Trace Width, Current, and Resistance

Section Overview

Chapter 1 gave you a first, practical read of a trace's size — its width, its copper weight, and how those set the current it can carry (§1.5); this chapter has since examined the trace, via, and pad as features you can see and follow. This section returns to the trace to put numbers on it: its resistance, the voltage it drops, and how the current it carries heats it. Every trace has some resistance, set by the material and the trace's dimensions: the resistivity of copper — how strongly the metal itself opposes current — times the trace's length, divided by its cross-section (its width times its thickness), so a trace grows more resistant as it gets longer, narrower, or thinner. A handy shortcut, for a given copper weight, is sheet resistance, the resistance of one square of the copper: count how many squares long a trace is — its length divided by its width — and multiply, and you have its resistance without working in microns. That resistance has two consequences a repairer cares about. First, a trace carrying current drops a voltage along its length equal to the current times its resistance — a voltage drop that can starve a load or upset a circuit when a power trace is too thin. Second, that same current turns power into heat in the resistance, warming the trace to a temperature rise above its surroundings — and a trace's ampacity, the current it may safely carry, is really the current that produces an acceptable temperature rise, which is what published current tables capture from a trace's width and copper weight (§1.5). For a repairer the payoff is judgement: these numbers tell you whether a jumper or a rebuilt trace will carry the load without dropping too much voltage or overheating, and they let you size a repair to match the original (§5; §8). Put numbers on a trace, and you can repair its power paths with confidence rather than hope.

Why This Matters

A trace's resistance, voltage drop, and heating are what decide whether a power path works and whether a repair to it is safe — so putting numbers on a trace turns guesswork into judgement. This matters because a repair must carry the load: if you bridge or rebuild a current-carrying trace with copper that is too thin, its higher resistance drops more voltage and runs hotter, so it can fail — or start a fire — right where you fixed it, which is why you size a repair to the original's cross-section (§5; §8). This matters because voltage drop causes real faults: a thin or long power trace drops enough voltage that a load downstream misbehaves — a chip browns out, a motor runs weak — and recognizing an unexpected voltage drop as the symptom points you to the trace (Volume 3). It matters because heat is the limit on current: a trace's ampacity is set by how hot it may get, so understanding temperature rise is understanding why a given width carries only so much current before it overheats (§1.5). It matters because the numbers are readable from the board: width and copper weight are things you can measure or estimate, and from them a current table gives a safe current directly (§1.5). And it matters because it keeps a repair honest: estimating a jumper's resistance and its heating before you fit it is the difference between a repair that lasts and one that becomes the next fault. Learn to put numbers on a trace, and you can judge a power path, diagnose a voltage-drop fault, and size every repair to carry its load safely.

Required Prerequisites

• Copper Weight and Current Capacity — Section 1.5 introduced a trace's width, its copper weight, and how those set the current it can carry; this section puts numbers on the same trace — its resistance, voltage drop, and heating. You should be comfortable with Ohm's law from Volume 1 and the trace as a conductor from Section 2.1. This is a reasoning section — no hot work; measuring a voltage drop needs a powered board, so bring care if you do.

Recommended Consumables

• A few boards with visible power traces of different widths — to estimate resistance and relate width to the current a trace carries
• A trace-width or PCB current calculator, or a printed current table (IPC-2221 style) — to turn width and copper weight into a safe current (§1.5)
• A notebook and calculator — to work out squares, resistance, voltage drop, and heating
• Isopropyl alcohol and a brush — to clean a board so trace widths are easy to measure

Recommended Practice Hardware

• A ruler, calipers, or a scale on a magnifier — to measure a trace's width and length
• A multimeter — to measure a voltage drop along a powered trace (millivolts) or a low resistance, if you choose to
• A magnifier or loupe — to see and measure fine traces
• No iron, hot air, or hot work is needed — this section is measurement and reasoning, not procedure

Real-World Applications

Putting numbers on a trace guides how a repairer sizes jumpers, diagnoses voltage-drop faults, and judges whether a power path is healthy. A technician jumpering a broken power trace estimates the original trace's cross-section and picks a wire of at least equal capacity, so the repair carries the current without overheating (§8). Someone chasing a browning-out load measures the voltage drop from the supply to the load along a suspect trace and finds an unexpected drop that points to a thin, damaged, or corroded conductor (Volume 3). A repairer assessing a burned trace works out roughly how much current it must have carried to overheat, which helps find the overload behind it (§5). A builder checking a design counts the squares of a power trace to estimate its resistance and confirms the voltage drop to the load is acceptable. And anyone who has fixed a power trace with hookup wire that was too thin learns that resistance and heating are real, because the "repair" ran hot and failed again. The failures this skill prevents: an undersized jumper that overheats, a repair that drops too much voltage, missing a voltage-drop fault, and misjudging how much current a trace can safely carry (§5; §8).

Common Challenges

• Sizing a repair by looks, not by load. A jumper must carry the original's current — estimate the cross-section and match it, or the repair overheats (§8).
• Forgetting that length adds resistance. A long jumper has more resistance and drops more voltage than the short trace it replaced — keep power jumpers short and adequately thick (§8).
• Ignoring where heat goes. A trace's safe current depends on an allowed temperature rise and on cooling — a table's current assumes a rise you must be willing to accept (§1.5).

