Amp Fuse Size Calculator
Estimate a preliminary fuse or overcurrent-device size from continuous load, noncontinuous load, an editable design factor and a chosen standard rating. Enter your own values and review every assumption shown with the result.
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The primary result is displayed first, followed by supporting values that make the estimate easier to verify.
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How to Use This Calculator
Enter each requested value using the displayed unit. Replace the example defaults with source-specific values and review all assumptions.
Formula & Methodology
The calculator applies the displayed formulas directly to the entered values. Supporting outputs expose important intermediate calculations.
Worked example
A 20 A continuous load at 125% plus a 5 A noncontinuous load produces a preliminary minimum of 30 A before checking conductor ampacity and equipment rules.
Actual fuse sizing depends on jurisdiction, conductor ampacity, terminals, ambient temperature, equipment listing, motor rules, inrush, fault current, fuse class and applicable electrical code.
Do not select protection solely from this calculator. Electrical work must be designed and installed by qualified persons under the applicable code and manufacturer instructions.
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Related Guides
- How to Calculate the Right Fuse Size for Electrical CircuitsLearn how to determine the correct fuse or circuit breaker size for any electrical circuit — based on load current, conductor size, NEC overcurrent protection rules, and motor starting current.
- How Three-Phase Motor Amperage Is Calculated from HP and VoltageLearn the three-phase motor current formula — how to calculate full-load amps from horsepower, voltage, motor efficiency, and power factor, with NEC table values for quick reference.
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Frequently Asked Questions
- What does the Amp Fuse Size Calculator calculate?
- Estimate a preliminary fuse or overcurrent-device size from continuous load, noncontinuous load, an editable design factor and a chosen standard rating.
- Can I change the assumptions?
- Yes. Replace the example defaults with values from the current game, project, product, drawing, data sheet or market source.
- Is the result exact?
- No. It is an estimate produced by the displayed formulas and entered assumptions. Real results may differ because of factors described in the limitations section.
- Should an important result be verified?
- Yes. Verify important decisions with current source data, official documentation, manufacturer information or a qualified professional.
Last updated 7/22/2026