Electrical reference chart
Fuse Size Chart
Use this fuse size chart after the calculator result to document load type, fuse class, conductor protection, interrupting rating, and equipment instructions before selecting an overcurrent device.
Quick reference table
Fuse sizing is a protection worksheet, not just a normal-load ampere lookup. Use the calculator with load current, continuous duty, fuse class, conductor ampacity, equipment listing, available short-circuit current, coordination needs, manufacturer instructions, adopted NEC requirements, and AHJ review.
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
| 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 |
How to use this chart
Classify the load
Identify whether the load is continuous, motor, transformer, HVAC, capacitor, service, or equipment-specific before selecting a fuse path.
Document equipment limits
Write any manufacturer maximum fuse size, required fuse class, voltage rating, or protection instruction before relying on a general estimate.
Check fault and coordination context
Use the calculator result with available fault current, interrupting rating, SCCR, conductor protection, and any selective-coordination requirement.
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 instructions
If 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 review
A transformer load may need inrush tolerance, conductor protection, available fault-current review, and manufacturer guidance documented before selecting a fuse class and rating.
Related calculators
Fuse Sizing Calculator
Screen general branch-load, motor branch-fuse, transformer primary-fuse, and capacitor-bank fuse ampere ratings from a visible current basis.
Circuit Breaker Sizing Calculator
Calculate standard breaker sizes for general loads, continuous loads, and simplified motor branch-circuit reviews
Short Circuit Current Calculator
Estimate available fault current from transformer impedance, feeder impedance, or known source impedance.
Related charts
Breaker Size Chart
Plan breaker size from load category, continuous duty, conductor protection, interrupting rating, equipment listing, and calculator result notes.
Short Circuit Current Chart
Plan short-circuit current checks from utility data, transformer kVA and impedance, voltage, conductor path, motor contribution, and equipment SCCR/AIC.
Wire Size Chart
Screen conductor size from calculated load, copper or aluminum material, terminal rating, derating, voltage drop, and equipment notes.
Frequently asked questions
These answers explain how to use the chart without turning a quick reference into a final design decision.