFoot = resistanceData[awgNum];
// Apply temperature correction
var correctedResistancePerFoot = refResistancePerFoot * (1 + tempCoeffValue * (temperatureC – referenceTemp));
// Check if this wire gauge meets the resistance requirement
if (correctedResistancePerFoot <= maxResistancePerFoot) {
// This gauge is sufficient for voltage drop. Now check ampacity.
// Find the string representation of the AWG for ampacity lookup
var currentAWGString = String(awgNum);
if (awgNum === 0) currentAWGString = "0"; // Handle AWG 0
else if (awgNum < 0) { // Handle larger gauges like 2/0, 3/0 etc.
currentAWGString = String(Math.abs(awgNum)) + "/0";
}
var ampCapacity = ampacityTable[currentAWGString] ? ampacityTable[currentAWGString][wireMaterial] : null;
// If ampacity is available and sufficient, this is our recommended size
if (ampCapacity !== undefined && currentAmps <= ampCapacity) {
recommendedAWG = awgNum;
actualResistancePerFoot = correctedResistancePerFoot;
actualWireSize = currentAWGString; // Store the string version
// Calculate the actual voltage drop with this wire size
var totalResistance = correctedResistancePerFoot * wireLengthFeet;
calculatedVoltageDrop = currentAmps * totalResistance;
break; // Found the smallest wire that meets both criteria
} else if (ampCapacity === undefined) {
// If ampacity data isn't readily available for this gauge, but voltage drop is met,
// we still consider it a potential candidate if no other options found, but flag it.
// For simplicity, we'll prioritize ones with ampacity data first.
// If this is the last iteration and nothing else worked, use it.
if (i === 0) {
recommendedAWG = awgNum;
actualResistancePerFoot = correctedResistancePerFoot;
actualWireSize = currentAWGString;
var totalResistance = correctedResistancePerFoot * wireLengthFeet;
calculatedVoltageDrop = currentAmps * totalResistance;
}
}
}
}
// Display results
if (recommendedAWG !== null) {
document.getElementById("wireSizeResult").textContent = actualWireSize + " AWG";
document.getElementById("voltageDropResult").textContent =
`Calculated Voltage Drop: ${calculatedVoltageDrop.toFixed(2)} V (${(calculatedVoltageDrop / voltageVolts * 100).toFixed(2)}%)`;
} else {
document.getElementById("errorMessages").textContent = "Could not determine a suitable wire size. Consider increasing voltage, reducing length, or accepting higher voltage drop.";
}
}