This professional Fuse Size Calculator helps you determine the correct amperage for fuses in electrical circuits. By considering power load, operating voltage, and the NEC-recommended safety factor, you can ensure both equipment safety and circuit reliability.
Fuse Size Calculator
Fuse Size Calculator Formula
Formula Source: Electrical Technology | NEC (NFPA 70)
Variables Explained
- P (Power): The total wattage consumed by the device or circuit.
- V (Voltage): The electrical potential difference (e.g., 12V, 110V, 230V).
- F (Safety Factor): A multiplier to handle continuous loads and inrush current (Standard is 1.25).
What is a Fuse Size Calculator?
A Fuse Size Calculator is a specialized tool used by electricians and hobbyists to select the correct protection device for an electrical circuit. Using an undersized fuse leads to nuisance blows, while an oversized fuse creates a fire hazard by allowing the wires to overheat before the fuse can pop.
This tool automates the Ohm’s Law calculation by dividing the total power by the voltage to find the base current, then applying a safety margin to ensure the fuse doesn’t blow during normal operating fluctuations.
How to Calculate Fuse Size (Example)
- Identify your device wattage (e.g., a 600W heater).
- Check your mains voltage (e.g., 120V).
- Calculate the current: $600 / 120 = 5$ Amps.
- Apply the 125% safety factor: $5 \times 1.25 = 6.25$ Amps.
- Choose the next standard fuse size available (e.g., 7A or 8A).
Frequently Asked Questions (FAQ)
Q: Why is the safety factor 1.25?
The NEC (National Electrical Code) recommends a 125% rating for continuous loads to prevent the fuse from reaching its thermal trip point during normal operation.
Q: Can I use a larger fuse if mine keeps blowing?
No. A blowing fuse indicates an overload or short circuit. Increasing fuse size without upgrading wire gauge can cause electrical fires.
Q: Does the fuse size depend on DC or AC voltage?
The basic current calculation is similar, but you must use a fuse rated for the specific voltage type (AC vs DC) due to arc extinguishing differences.
Q: What if my calculated size isn’t a standard fuse rating?
Always round up to the nearest standard manufacturer rating (e.g., if you calculate 4.2A, use a 5A fuse).