Fuse Selection Calculator
Recommends fuse current rating (nearest standard value), voltage rating, and fuse type based on load type, operating voltage, and normal current, using industry derating factors, with optional inrush I²t calculation.
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How to Use
- Select the load type: general electronics, resistive, motor, transformer, or LED driver — each maps to a different derating factor and recommended fuse type.
- Enter the working voltage (V) and normal operating current (A).
- Optional: enter the inrush current (A) and inrush duration (ms) to additionally calculate the I²t withstand requirement; leave both blank to skip this.
- Click "Calculate" (or edit any field to trigger automatic recalculation).
- The "Recommendation" section shows a full text description of the recommended fuse (current rating, type, voltage rating requirement, and I²t requirement if surge parameters were entered), plus the derating factor used.
- Parameter cards below show: recommended current rating (standard value), voltage rating requirement (rounded up to the nearest multiple of 10V), fuse type (fast-blow/time-delay), and supporting data such as the I²t withstand requirement or the base calculation current and derating factor.
- "Nearby Standard Fuse Values" lists standard current ratings around the recommendation as tags (the current recommendation is highlighted), helping you pick a rating that's actually available to purchase.
- The "Common Fuse Package Reference" table lists current range and voltage rating for five common package types: 3×6mm SMD, 5×20mm/6×30mm glass tube, 10×38mm cylindrical, and NH blade fuses.
- Click "Load Sample Data" to restore the default: motor load, 24V, 3A, 30A/10ms inrush.
Features
- Supports five load-type selections, each with a fixed industry-standard derating factor: general electronics (×1.25), resistive load (×1.25), motor load (×1.5, accounting for starting current), transformer load (×2.0, accounting for inrush magnetizing current), and LED driver (×1.3, accounting for surge current).
- Calculates the base recommended current using "recommended rating = normal current × derating factor," then automatically rounds up to the nearest standard fuse current rating (from a 31-step IEC standard scale spanning 0.1A to 100A).
- Gives the voltage rating requirement by rounding the working voltage up to the nearest multiple of 10V (Math.ceil(voltage/10)×10).
- Automatically recommends a fuse type based on load: general/resistive/LED loads get fast-blow fuses, while motor/transformer loads get time-delay (slow-blow) fuses, since the latter experience normal starting/inrush surge currents.
- When inrush current and duration are provided, calculates the minimum I²t the fuse must withstand using I²t = I_inrush² × t_inrush (duration converted to seconds), for cross-checking against fuse datasheet let-through ratings.
- Provides a reference table of typical current ranges and voltage ratings for five common fuse package types: 3×6mm SMD, 5×20mm/6×30mm glass tube, 10×38mm cylindrical, and NH blade fuses.
Use Cases
Power input fuse selection
Select an appropriately rated fuse for a product's 12V/24V DC power input, determining the standard rating from normal operating current and the 1.25x derating factor for general electronic loads.
Motor drive circuit fuse selection (accounting for starting current)
When selecting a fuse for a motor drive circuit, choose the motor load type (1.5x derating factor) and get a time-delay fuse recommendation, avoiding nuisance blowing from the motor's normal starting surge.
Transformer primary-side fuse selection (accounting for inrush magnetizing current)
Transformers draw a large inrush magnetizing current at power-on; select the transformer load type (2.0x derating factor) and get a time-delay fuse recommendation to prevent nuisance blowing during normal power-up.
I²t withstand verification for surge-prone circuits
For circuits with significant inrush current, such as capacitor-input power supplies, enter the measured or estimated peak inrush current and duration to calculate the minimum I²t the fuse must withstand, for comparison against manufacturer datasheet specs.
FAQ
Why do motor and transformer loads have much higher derating factors than general loads?
Because motors draw a starting current far above rated current at startup (often several times rated current), and transformers draw a significant inrush magnetizing current at power-on. If a fuse were sized using the general-load 1.25x derating factor, these normal brief surges would nuisance-trip the fuse. The tool applies a 1.5x factor for motor loads and 2.0x for transformer loads, and recommends time-delay (slow-blow) fuses that can tolerate brief surges without tripping, blowing only on genuine sustained overcurrent faults.
How is the recommended current rounded to a "standard value"?
The tool has a built-in table of common IEC-standard fuse current ratings (0.1A, 0.125A, 0.16A... up to 100A, 31 steps total). After computing the theoretical recommended current (normal current × derating factor), it finds the smallest standard rating in this table that is greater than or equal to that theoretical value. Fuse manufacturers typically only produce these standard current ratings, and choosing the nearest standard value at or above the theoretical figure ensures normal operating current won't cause nuisance blowing.
What happens if I don't enter inrush current and duration?
These are optional parameters. Leaving them blank still gives you the base recommendation (current rating, voltage rating, fuse type), just without the I²t calculation — the results area shows "I²t (no surge data)" instead. If your load has a notable inrush at power-on (e.g., a power supply with large capacitors, a motor, or a transformer), it's recommended to fill in both fields for a more complete selection reference.
How is the voltage rating requirement calculated?
The tool rounds the working voltage up to the nearest multiple of 10V as the minimum voltage rating requirement (Math.ceil(working voltage/10)×10). For example, a 12V working voltage rounds up to 20V, and a 24V working voltage rounds up to 30V (24/10=2.4 → 3). This builds in a safety margin on the fuse's voltage rating, avoiding a fuse whose rated voltage is exactly equal to or below the actual working voltage.