Wire Size Calculator

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Wire Size Calculator
Copper (CU)Aluminum (AL)
Single Phase (120/240V)Three Phase
Results:
function calculateWireSize(){var K=parseFloat(document.getElementById('material').value);var V=parseFloat(document.getElementById('voltage').value);var I=parseFloat(document.getElementById('current').value);var L=parseFloat(document.getElementById('distance').value);var P=parseFloat(document.getElementById('drop').value);var Phase=parseFloat(document.getElementById('phase').value);if(isNaN(V)||isNaN(I)||isNaN(L)||isNaN(P)){alert('Please enter valid numerical values');return;}var allowedVD=V*(P/100);var cmils=(Phase*K*L*I)/allowedVD;var awg="Unknown";var sizeMap=[{awg:"14 AWG",cm:4110},{awg:"12 AWG",cm:6530},{awg:"10 AWG",cm:10380},{awg:"8 AWG",cm:16510},{awg:"6 AWG",cm:26240},{awg:"4 AWG",cm:41740},{awg:"3 AWG",cm:52620},{awg:"2 AWG",cm:66360},{awg:"1 AWG",cm:83690},{awg:"1/0″,cm:105600},{awg:"2/0″,cm:133100},{awg:"3/0″,cm:167800},{awg:"4/0″,cm:211600},{awg:"250 MCM",cm:250000},{awg:"300 MCM",cm:300000},{awg:"350 MCM",cm:350000},{awg:"400 MCM",cm:400000},{awg:"500 MCM",cm:500000}];for(var i=0;i<sizeMap.length;i++){if(cmils 500000) awg = "Over 500 MCM (Parallel lines required)";document.getElementById('finalResult').innerHTML="Recommended Size: "+awg;if(document.getElementById('steps').checked){var stepHtml="Step-by-Step Calculation:
";stepHtml+="1. Allowable Voltage Drop = "+V+"V × "+(P/100)+" = "+allowedVD.toFixed(2)+"V
";stepHtml+="2. K Factor (Material) = "+K+"
";stepHtml+="3. Circular Mils = ("+Phase+" × "+K+" × "+L+"ft × "+I+"A) / "+allowedVD.toFixed(2)+"V
";stepHtml+="4. Total Circular Mils = "+Math.round(cmils).toLocaleString()+" cmils
";stepHtml+="5. Standard wire size matching "+Math.round(cmils).toLocaleString()+" cmils is "+awg+".";document.getElementById('stepDetails').innerHTML=stepHtml;document.getElementById('stepDetails').style.display='block';}else{document.getElementById('stepDetails').style.display='none';}}

Using the Wire Size Calculator

Choosing the correct conductor gauge is critical for safety and efficiency in any electrical installation. This wire size calculator helps you determine the minimum American Wire Gauge (AWG) or MCM size needed based on electrical load, distance, and allowable voltage drop. Using an undersized wire can lead to overheating, equipment failure, and even fire hazards.

System Voltage
The nominal voltage of your circuit (e.g., 12V for DC automotive, 120V for standard household, 240V for heavy appliances).
Current (Amps)
The maximum load current that will flow through the circuit under normal operation.
One-way Distance
The physical length of the run from the power source to the load in feet.
Allowable Drop
The percentage of voltage lost due to resistance. The NEC recommends no more than a 3% drop for branch circuits.

How the Calculation Works

The wire size calculator uses the circular mil formula to find the cross-sectional area required to keep voltage drop within a specific range. Electrical resistance increases with distance and decreases with wire thickness.

CM = (Phase × K × L × I) / VD

  • CM: Circular Mils (area of the wire).
  • Phase: Constant of 2 for single-phase or 1.732 for three-phase.
  • K: Specific resistivity (approx. 12.9 for Copper, 21.2 for Aluminum).
  • L: Distance of the run in feet.
  • I: Current in Amperes.
  • VD: Permissible Voltage Drop in Volts.

Calculation Example

Scenario: You are installing a 120V outdoor lighting circuit that draws 15 Amps and is located 150 feet away from the breaker panel. You want to limit voltage drop to 3%.

Step-by-step solution:

  1. Calculate allowable drop: 120V × 0.03 = 3.6 Volts.
  2. Determine constant (Copper): 12.9.
  3. Apply formula: (2 × 12.9 × 150ft × 15A) / 3.6V.
  4. Result: 58,050 / 3.6 = 16,125 Circular Mils.
  5. Lookup: 16,125 cmils falls between 10 AWG (10,380) and 8 AWG (16,510).
  6. Final Wire Size: 8 AWG.

Frequently Asked Questions

Why is voltage drop important?

Excessive voltage drop causes motors to run hot and inefficiently, reduces the brightness of lighting, and can cause electronic equipment to malfunction or reboot randomly.

What is the difference between AWG and MCM?

AWG (American Wire Gauge) is used for smaller wires (up to 4/0). MCM (or kcmil) is used for very large wires, where 1 MCM equals 1,000 circular mils. Sizes larger than 4/0 are labeled as 250 MCM, 300 MCM, and so on.

Can I use Aluminum instead of Copper?

Yes, but aluminum has higher resistance (K factor of 21.2 vs 12.9). To carry the same current over the same distance, you will typically need an aluminum wire two sizes larger than the equivalent copper wire.

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