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Fuse Sizing Calculator

Calculate recommended fuse current ratings for continuous and non-continuous electrical circuits.

Continuous Load Current

Non-Continuous Load Current

Recommended Fuse Rating

Recommended Fuse Size

25 Amps

Minimum calculated threshold: 25 A

Fuse Characteristics

Fuse selection also depends on the fuse speed/characteristic (fast-acting, time-delay / slow-blow, semiconductor) to properly handle inrush currents without nuisance tripping while providing necessary short-circuit protection.

Step-by-Step Engineering Calculation

1Non-continuous load: $I_{\text{non-cont}} = 0\text{ A}$
2Continuous load (runs $\ge 3$ hrs): $I_{\text{cont}} = 20\text{ A} \times 125\% = 25.00\text{ A}$
3Minimum Fuse Ampacity: $I_{\text{min}} = 25.00\text{ A}$
4Standard commercial fuse rating: 25 A (NEC 240.6(A))

Formula & Theory

Min Fuse Rating = Non-Continuous Load + (Continuous Load × 125%)

Estimates educational overcurrent protective fuse ratings.

Variable Definitions

I_cont:: Continuous Load (Amperes (A)) – Load running for 3 hours or more
I_noncont:: Non-Continuous Load (Amperes (A)) – Intermittent load
Fuse:: Recommended Rating (Amperes (A)) – Standard commercial fuse rating

Worked Engineering Example

12V 24A DC Continuous Bilge Pump

A continuous 24A DC load requiring short circuit and overload protection.

Inputs:

Continuous Load: 24 A
Safety Multiplier: 125%

Result: Calculated Ampacity = 30.00 A | Nearest Standard Fuse = 30 A

Key Assumptions

•Standard continuous 125% overcurrent multiplier.

Limitations & Safety Scope

•Fuses must protect the downstream conductor ampacity. Selection must account for fuse time-current curves (fast-acting vs slow-blow/time-delay).

Frequently Asked Questions

What is the purpose of a fuse?

A fuse is a sacrificial overcurrent safety device designed to melt and open the circuit when excessive current threatens cable insulation or equipment.

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Educational & Reference Notice: This calculator provides technical estimates based on standard physical formulas. Actual electrical installation, protective device sizing, and conductor selection must be verified against the applicable electrical code (NEC, IEC, CEC) by a licensed electrician or qualified professional engineer.