PCB Trace Width and Current Capacity: IPC-2221 vs IPC-2152
How to calculate PCB trace width for a given current. Compares IPC-2221 and IPC-2152 standards, explains temperature rise, and covers external vs internal layer differences.
IPC-2221 vs IPC-2152: Which Should You Use?
IPC-2221 (1998) is the legacy standard. It's conservative, based on measurements from 1954, and uses a simple empirical formula:Where *k* = 0.048 for external traces, 0.024 for internal; *ΔT* is temperature rise (°C); *A* is cross-sectional area in mil².
IPC-2152 (2009) is the current standard. It's based on modern measurements and is less conservative — it allows narrower traces or higher currents than IPC-2221 for the same temperature rise. For a 10A external trace with 10°C rise, IPC-2152 permits a trace approximately 30–40% narrower than IPC-2221. Use IPC-2152 for new designs. Use IPC-2221 only if your customer requires it by name.Temperature Rise Budget
The trace temperature is the sum of ambient temperature plus rise:
For FR4, the glass transition temperature (Tg) is typically 130–170°C. Stay below Tg by at least 20°C. In a 70°C ambient (inside a hot enclosure), your maximum trace temperature is ~110°C, leaving only 40°C of rise budget.
Typical design targets:
- Consumer electronics: 10°C rise
- Industrial: 20–30°C rise
- Power electronics: 30–40°C rise
External vs Internal Layers
Internal traces run hotter because they can't dissipate heat to air — only through the PCB laminate (poor thermal conductor, ~0.3 W/m·K vs ~150 W/m·K for copper). The IPC-2221 *k* factor of 0.024 for internal vs 0.048 for external reflects this directly. Internal traces need roughly 2× the cross-sectional area for the same current and temperature rise.
Copper Weight and Cross-Section
| Copper weight | Thickness | Area for 1mm wide trace |
|---|---|---|
| ½ oz | 17.5 µm (0.7 mil) | 0.7 mil² per mil width |
| 1 oz | 35 µm (1.4 mil) | 1.4 mil² per mil width |
| 2 oz | 70 µm (2.8 mil) | 2.8 mil² per mil width |
| 3 oz | 105 µm (4.2 mil) | 4.2 mil² per mil width |
Resistance and Voltage Drop
Even if thermal limits are met, check voltage drop:
Copper resistivity *ρ* = 1.72×10⁻⁸ Ω·m at 20°C, temperature coefficient *α* = 0.00393/°C.
For a 100mm, 1mm wide, 1oz trace carrying 3A:
- R = 0.049Ω
- V_drop = 0.15V
- P_loss = 0.44W
Practical Tips
- Pour copper on power rails rather than routing traces. A 10mm copper pour at 1oz carries 20A+ with <5°C rise.
- Thermal vias under hot traces improve heat spreading. Array them at 0.5–1mm pitch.
- Verify with IR camera on your first prototype. Calculated values assume ideal conditions — real boards often run cooler or hotter due to adjacent components and airflow.