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PCB Trace Width and Current: Temperature Rise, Voltage Drop and Loss

A trace can meet a temperature-rise estimate and still lose too much voltage. Size the conductor for both thermal and electrical requirements, using the actual copper thickness, route length and operating conditions. A calculator width is an estimate for its stated model, not a board-wide current rating.

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What the IPC-2221 calculation does

The implemented relationship is:

I = k · ΔT^0.44 · A^0.725

I is in amperes, A in square mils, and ΔT is the rise above ambient in °C. PCBPlanner uses k = 0.048 for external layers and 0.024 for internal layers. It solves for cross-section, then divides by copper thickness to get width. Do not enter square millimetres into the square-mil equation. ^[2]

Worked example: 3 A through a 50 mm trace

Assume 35 µm copper, a 10 °C allowed rise, 25 °C ambient and 50 mm route length. The IPC-2221 model gives the following values; resistance is evaluated at the resulting 35 °C conductor temperature.

Quantity	External	Internal
Calculated width	1.367 mm	3.556 mm
Cross-section	0.04785 mm²	0.12447 mm²
Trace resistance	19.079 mΩ	7.334 mΩ
Voltage drop at 3 A	57.238 mV	22.002 mV
Trace dissipation	171.713 mW	66.007 mW

Reproduce the 3 A trace-width and voltage-drop example →

Check voltage drop separately

R = ρ(TCOND)L/(W tCU) VDROP = IR PLOSS = I²R

Here W is width, tCU is copper thickness and TCOND is conductor temperature. PCBPlanner uses copper resistivity 1.7241 × 10⁻⁸ Ω·m at 20 °C and a temperature coefficient of 0.00393/°C. The external example loses about 1.73% of a 3.3 V rail in this one conductor. The return path, connector and vias add further drop.

Changing only the length from 50 to 100 mm doubles this model's resistance, drop and loss. Its IPC width remains unchanged because route length is not an input to that empirical sizing relationship. If the drop is unacceptable, increase width or copper thickness and check again.

Check the temperature rise and drop of a chosen 2 mm trace →

What changes with the IPC-2152 option?

The tool's IPC-2152 option is the Brooks–Adam external-trace regression, rather than a reproduction of every IPC-2152 chart or modifier. It compares the fit with the IPC-2221 external result:

ΔT = 215.3 · I² · W^−1.15 · Th⁻¹

W and Th are width and thickness in mils. The fit was derived from 2 oz and 3 oz external-trace data; thinner copper is an extrapolation. The paper's plotted comparisons cover widths of 5–200 mil, current up to about 20 A and rises up to about 100 °C. Nearby copper, planes and the board construction affect cooling, so the fitted result does not model all the conditions of your board. ^[1]

Compare the implemented models using 70 µm copper →

Finish the current path, not just the straight trace

Check neck-downs at pads, vias and connector pins, and evaluate the complete supply and return loop. A current-density number alone does not describe heat removal. For pulsed loads, fault currents, strong airflow or closely coupled heat sources, choose an analysis that represents those conditions and confirm critical temperatures and drops on hardware. ^[3]

Carry the 3 A requirement into the Via Calculator →

Tools used in this guide

Trace Width, Current & Temperature Rise – IPC-2221: width for a current, current for a width, or temperature rise of a trace, with resistance, voltage drop and loss.
Via Calculator – Via current capacity, resistance, thermal resistance, capacitance, inductance and rise-time impact.

Sources

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Guides explain published methods and data with their sources; check the values for your own design. Terms · Privacy

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