PCB Design
PCB trace width for current
A PCB trace is a resistor. Its width sets how much current it can carry: too narrow, and it drops too many volts and runs hot. As a rule of thumb for standard 1 oz copper on an outer layer, 0.3 mm carries about 1 A, 0.8 mm about 2 A and 1.4 mm about 3 A, each for a 10 °C rise.
Count the squares
Standard board copper is 35 µm thick, called 1 oz. A square of it, as long as it is wide, has about 0.5 mΩ, whatever its size: a 1 mm square and a 5 mm square have the same resistance. So designers count squares along a trace:
Example: 100 mm of 0.25 mm trace is 400 squares, 0.2 Ω. Put 2 A through it and it takes 0.4 V before the load sees anything: a 5 V rail arrives as 4.6 V. Widen it to 2.5 mm and it's 40 squares, 0.02 Ω and 0.04 V.
Live experiment
Count the squares
A 5 V rail feeds Pip, a 2.5 Ω load drawing about 2 A, through 100 mm of trace. Pick the width and count its squares, its ohms and the volts the copper eats. At 0.25 mm the trace takes about 0.37 V; at 1 mm the load gets 4.9 V.
“A trace is a resistor” is lesson 1 of 6 in Copper Cay, PCB Design World 1. Free with an account. It’s open as soon as you sign up.
Heat is the real limit
The trace turns its share into heat: P = I² × R. Twice the current is four times the heat, and the heat goes where the ohms are. A trace that's wide along most of its length but squeezes through a narrow neck between two pads has most of its resistance, and most of its heat, in the neck. Burnt traces on damaged boards are almost always at a neck.
The IPC-2221 rule of thumb relates width, current and temperature rise; the newer IPC-2152 tables refine it. For 1 oz copper on an outer layer, at a 10 °C rise:
| Current | Width |
|---|---|
| 1 A | about 0.3 mm |
| 2 A | about 0.8 mm |
| 3 A | about 1.4 mm |
Inner layers are buried in the board and shed heat less easily, so they need two to three times the width. Heavier 2 oz copper halves the resistance.
Voltage drop, not just heat
A trace can stay cool and still drop too much voltage for a sensitive load. Work out both: the IPC width for temperature, then I × R for the volts the load can spare. Power traces on laptop and motor boards are wide for both reasons, and most boards past the simplest give ground a whole layer, a ground plane, so the return is milliohms.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“A trace is a resistor” is lesson 1 of 6 in Copper Cay, PCB Design World 1. Free with an account. It’s open as soon as you sign up.
Or start from the very first lesson, no account neededWork it out
Practice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: Halves its resistance. Twice the width is half the squares for the same length, and half the squares is half the resistance.
- Why not “Quarters it”? Resistance goes with length ÷ width. Doubling the width halves it; it doesn't quarter it.
- Why not “Doubles it”? A wider trace gives the current more copper to flow through, so its resistance goes down, not up.
- Why not “Leaves it”? Width changes the number of squares along the trace, so it changes the resistance.
Show the answer
Answer: 20 mm long and 2 mm wide. Both are 10 squares. Only length ÷ width matters, not the size of the squares.
- Why not “5 mm long and 2 mm wide”? 5 mm by 2 mm is only 2.5 squares, a quarter of the resistance.
- Why not “10 mm long and 2 mm wide”? 10 mm by 2 mm is 5 squares, half as many, so half the resistance.
- Why not “20 mm long and 1 mm wide”? 20 mm by 1 mm is 20 squares, twice as many as 10 mm by 1 mm, so twice the resistance.
Show the answer
Answer: 100 times more. P = I² × R, so 10 times the current is 10² = 100 times the heat. That's why faults burn copper.
- Why not “10 times more”? Heat goes with current squared, not current. Ten times the current gives a hundred times the heat.
- Why not “1,000 times more”? A thousand times would be current cubed. Heat in a resistance goes with current squared.
- Why not “The same”? Heat is I² × R, so more current through the same trace always makes more heat.
“A trace is a resistor” is lesson 1 of 6 in Copper Cay, PCB Design World 1. Free with an account. It’s open as soon as you sign up.
Questions people ask
How wide should a PCB trace be for 1 A?
On an outer layer of standard 1 oz copper, about 0.3 mm (12 mil) for a 10 °C temperature rise, by the IPC-2221 rule. Roughly 0.8 mm carries 2 A and 1.4 mm carries 3 A. Inner layers need two to three times the width, because they shed heat less easily.
How do I calculate a PCB trace's resistance?
Count squares: length ÷ width. Standard 1 oz copper, 35 µm thick, is about 0.5 mΩ per square. A trace 100 mm long and 0.25 mm wide is 400 squares, or 0.2 Ω. Doubling the copper weight to 2 oz halves it.
What is 1 oz copper on a PCB?
One ounce of copper spread over one square foot, which works out about 35 µm thick. It's the standard outer-layer copper. Heavier 2 oz copper, about 70 µm, halves the resistance and carries more current in the same width.
Does the ground return trace matter too?
Yes. The current flows out along the power trace and back along the return, and both drop voltage. A wide power trace with a thin return still loses volts and heats the return. A ground plane solves this for most boards.
Related topics
- How to calculate voltage drop in a wireA wire is a resistor: how many volts a cable steals, the heat it makes, and why the grid runs at high voltage.
- What is a ground plane on a PCB?A whole layer of copper for ground: milliohms for steady current, about a nanohenry for fast current.
- How to calculate junction temperatureHeat flows like current: watts through °C/W give a temperature rise, so size the copper before it cooks.
- What is a decoupling capacitor?Why every chip has a capacitor beside its power pins, how big, and why it must be close.