PCB Trace Fusing Current Calculator

Calculator

Trace width

Width unit

Copper thickness

Thickness unit

Trace length

Length unit

Selected fusing time, seconds

Ambient temperature, °C

Copper melting temperature, °C

Expected load or fault current, A

Derating factor, %

Copper resistivity at 20°C, Ω·m

Resistance temperature coefficient

Formula Used

The main estimate uses the Onderdonk copper fusing equation.

I = A × √[ ln(1 + (Tm - Ta) / (234 + Ta)) / (33 × t) ]

Here, I is fusing current in amperes. A is copper area in circular mils. Tm is copper melting temperature in °C. Ta is ambient temperature in °C. t is time in seconds.

Area mm² = trace width mm × copper thickness mm

Area circular mils = Area mm² × 1973.52524139

R = ρ × L / A

Voltage drop = Iload × R

Power loss = Iload² × R

How to Use This Calculator

Enter the trace width from your board layout.
Enter copper thickness in oz, µm, mil, or mm.
Add trace length for resistance and voltage drop checks.
Set ambient temperature and copper melting temperature.
Enter the time period for the fault event.
Enter the expected load or fault current.
Choose a derating percent for safer design review.
Press calculate and review the result above the form.
Use CSV or PDF download for your records.

Example Data Table

Trace Width	Copper	Time	Ambient	Load	Use Case
40 mil	1 oz	10 s	25 °C	5 A	Small power trace check
80 mil	2 oz	5 s	40 °C	20 A	Battery fault review
1.5 mm	70 µm	2 s	60 °C	30 A	Motor driver surge path

Understanding PCB Trace Fusing Current

PCB trace fusing current is the estimated current that can melt a copper trace during a fault. It is not the same as normal current rating. Normal rating focuses on temperature rise. Fusing current focuses on failure during a short time pulse.

Why This Calculator Matters

A thin trace can act like a weak link. That can protect a wider circuit in some designs. It can also create smoke, carbon paths, or board damage. A careful estimate helps designers review fault paths before layout release.

Important Inputs

Trace width and copper thickness define cross sectional area. Fusing time defines how long the fault current lasts. Ambient temperature affects stored thermal headroom. The copper melting point is normally near 1083 degrees Celsius. A derating factor lowers the theoretical value for practical design work.

Formula Behavior

This calculator uses the Onderdonk copper fusing method. It assumes heat stays mostly inside the copper during the pulse. That means it is best for short events. Long events need thermal modeling, airflow data, board copper spreading, and laminate limits.

Resistance and Loss Checks

The tool also estimates trace resistance from copper resistivity. Length affects resistance, voltage drop, and power loss. It does not directly change the basic fusing current equation. A long trace can still fail because extra heat is created along its path.

Using Results Safely

Treat the result as an engineering estimate. Manufacturing tolerance changes width and thickness. Solder mask, plating, vias, and nearby copper also change behavior. Internal traces cool differently from external traces. Always compare the result with standards, tests, and protection device curves.

Design Review Tips

Use a conservative derating factor when the board is costly or critical. Check the estimated time to fuse at the expected fault current. Verify that upstream fuses or current limiters operate first when needed. Keep enough clearance around sacrificial traces. Avoid placing them near plastics, batteries, or connectors.

Final Notes

A calculator cannot replace testing. It can make early decisions faster. Save the CSV or PDF record with your design notes. Then review it during layout, prototype bring up, and safety checks. Use repeated calculations to compare revisions and copper weights before final drawings are approved by your team.

FAQs

1. What is PCB trace fusing current?

It is the estimated current that can melt a copper PCB trace during a fault. It depends on copper area, ambient temperature, melting temperature, and fault duration.

2. Is fusing current the same as normal current capacity?

No. Normal current capacity checks temperature rise during operation. Fusing current checks melting during a short fault. Use both checks when reviewing a power trace.

3. Does trace length change fusing current?

In the Onderdonk estimate, length does not directly change fusing current. Length affects resistance, voltage drop, and power loss. Those values still matter for design safety.

4. What copper melting point should I use?

Pure copper melts near 1083 °C. The default value uses that number. Real PCB behavior can differ because copper is bonded to laminate and nearby materials.

5. Why is a derating factor included?

Derating lowers the theoretical fusing current. It adds design conservatism for copper tolerance, manufacturing variation, heating uncertainty, aging, and fault prediction errors.

6. Can a PCB trace be used as a fuse?

Sometimes, but it needs careful design and testing. A trace fuse can damage laminate or leave carbon residue. Certified fuses are safer for many products.

7. Is this calculator suitable for long overloads?

It is better for short fault events. Long overloads need thermal modeling, copper spreading data, airflow, board stackup details, and material temperature limits.

8. Why include resistance and voltage drop?

Resistance and voltage drop help check normal operation. A trace may survive fusing limits but still waste power, heat locally, or reduce load voltage.