Fuse Size Calculator

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Fuse Size Calculator
Common values: 12V (Automotive), 120V (US Mains), 230V (EU Mains)
Standard Load (25% Safety Margin) Simple Resistive (Lighting/Heaters – 10% Margin) Inductive/Motor (High Startup Surge – 50% Margin)
Current Draw: 0 Amps
Minimum Fuse Rating: 0 Amps
Note: Always round up to the nearest standard fuse size available (e.g., if result is 13.5A, use a 15A fuse).

How to Calculate Correct Fuse Size

Selecting the correct fuse size is critical for the safety of any electrical circuit, whether you are wiring a car audio system, installing a new appliance, or designing a DIY electronics project. A fuse acts as the weak link in a circuit, designed to fail (blow) before the wiring overheats and causes a fire.

The Core Formula: Amps = Watts / Volts

To determine the size of the fuse you need, you first need to calculate the current (measured in Amps) that your device draws. Most electrical devices list their power consumption in Watts (W). The relationship is governed by Ohm's Law:

Current (I) = Power (P) รท Voltage (V)

For example, if you are installing a 600 Watt amplifier in a car with a 12 Volt system:

  • 600 Watts / 12 Volts = 50 Amps

This means the device draws 50 Amps of current during operation.

Applying a Safety Margin

You should never size a fuse exactly at the operating current. If a device draws 10 Amps and you use a 10 Amp fuse, it is likely to blow during normal fluctuations or minor surges. Electrical codes and best practices suggest adding a safety margin.

  • Standard Load (125% Rule): For most continuous loads, multiply the current by 1.25. This allows for a 25% overhead.
  • Inductive Loads (Motors/Compressors): Devices with motors have a high "in-rush" current when starting. You may need a higher margin (150% or more) or a "Slow Blow" fuse.

Standard Fuse Ratings

Once you calculate your minimum fuse rating, you must round up to the nearest standard fuse size. Common automotive (blade) and household fuse sizes include:

  • Automotive: 5A, 7.5A, 10A, 15A, 20A, 25A, 30A, 40A, 50A, 60A.
  • Household (Mains): 13A, 15A, 20A, 30A, 50A.

Warning: Never use a fuse rated higher than the current carrying capacity (ampacity) of your wire. If your wire can only handle 20 Amps, you cannot use a 30 Amp fuse, even if your device requires it. You must upgrade the wire first.

Example Scenarios

1. LED Light Bar (Automotive):
Power: 120 Watts
Voltage: 12 Volts
Current = 120W / 12V = 10 Amps.
Recommended Fuse: 10 Amps x 1.25 = 12.5 Amps.
Action: Use a 15 Amp fuse.

2. Space Heater (Home):
Power: 1500 Watts
Voltage: 120 Volts
Current = 1500W / 120V = 12.5 Amps.
Recommended Fuse: 12.5 Amps x 1.25 = 15.625 Amps.
Action: This requires a 20 Amp circuit and fuse/breaker.

function calculateFuseSize() { // 1. Get input values var wattsInput = document.getElementById('deviceWatts'); var voltsInput = document.getElementById('circuitVolts'); var loadTypeInput = document.getElementById('loadType'); var resultDiv = document.getElementById('fuseResult'); // 2. Parse values var watts = parseFloat(wattsInput.value); var volts = parseFloat(voltsInput.value); var safetyFactor = parseFloat(loadTypeInput.value); // 3. Validation if (isNaN(watts) || watts <= 0) { alert("Please enter a valid Wattage (Power) greater than 0."); return; } if (isNaN(volts) || volts <= 0) { alert("Please enter a valid Voltage greater than 0."); return; } // 4. Calculate Current (Amps) var amps = watts / volts; // 5. Calculate Recommended Fuse Size (with safety margin) var recommendedFuse = amps * safetyFactor; // 6. Display Results document.getElementById('currentDraw').innerText = amps.toFixed(2); document.getElementById('minFuse').innerText = recommendedFuse.toFixed(2); // Show result div resultDiv.style.display = "block"; }

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