Choosing the correct circuit breaker size is critical for electrical safety. A breaker that is too small will trip constantly, while a breaker that is too large may fail to trip during an overload, potentially causing a fire. This calculator uses the National Electrical Code (NEC) guidelines to help you determine the appropriate protection for your electrical circuit.
The 80% Rule and Continuous Loads
In electrical engineering, the "80% Rule" states that a circuit breaker should not be loaded to more than 80% of its capacity for continuous loads. A continuous load is defined as any load that runs for three hours or more (such as space heaters, air conditioners, or lighting systems).
Mathematically, this is handled by multiplying the actual amperage by 125% (1.25) to find the minimum breaker rating. For example, a 16-amp continuous load requires a 20-amp breaker (16 x 1.25 = 20).
Standard Circuit Breaker Sizes
Standard circuit breakers in the United States come in specific increments. If your calculation results in a non-standard number, you must round up to the next available standard size. Common sizes include:
15 Amp
20 Amp
30 Amp
40 Amp
50 Amp
60 Amp
100 Amp
Breaker Size and Wire Gauge Reference
Breaker Size
Copper Wire Gauge (AWG)
Max Continuous Load (Watts at 120V)
15 Amps
14 AWG
1,440 Watts
20 Amps
12 AWG
1,920 Watts
30 Amps
10 AWG
2,880 Watts
40 Amps
8 AWG
3,840 Watts
50 Amps
6 AWG
4,800 Watts
Step-by-Step Calculation Formula
To calculate the breaker size manually, follow these steps:
Find the Amperage: Divide the total Watts by the Voltage (Amps = Watts / Volts).
Apply Safety Factor: If it's a continuous load, multiply the Amps by 1.25.
Select Breaker: Choose the next standard breaker size that is equal to or greater than your result.
Disclaimer: Always consult with a licensed electrician before performing any electrical work. This calculator is for informational purposes and does not replace professional advice or local building codes.
function calculateBreaker() {
var watts = parseFloat(document.getElementById('loadWatts').value);
var volts = parseFloat(document.getElementById('voltage').value);
var multiplier = parseFloat(document.getElementById('loadType').value);
var resultDiv = document.getElementById('breakerResult');
if (isNaN(watts) || watts <= 0) {
alert("Please enter a valid wattage.");
return;
}
// Calculate actual current
var actualAmps = watts / volts;
// Calculate required capacity based on load type (NEC 80% rule / 125% factor)
var safetyAmps = actualAmps * multiplier;
// Standard breaker sizes array
var standardBreakers = [15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200];
var selectedBreaker = 0;
for (var i = 0; i = safetyAmps) {
selectedBreaker = standardBreakers[i];
break;
}
}
// If load is too high for standard 200A residential
if (selectedBreaker === 0) {
selectedBreaker = "Over 200A (Consult Engineer)";
} else {
selectedBreaker = selectedBreaker + " Amp";
}
// Determine Wire Gauge (Copper, approximate for short runs)
var gauge = "Unknown";
var ampVal = parseFloat(selectedBreaker);
if (ampVal <= 15) gauge = "14 AWG";
else if (ampVal <= 20) gauge = "12 AWG";
else if (ampVal <= 30) gauge = "10 AWG";
else if (ampVal <= 40) gauge = "8 AWG";
else if (ampVal <= 55) gauge = "6 AWG";
else if (ampVal <= 70) gauge = "4 AWG";
else if (ampVal <= 85) gauge = "3 AWG";
else if (ampVal <= 95) gauge = "2 AWG";
else if (ampVal <= 110) gauge = "1 AWG";
else if (ampVal <= 130) gauge = "1/0 AWG";
else if (ampVal <= 150) gauge = "2/0 AWG";
else if (ampVal <= 175) gauge = "3/0 AWG";
else if (ampVal <= 200) gauge = "4/0 AWG";
// Update Display
document.getElementById('actualAmps').innerText = actualAmps.toFixed(2) + " A";
document.getElementById('safetyAmps').innerText = safetyAmps.toFixed(2) + " A";
document.getElementById('recBreaker').innerText = selectedBreaker;
document.getElementById('wireGauge').innerText = gauge;
resultDiv.style.display = 'block';
}