Fuse Rating Calculator

Step-by-Step Guide

1. 1Choose an Input Mode: Select Power + Voltage if you know the appliance's wattage and supply voltage, or Direct Current if you already have a measured or nameplate current value. The calculator computes current as I = P divided by V automatically in power/voltage mode.
2. 2Enter Circuit Values: Type the wattage and voltage, or the current directly, depending on the selected mode. Use the built-in appliance presets to auto-fill common values like a microwave, kettle, or air conditioner.
3. 3Select a Safety Factor: Choose 1.25 for standard resistive loads, 1.5 for occasional surge loads, or 1.6 for motors and other high-inrush equipment. This factor is applied to the calculated current before a fuse size is selected.
4. 4Choose Fast Blow or Slow Blow: Select Fast Blow for lighting, heating, and steady electronic loads. Select Slow Blow for motors, compressors, and transformers that need to tolerate a brief startup current surge.
5. 5Read the Recommended Fuse Rating: The calculator returns the nearest standard fuse rating at or above the adjusted current, the safety margin percentage, and the next higher standard size as a backup option.
6. 6Save or Export the Result: Copy the recommended rating to your clipboard, save the calculation to local history, or export a full text report showing every calculation step.

What This Tool Provides

• ✓Recommended fuse rating from the standard IEC ladder
• ✓Calculated and safety-factor-adjusted current
• ✓Safety margin percentage with over/under warnings
• ✓Next-size-up alternative rating
• ✓Power/voltage and direct current input modes
• ✓Fast blow and slow blow type selection
• ✓8 built-in common appliance presets
• ✓Calculation history (last 20 entries)
• ✓Export results as a text report
• ✓Copy result to clipboard
• ✓Private: your inputs are not collected or stored
• ✓No registration required

Pro Tips

• 💡Use the 1.6 safety factor for anything with a motor, compressor, or transformer. Inrush current on startup can briefly spike to 5–8 times the running current, and a fuse sized only for steady-state current will blow the instant the equipment is switched on.
• 💡If you already know the appliance's nameplate current, use Direct Current mode instead of calculating power and voltage manually. Nameplate FLA (full-load amps) is more accurate than a wattage-based estimate, since real-world efficiency and power factor affect actual current draw.
• 💡Always match the fuse's voltage rating to the circuit, not just its amperage. A fuse rated for 32V DC installed in a 230V AC circuit may fail to safely interrupt the arc during a fault, even if the amperage rating looks correct.
• 💡When a fuse blows repeatedly at the recommended rating, don't just size up — investigate for a short circuit, ground fault, or failing component first. Repeated blowing at a correctly calculated rating is a symptom of a fault, not proof the fuse is undersized.
• 💡For DC systems like solar arrays and battery banks, use fuses specifically rated for DC interruption. AC fuses are not designed to extinguish the sustained arc that forms when DC current is broken, and substituting one is a genuine fire risk.
• 💡Keep a couple of the next-size-up fuse on hand as a backup, but never install it as a permanent fix. The calculator's next-higher-fuse figure is there for reference and temporary bridging only — always fit the specifically recommended rating for daily use.

Common Mistakes to Avoid

• ✕Don't round the calculated rating down to the nearest standard size. Rounding down defeats the safety margin entirely — always round up, even if the calculated value is only slightly above a lower standard rating.
• ✕Don't use a fast blow fuse on a motor or compressor circuit. The startup inrush current will trip it almost every time the equipment starts, even though the fuse rating is technically adequate for steady-state operation.
• ✕Don't ignore a low safety margin warning. A margin under 10% means normal voltage fluctuations or slightly higher-than-expected load can push the circuit over the fuse rating, causing frequent nuisance blowing that's easy to mistake for a fault.
• ✕Don't substitute a higher-amperage fuse just because the correct size isn't in stock. An oversized fuse won't open in time during a real overcurrent event, allowing wiring to overheat well past the point the original design intended to protect against.
• ✕Don't assume the fuse alone protects the wiring. The fuse rating must also be checked against the ampacity of the wire feeding the circuit — a fuse sized correctly for the appliance can still be too large for undersized wire. Cross-check with a wire size or circuit breaker calculator for the full circuit.