Continuous (3+ Hours – NEC Rule)
Non-Continuous (Intermittent)
Calculation Results
Actual Current Flow: 0 Amps
Minimum Required Capacity: 0 Amps
Recommended Breaker Size: 0 Amps
*Calculated based on standard NEC breaker sizes (rounding up to next standard size).
Disclaimer: This calculator is for educational and estimation purposes only. Electrical work is dangerous. Always consult with a licensed electrician and follow local building codes (like NEC) before modifying any electrical systems.
Understanding Breaker Sizing Calculations
Choosing the right circuit breaker is critical for preventing electrical fires and protecting your home's wiring. A breaker that is too small will constantly trip, while one that is too large may fail to trip during an overload, leading to overheated wires and potential fire hazards.
The 80% Rule (The Continuous Load Principle)
In electrical engineering and according to the National Electrical Code (NEC), you should not load a circuit breaker to more than 80% of its rated capacity if the load is "continuous" (running for 3 hours or more). This is why we apply a 125% multiplier to the actual load during the calculation phase.
The Formula for Breaker Sizing
To calculate the required amperage, we use Ohm's Law and the safety factor:
Step 1: Calculate Amps (I = P / V). Divide Total Watts by Voltage.
Step 2: Apply Safety Factor. Multiply the Amps by 1.25 for continuous loads.
Step 3: Select the Breaker. Round up to the next standard circuit breaker size.
Standard Breaker Sizes
Standard circuit breakers do not come in every possible amperage. Common residential and commercial sizes include:
15 Amp (Standard lighting/outlets)
20 Amp (Kitchen/Bathroom outlets)
30 Amp (Clothes dryers, water heaters)
40-50 Amp (Electric ranges, EV chargers)
60-100 Amp (Sub-panels)
Example Calculation
If you have a space heater that pulls 1,500 Watts on a 120V circuit and it runs for several hours:
Load: 1,500W / 120V = 12.5 Amps
Adjustment: 12.5A x 1.25 = 15.625 Amps
Selection: The next standard size is 20 Amps.
Wire Gauge Considerations
Never upgrade a breaker without ensuring the wiring can handle the increased load. Using a 20A breaker on 14-gauge wire (rated for 15A) is a severe fire hazard. Always match your breaker size to the wire gauge installed in the walls.
function calculateBreaker() {
var watts = parseFloat(document.getElementById("totalWatts").value);
var volts = parseFloat(document.getElementById("voltage").value);
var factor = parseFloat(document.getElementById("loadType").value);
var margin = parseFloat(document.getElementById("safetyMargin").value);
if (isNaN(watts) || watts <= 0) {
alert("Please enter a valid wattage.");
return;
}
// Standard breaker sizes array
var standardBreakers = [15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600];
// Calculate actual current flow
var actualAmps = watts / volts;
// Calculate required capacity based on load type and extra safety margin
var safetyMultiplier = factor + (margin / 100);
var requiredAmps = actualAmps * safetyMultiplier;
// Find the recommended breaker size (round up to next standard size)
var recommended = standardBreakers[0];
for (var i = 0; i = requiredAmps) {
recommended = standardBreakers[i];
break;
}
// If it exceeds the largest standard breaker in the list
if (i === standardBreakers.length – 1 && requiredAmps > standardBreakers[i]) {
recommended = "Special Order / High Capacity (>600)";
}
}
// Display results
document.getElementById("breakerResult").style.display = "block";
document.getElementById("actualAmps").innerText = actualAmps.toFixed(2);
document.getElementById("requiredAmps").innerText = requiredAmps.toFixed(2);
document.getElementById("recBreaker").innerText = recommended;
// Smooth scroll to results
document.getElementById("breakerResult").scrollIntoView({ behavior: 'smooth', block: 'nearest' });
}