A copper trace is a resistor, and it warms up when current flows. Board designers size power traces so they stay cool and drop little voltage. In repair, the same calculation tells you whether a damaged trace was carrying real power — and how much copper a replacement path needs. This calculator uses the widely used IPC-2221 approximation.
How to use it
- Enter the current the trace carries.
- Choose the temperature rise you accept — 10 °C is a common design target.
- Choose the copper weight and whether the trace is on an outer or inner layer.
- Optionally enter the length to get resistance, voltage drop and heat.
The formula
IPC-2221 relates current, temperature rise and cross-section:
A = (I ÷ (k × ΔT^0.44))^(1 ÷ 0.725), with A in square mils, k = 0.048 for outer layers and 0.024 for inner layers.
Width follows from the copper thickness: 1 oz copper is about 1.378 mil (35 µm) thick. The formula is a conservative estimate from older test data; modern design tools give more detailed results, but for repair decisions it is more than accurate enough.
Worked example: a burnt trace on the adapter input
A trace carrying 2 A from the DC jack burned through after a short. On an outer layer with 1 oz copper and a 10 °C rise, the calculator gives a minimum width of about 0.8 mm. Over 20 mm, that trace has around 12 mΩ and drops about 25 mV. A single 30 AWG jumper of the same length has a similar resistance; for a heavier current, use thicker wire or several in parallel, as shown by Jumper Wire AWG Calculator. The board's layer structure is explained under pcb layers, and copper paths under trace.
Where it matters in repair
- Judging whether a damaged trace was a power path or a signal line.
- Choosing jumper size when rebuilding a power path.
- Understanding why a thin trace near a short burned before the fuse blew.
FAQ
Why do inner layers need wider traces?
They cannot shed heat to the air, so the same current makes them warmer. The formula uses half the k value for inner layers.
How thick is the copper on a laptop board?
Typically 0.5 to 1 oz on signal layers, sometimes more on power planes. Large currents use whole planes rather than traces.
Is IPC-2221 still used?
It is a common quick estimate. Newer standards give more refined results, but the difference rarely matters for a repair decision.