Fuse Size Calculator

A fuse protects your electrical circuit by breaking the connection if current exceeds a safe level — choosing the wrong size can damage wiring or leave the circuit unprotected. Enter your load current, circuit voltage, circuit type, and wire size, then specify whether it's a motor load, located in an engine room, or has bundled wires. The Fuse Size Calculator returns your recommended fuse size, along with the minimum fuse size, maximum fuse size, and safety margin.

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The current draw of your device or circuit

Branch circuits feed individual devices, feeder circuits supply multiple branch circuits

Motor Load? *

Motors require higher inrush current protection

Higher temperature environment affects fuse rating

Wire gauge affects maximum safe current

More bundled wires reduce current capacity due to heat buildup

Results

Recommended Fuse Size

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Minimum Fuse Size

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Maximum Fuse Size

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Safety Margin

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Frequently Asked Questions

How do I determine if I need a branch or feeder circuit fuse?

Use a branch circuit fuse for individual loads like lights or radios. Use a feeder circuit fuse for main power lines that supply multiple branch circuits or devices.

Why does motor load require a different fuse size?

Motors draw much higher current during startup (inrush current) than their normal operating current. The fuse must handle this temporary surge without blowing.

How does wire bundling affect fuse sizing?

Multiple wires bundled together generate more heat and reduce current carrying capacity. More bundled wires require lower fuse ratings to prevent overheating.

What's the difference between minimum and maximum fuse size?

Minimum fuse size protects the load from damage. Maximum fuse size protects the wire from overheating. Your fuse should be between these values.

Should I use the exact calculated fuse size or round up?

Always round up to the next available standard fuse size. Never use a fuse smaller than the calculated minimum or larger than the calculated maximum.

How does engine room installation affect fuse selection?

Engine rooms have higher ambient temperatures, which reduces wire current capacity and may require derating the fuse size for safety.

Can I use this calculator for AC circuits?

This calculator is designed for DC circuits (12V, 24V, 32V). AC circuits have different safety requirements and should follow AC electrical codes.

What happens if I use too large or too small a fuse?

Too small: fuse blows unnecessarily during normal operation. Too large: won't protect wire from overheating and fire risk, potentially causing dangerous conditions.