Correctly sizing a circuit breaker is critical for electrical safety. A breaker that is too small will trip unnecessarily, causing nuisance outages. A breaker that is too large may fail to trip during an overload, leading to wire overheating and potential electrical fires.
Use this calculator to determine the appropriate amperage rating for your circuit breaker in accordance with the National Electrical Code (NEC).
How to Calculate Circuit Breaker Size
The calculation involves determining the total current (Amperage) drawn by the connected load and applying a safety factor based on how long the load will be running.
1. Calculate Amperage
First, convert the total power (Watts) of all devices on the circuit to current (Amps) using the formula:
Amps (I) = Watts (P) / Volts (V)
2. Apply the NEC 80% Rule (Continuous Loads)
The National Electrical Code (NEC) distinguishes between continuous and non-continuous loads:
Non-Continuous Load: Equipment expected to run for less than 3 hours continuously. The breaker is sized at 100% of the load.
Continuous Load: Equipment running for 3 hours or more (e.g., lighting systems, heaters). The circuit breaker must be sized at 125% of the continuous load. This effectively means you should only load the breaker to 80% of its rating.
Standard Circuit Breaker Sizes
Once the required capacity is calculated, you must select the next standard size up. Common standard breaker sizes (in Amps) include:
Amperage
Typical Application
15 Amp
Standard lighting circuits, outlets
20 Amp
Kitchen outlets, laundry, garage
30 Amp
Electric dryers, water heaters, RV hookups
40/50 Amp
Electric ranges, ovens, EV chargers
60+ Amp
Sub-panels, electric furnaces
Safety Warning
Wire Gauge Matters: Increasing the breaker size without upgrading the wire gauge (thickness) is dangerous. The wire must be rated to handle the amperage of the breaker. For example, a 20 Amp breaker typically requires 12 AWG copper wire, while a 15 Amp breaker uses 14 AWG.
Always consult a licensed electrician or the current NEC guidelines before installing electrical components.
function calculateBreakerSize() {
// 1. Get Input Values
var wattsInput = document.getElementById("cbs_watts");
var voltageSelect = document.getElementById("cbs_voltage");
var loadTypeSelect = document.getElementById("cbs_load_type");
var watts = parseFloat(wattsInput.value);
var volts = parseFloat(voltageSelect.value);
var loadType = loadTypeSelect.value;
// 2. Validate Inputs
if (isNaN(watts) || watts <= 0) {
alert("Please enter a valid number for Total Load (Watts).");
return;
}
// 3. Calculate Actual Current (I = P/V)
var actualAmps = watts / volts;
// 4. Apply Safety Factor based on Load Type
var multiplier = 1.0;
if (loadType === "continuous") {
multiplier = 1.25; // 125% rule for continuous loads
}
var requiredAmps = actualAmps * multiplier;
// 5. Determine Standard Breaker Size
// Common standard sizes based on NEC 240.6(A)
var standardSizes = [15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400];
var recommendedSize = 0;
var found = false;
for (var i = 0; i = requiredAmps) {
recommendedSize = standardSizes[i];
found = true;
break;
}
}
// Handle case where load exceeds standard residential/commercial sizes listed
if (!found) {
recommendedSize = "Custom/Industrial (>400A)";
}
// 6. Display Results
document.getElementById("result-box").style.display = "block";
document.getElementById("res_actual_amps").innerHTML = actualAmps.toFixed(2) + " Amps";
document.getElementById("res_multiplier").innerHTML = (multiplier * 100) + "%";
document.getElementById("res_required_amps").innerHTML = requiredAmps.toFixed(2) + " Amps";
document.getElementById("res_breaker_size").innerHTML = recommendedSize;
// 7. Contextual Alerts (Wire Wire Sizing Hints)
var alertBox = document.getElementById("cbs_alert");
var wireMsg = "";
if (typeof recommendedSize === 'number') {
if (recommendedSize === 15) wireMsg = "Requires minimum 14 AWG Copper wire.";
else if (recommendedSize === 20) wireMsg = "Requires minimum 12 AWG Copper wire.";
else if (recommendedSize === 30) wireMsg = "Requires minimum 10 AWG Copper wire.";
else if (recommendedSize > 30) wireMsg = "Consult NEC Table 310.16 for correct wire gauge.";
alertBox.style.display = "block";
alertBox.innerHTML = "Note: " + wireMsg + " Ensure wire rating matches the breaker.";
} else {
alertBox.style.display = "none";
}
}