Fuse Rating Calculator

A fuse is a safety device that protects electrical circuits from dangerous overcurrents — choosing the wrong rating risks equipment damage or fire. Select your electrical standard (NEC or IEC), system type (AC or DC), and load type, then enter your system voltage and load current to get the recommended fuse size. The Fuse Rating Calculator also returns time-delay and non-time-delay fuse ratings, plus the minimum and maximum allowable fuse sizes for your circuit.

Electrical Standard *

V
A

For continuous loads: operating current. For motors: Full Load Current from NEC tables.

A

Required for IEC motor calculations only

Results

Recommended Fuse Size

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Time-Delay Fuse

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Non-Time-Delay Fuse

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Minimum Fuse Size

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Maximum Fuse Size

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Results Table

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Frequently Asked Questions

How do you calculate fuse size?

Fuse size depends on load type and electrical standard. For continuous loads, multiply current by 125% (NEC). For motor loads, use 175% of Full Load Current for time-delay fuses or 300% for non-time-delay fuses.

What's the difference between NEC and IEC fuse sizing?

NEC (North American) uses specific multipliers based on load type, while IEC (International) provides ranges based on motor nameplate current and fuse characteristics (gG/gL or aM types).

What size fuse for a 3hp motor?

A 3HP motor typically has 17.5A Full Load Current (230V). Using NEC standards, you'd need a 30A time-delay fuse (17.5 x 1.75 = 30.6A, rounded to next standard size).

Should I use time-delay or non-time-delay fuses?

Time-delay fuses are preferred for motor loads as they allow for motor starting currents. Non-time-delay fuses are used for resistive loads or when faster protection is needed.

What size fuse for a 2000W inverter?

For a 2000W inverter at 12V DC: 2000W ÷ 12V = 167A. Adding 25% safety margin gives approximately 208A, so you'd use a 200A or 225A fuse depending on available sizes.

Can I use a larger fuse than calculated?

Never exceed the maximum calculated fuse size, as this compromises safety and code compliance. The wire and equipment ampacity must also support the fuse size chosen.

What happens if my fuse is too small?

An undersized fuse will blow frequently during normal operation, especially during motor starting or equipment inrush currents, causing unnecessary downtime and nuisance tripping.

Do I need different calculations for DC systems?

DC systems follow similar principles but may have different safety factors. Always consult equipment manuals and local codes, as DC arc characteristics differ from AC systems.