*Based on NEC Chapter 9, Table 1 (40% fill for 3 or more conductors).
function calculateConduitSize() {
var wireArea = parseFloat(document.getElementById('wireGauge').value);
var insulationMultiplier = parseFloat(document.getElementById('insulationType').value);
var count = parseInt(document.getElementById('wireCount').value);
if (isNaN(count) || count 40% Fill Area (sq inches)
var conduitCapacities = [
{ size: '1/2″', cap: 0.122 },
{ size: '3/4″', cap: 0.213 },
{ size: '1″', cap: 0.346 },
{ size: '1-1/4″', cap: 0.598 },
{ size: '1-1/2″', cap: 0.814 },
{ size: '2″', cap: 1.342 },
{ size: '2-1/2″', cap: 1.926 },
{ size: '3″', cap: 2.950 },
{ size: '3-1/2″', cap: 3.960 },
{ size: '4″', cap: 5.093 }
];
var recommendedSize = "Exceeds 4 inch";
var recommendedCap = 0;
for (var i = 0; i < conduitCapacities.length; i++) {
var limit = conduitCapacities[i].cap;
// NEC Table 1: If 1 wire, 53% fill. If 2 wires, 31% fill. If 3+ wires, 40% fill.
var fillModifier = 1.0;
if (count === 1) fillModifier = 1.325; // (53/40)
if (count === 2) fillModifier = 0.775; // (31/40)
if (totalWireArea <= (limit * fillModifier)) {
recommendedSize = conduitCapacities[i].size;
recommendedCap = limit * fillModifier;
break;
}
}
document.getElementById('resTotalArea').innerHTML = totalWireArea.toFixed(4) + " sq. in.";
document.getElementById('resConduitSize').innerHTML = recommendedSize;
document.getElementById('resFillLimit').innerHTML = recommendedCap.toFixed(4) + " sq. in.";
document.getElementById('conduitResult').style.display = 'block';
}
How to Use the Conduit Size Calculator
Choosing the correct conduit size is critical for electrical safety and compliance with the National Electrical Code (NEC). This calculator helps you determine the appropriate Trade Size for EMT, PVC, or Rigid conduit based on the cross-sectional area of the wires you plan to pull.
Understanding the NEC 40% Fill Rule
The NEC Chapter 9, Table 1 dictates the maximum percentage of a conduit's interior volume that can be occupied by conductors. This rule exists to prevent excessive heat buildup and to ensure that wires can be pulled through the pipe without damaging the insulation.
1 Conductor: 53% Fill allowed.
2 Conductors: 31% Fill allowed.
3 or More Conductors: 40% Fill allowed.
Common Wire Types and Their Impact
Not all wires of the same gauge have the same outer diameter. The insulation type significantly impacts the total area:
THHN/THWN: These have a thin, heat-resistant thermoplastic jacket, allowing more wires in a single conduit.
XHHW: Slightly thicker insulation often used in commercial applications.
RHH/RHW: These feature much thicker rubber insulation and typically require larger conduit sizes for the same number of conductors.
Step-by-Step Calculation Example
Suppose you are running a 240V circuit that requires three 6 AWG THHN wires.
Identify the area of one 6 AWG THHN wire (approx. 0.0507 sq. in.).
Multiply by the number of wires: 0.0507 x 3 = 0.1521 total sq. in.
Check conduit capacities:
1/2″ EMT (40% fill) = 0.122 sq. in. (Too small)
3/4″ EMT (40% fill) = 0.213 sq. in. (Fits!)
Result: Use a minimum of 3/4″ conduit.
Pro-Tip for Professional Electricians
While the calculator provides the minimum legal size, it is often wise to "upsize" by one trade size if the run contains several 90-degree bends or is exceptionally long. This makes the wire pull significantly easier and reduces the risk of friction-related heat or wire jamming.