PlatformProductsMarine SystemsPricingEnterpriseDocs
Launch Demo

Fuse Sizing Calculator

NEC/SAE 125% continuous current. Standard A sizes. No profile required.

Recommended fuse

15 A

Ready to design the entire system?

Open DemoBook Technical ReviewRequest Early Access

Dive deeper: Read our technical whitepapers

These calculators are engineering design aids intended for estimation and educational purposes. They do not replace professional engineering judgment or compliance with applicable standards (ABYC, NEC, IEC, SAE, or local regulations). Always verify final designs against relevant codes and manufacturer specifications.

Purpose

This calculator estimates appropriate overcurrent protection size based on continuous current. Size the wire first with the AWG sizing calculator, then size the fuse for the wire ampacity and load.

The 125% rule

Many electrical standards recommend sizing overcurrent protection at up to 125% of continuous load current, provided the conductor ampacity supports that rating and the protection device does not exceed wire limits. Example: 10A continuous load → 12.5A minimum → next standard size (e.g. 15A).

Fast-blow vs time-delay

Fast-blow fuses: suitable for resistive loads (lighting, heaters). Time-delay fuses: better for motors and inrush loads. Motors may briefly draw 2–6× running current during startup.

Important limitations

Fuse sizing must never exceed conductor ampacity. This tool does not replace applicable code compliance or manufacturer specifications. Verify voltage drop stays within limits.

FAQ

What is the 125% fuse rule?
Many electrical standards size overcurrent protection at up to 125% of continuous load, subject to conductor ampacity limits. A 10A continuous load needs at least 12.5A; round up to nearest standard size (15A). The fuse must not exceed wire ampacity.
How do I size a fuse for a marine circuit?
Size the fuse for the wire, not the load. The fuse protects the wire from overload. Use 125% of continuous load, then verify the wire can carry that current. See AWG ampacity tables.
What is the difference between fast-blow and slow-blow fuses?
Fast-blow open quickly on overload; slow-blow (time-delay) tolerate brief surges (e.g. motor start). For motors and inductive loads with high inrush, slow-blow is often recommended. This calculator gives continuous sizing.
Can I use a larger fuse than recommended?
No. Oversizing the fuse defeats the purpose — the wire can overheat before the fuse opens. Always match fuse to wire ampacity and load.
What happens if my fuse is too small?
The fuse may nuisance-trip under normal load. Size at 125% of continuous current; for motors with high inrush, consider time-delay fuses.

Related tools

Voltage Drop CalculatorAWG Sizing CalculatorBattery Runtime CalculatorMarine electrical guideMotorsport electrical guideTechnical whitepapersHome

Save this to a LoomLab project for full harness validation.

Or Request Early Access.