Every PCB trace width current chart is really a temperature-rise chart. It answers the question of how wide a conductor must be to stay within a chosen temperature increase under a defined set of conditions, and those conditions include copper thickness, whether the trace is on an outer or inner layer, and an assumption of undisturbed air. Change any of them and the answer changes.
What the charts are actually telling you
The widely used guidance derives from empirical work relating conductor cross-section, current and temperature rise. You choose an acceptable rise above ambient, find the cross-sectional area required, and convert that to a width for your copper thickness. Nothing in the process defines a maximum current in an absolute sense; a narrower trace simply runs hotter. That means the designer, not the chart, decides what is acceptable, and the decision depends on the materials around the trace, the temperature rating of the laminate, the components nearby, and whether the rise is tolerable on top of an already warm ambient. Inner-layer traces are treated separately because they are surrounded by laminate rather than exposed to air, so they need more copper for the same rise.
The assumptions that rarely hold
A chart assumes a single isolated trace in still air at a stated ambient. A real board has neighboring traces carrying their own current, planes and components that both conduct heat away and add heat of their own, an enclosure that raises the local ambient, and possibly a conformal coating or potting compound that changes the thermal picture entirely. Several parallel current-carrying traces heat one another. A trace running under a hot component receives heat rather than shedding it. Conversely, a trace connected to a large copper pour or flooded with vias into a plane will run cooler than the chart predicts. Because these effects push in both directions, the honest approach is to use the chart for an initial size and then measure the finished assembly under load.
Voltage drop often decides before temperature does
For long traces the limiting factor is frequently resistance rather than heat. A conductor sized to keep temperature rise acceptable may still drop more voltage than a low-voltage rail can tolerate, particularly on boards operating from a small supply where a fraction of a volt matters. Calculate the resistance from the trace geometry and copper resistivity, remembering that resistivity rises with temperature so the hot resistance exceeds the cold one. Then check the drop at maximum current against the tolerance of whatever the trace feeds. Sense connections deserve the same thought: measuring a voltage through a conductor that also carries the load current builds the drop into your measurement, which is why four-terminal sensing exists.
Vias, connectors and the rest of the path
A trace is only one element of a current path. A via has far less copper than the trace it connects, since its conducting barrel is a thin plated cylinder, so a single via is usually the narrowest point in the path and multiple vias in parallel are standard practice for carrying real current between layers. Connector contacts, solder joints, fuse clips and component leads all contribute resistance and all have their own current ratings that may be lower than the trace’s. Thermal management has to consider the whole chain, and a thermal camera on a loaded board finds the true bottleneck much faster than a spreadsheet. Where currents are significant, the hottest point is often a via field or a connector rather than the conductor everyone worried about.
Where this becomes a safety matter
On a low-voltage signal board an overheated trace is a reliability problem. On a board carrying significant power, or one connected to a mains-derived supply, an overheated conductor is an ignition source and a potential path to a hazard. Boards in those applications are subject to construction requirements covering conductor spacing, creepage and clearance, and insulation coordination, and those requirements come from the applicable product safety standard rather than from a trace width chart. Fusing, whether by a proper protective device or by inadvertent reliance on the trace itself, must be designed deliberately. If a board interfaces with mains power or with a source capable of delivering large fault currents, treat the conductor sizing as part of a safety design and follow the applicable standards and the equipment manufacturer’s requirements.
What to check before you act
- Decide the acceptable temperature rise before reading any chart.
- Use the inner-layer figures for buried traces, not the outer-layer ones.
- Check voltage drop separately, especially on long low-voltage runs.
- Count vias, connectors and joints as part of the current path.
Common questions
Does doubling the trace width double the current capacity?
Not quite, because heat removal does not scale linearly with width and the surrounding conditions change. It helps substantially but the relationship is not proportional.
How many vias do I need for a given current?
It depends on the plating thickness and the via geometry, so use the fabricator’s data. Using several vias in parallel is normal practice wherever meaningful current changes layers.
The practical takeaway
Use a width chart as a starting estimate, then check voltage drop, count the vias and connectors in the path, and measure the assembled board under full load in its real enclosure. Where the board handles power or connects to a mains-derived supply, size conductors as part of a safety design rather than a routing convenience.
References and further reading
- NFPA: electrical safety research and guidance on overheating equipment and fire risk
- OSHA: wiring design and protection requirements for electrical installations
- CPSC: consumer electrical safety guidance for equipment in the home
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
