Fuse Size Chart

Use this fuse size chart after you open the Fuse Sizing Calculator and have a calculator result to document load type, fuse class, conductor protection, interrupting rating, and equipment instructions before selecting an overcurrent device.

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Quick reference table

Fuse sizing is a protection worksheet, not just a normal-load ampere lookup. Start with the Fuse Sizing Calculator for load current, continuous duty, application type, fuse class, conductor ampacity, equipment listing, available short-circuit current, coordination needs, manufacturer instructions, adopted NEC requirements, and AHJ review.

Fuse sizing worksheet inputs

Fuse sizing worksheet inputs
Input	Record	Why it matters
Load basis	Calculated amps, nameplate data, duty, and source of value	Sets the initial current screen
Fuse class	Class, voltage rating, time-delay behavior, and manufacturer requirement	Affects clearing behavior and equipment compatibility
Conductor protection	Conductor size, material, ampacity, and terminal rating	Keeps the fuse tied to the protected conductors
Fault current	Available short-circuit current at the installation point	Checks interrupting rating and equipment duty
Coordination need	Service, emergency, life-safety, process, or downstream selectivity goal	Can change fuse class or rating choice

Fuse result-area follow-up

Fuse result-area follow-up
Calculator result shows	Field follow-up	Why it matters
Standard fuse rating selected	Check equipment maximum fuse size and conductor protection	Equipment labels can override a generic result
Time-delay fuse considered	Document inrush, motor starting, or transformer energization reason	Time-current behavior must match the application
High available fault current	Verify interrupting rating and SCCR of the assembly	Normal load current does not prove short-circuit suitability
Selective coordination required	Compare upstream and downstream device behavior	Coordination can control the final fuse choice

Formula basis

Fuse planning screen = calculated load current adjusted for duty and equipment type, then compared with conductor protection limits, permitted fuse class, standard ratings, and interrupting rating.

Calculated load current is the normal operating current, nameplate current, or code-calculated load.
Load type can be continuous, motor, transformer, HVAC, capacitor, service, or equipment-specific.
Interrupting rating and fuse class must be reviewed against available fault current and equipment instructions.

Worked examples

Equipment with manufacturer fuse instructionsIf equipment instructions call for a fuse class or maximum fuse size, record that requirement before using the calculator result as the planning basis.

Transformer primary fuse reviewA transformer load may need inrush tolerance, conductor protection, available fault-current review, and manufacturer guidance documented before selecting a fuse class and rating.

Assumptions. Balanced load and line-to-line voltage assumptions behind this chart.

This chart is a planning worksheet and does not approve any specific fuse rating or class.
Fuse class, time-current behavior, selective coordination, equipment listing, and available fault current can control the final choice.
The worksheet does not replace a coordination study, equipment label, or manufacturer protection instruction.

Code and standard notes. Planning limits that should be checked before final equipment selection.

Verify the adopted NEC edition, equipment instructions, manufacturer data, local amendments, and the Authority Having Jurisdiction (AHJ) before final installation decisions.
Verify fuse interrupting rating, selective coordination needs, conductor protection, SCCR, and equipment manufacturer instructions before installation.

How to use this chart

1Classify the loadIdentify whether the load is continuous, motor, transformer, HVAC, capacitor, service, or equipment-specific before selecting a fuse path.

2Document equipment limitsWrite any manufacturer maximum fuse size, required fuse class, voltage rating, or protection instruction before relying on a general estimate.

3Check fault and coordination contextUse the calculator result with available fault current, interrupting rating, SCCR, conductor protection, and any selective-coordination requirement.

Worksheet checklist. Record source basis, review gaps, and assumptions before using the chart result.

Record current basisIdentify whether current comes from calculated load, nameplate data, measured load, equipment schedule, or transformer/motor calculation.
Add protection contextDocument fuse class, conductor ampacity, terminal rating, available fault current, equipment SCCR, and manufacturer instructions.
Verify downstream behaviorCheck whether selective coordination, motor starting, transformer inrush, downstream protection, or service requirements affect the final fuse choice.

Common mistakes to avoid. Review these before turning chart current into an equipment decision.

Selecting a fuse from load amps alone without checking fuse class, conductor limits, equipment listing, interrupting rating, and SCCR.
Ignoring manufacturer maximum fuse instructions because a general calculation suggests a different standard rating.
Matching ampere rating while overlooking voltage rating, time-current behavior, current limitation, or coordination with downstream devices.

Frequently asked questions

These answers explain how to use the chart without turning a quick reference into a final design decision.

Is fuse size the same as breaker size?

Not always. Both are overcurrent protective devices, but fuse class, time-current behavior, equipment instructions, and interrupting rating can lead to different decisions.

Why does available fault current matter?

The fuse must be suitable for the available short-circuit current at the installation point, not only the normal load current.

Can I ignore fuse class if the amperes match?

No. Fuse class, voltage rating, time-delay behavior, current limitation, and equipment listing can all matter even when the ampere rating appears correct.