Safety Notes

Risk Level: Low. This is a measurement-and-reasoning section, but what it teaches is a safety essential — the numbers are how you keep a repair from overheating.

Professional Tips Before Starting

• Think in squares. For a given copper weight, a trace's resistance is just its sheet resistance times its length in squares — counting squares is faster than working in microns.
• Size a jumper to the trace, then a little over. Match at least the original's cross-section and keep the jumper short, so its resistance and heating are no worse than the trace it replaces (§8).
• Use a current table, and know its rise. A published current for a width and copper weight assumes an allowed temperature rise — read what rise it uses before you trust the number (§1.5).

The Trace, by the Numbers

Recap and Frame

Before working the numbers, it helps to see what this section adds to what you already know about a trace's size. In Section 1.5 you learned a trace's width and copper weight, and used them as a first, practical read of how much current it can carry — wide, heavy copper carries more (§1.5). That was deliberately a read-at-a-glance skill: no arithmetic, just "wider and thicker means more current." This section goes one level deeper and puts actual numbers behind that read: why a trace has the resistance it does, how much voltage it drops when it carries current, and how that current heats it. The point is not to turn you into a board designer: it is to give a repairer enough quantitative judgement to size a repair and diagnose a power path. Three ideas do most of the work — resistance, voltage drop, and heating — and they are linked: a trace's dimensions set its resistance; its resistance and the current set the voltage it drops and the heat it makes; and the heat it may make sets the current it can safely carry. You have met the last of these already as ampacity, from the outside (§1.5); here you will see the machinery underneath it. Keep the repairer's use in view throughout — will this conductor carry the load without dropping too much or getting too hot? — and start with where a trace's resistance comes from.

Trace Resistance

Everything in this section rests on one fact: a trace, like any conductor, has resistance, and its value follows a simple rule. A trace's resistance is the resistivity of its material — a fixed property of copper that says how strongly the metal itself opposes current — multiplied by the trace's length and divided by its cross-sectional area (conductor, §2.1). Copper's resistivity is low, which is why it is used, but it is not zero, so a real trace always has some resistance. The dimensions do the rest. Length works against you: twice as long is twice the resistance, because the current must push through twice as much copper. Cross-section works for you: the wider the trace and the thicker its copper, the more room the current has, and the lower the resistance — double the width or double the copper weight, and you halve the resistance (§1.5). So the rule is short: resistance rises with length and falls with width and thickness. In practice the numbers are small — a short, wide power trace might be a few thousandths of an ohm — but small resistances matter when large currents flow through them, because, as you will see, both the voltage a trace drops and the heat it makes grow with that resistance. This is also why the traces you most need to think about are the long, thin, high-current ones: there, a small resistance times a large current becomes a voltage drop and a heating you can measure and must respect. Know that resistance equals resistivity times length over cross-section, and you know the root of everything else in this section.

Sheet Resistance — Counting Squares

Working a trace's resistance from microns and material constants is fiddly, so there is a shortcut that fits how boards are actually built: sheet resistance. For a given copper weight, the copper layer has a fixed sheet resistance, the resistance of one square of it — and here a "square" means any patch of the trace that is exactly as long as it is wide, whatever the size (copper weight, §1.5). The neat part is that a square's resistance does not depend on how big the square is: a big square and a small square of the same copper have the same resistance, because making it bigger widens the current path exactly as much as it lengthens it. So to find a trace's resistance you simply count how many squares long it is — its length divided by its width — and multiply by the sheet resistance. A trace ten times as long as it is wide is "ten squares," and its resistance is ten times the sheet resistance. For common 1 oz copper the sheet resistance is roughly half a milliohm per square, so that ten-square trace is about five milliohms (§1.5). This is how a repairer can estimate resistance in their head: judge the trace's length-to-width ratio, multiply by a familiar per-square figure for the copper weight, and you have a working number without a calculator. Heavier copper has a lower sheet resistance (more thickness, less resistance per square); lighter copper, higher. Counting squares turns the resistance rule into something you can actually use at the bench.

Voltage Drop Along a Trace

The first thing a trace's resistance does, once current flows, is drop a voltage — and that voltage drop is a common, real fault. By Ohm's law, the voltage lost along a trace is the current through it times its resistance (voltage drop, Volume 1). For a signal trace carrying almost no current this is negligible, which is why signal traces can be thin (§2.1). For a power trace carrying real current it can matter a great deal: a few milliohms times several amps is tens of millivolts or more, and along a long, thin supply trace that can grow to a drop the load actually feels. The consequence is that the voltage arriving at a part is a little lower than the voltage that left the supply, by the drop along the trace between them. Usually that is designed to be small and harmless. But when a power trace is too thin, too long, damaged, or corroded — or when a repair replaces it with something worse — the drop grows, and a load downstream can misbehave: a processor browns out and resets, a regulator struggles, a motor runs weak. This makes voltage drop a diagnostic tool: measuring the voltage at the supply and again at the load, on a powered board, reveals how much is being lost along the way, and an unexpectedly large drop points to a thin, damaged, or high-resistance conductor between them (Volume 3). It also sets a rule for repairs: a jumper on a power trace must be thick and short enough that it does not add a drop of its own (§8). Remember that current times resistance is a voltage lost, and you can both diagnose a starved load and avoid creating one.

Current and Temperature Rise

The second thing a trace's resistance does is turn some of the current's power into heat, and that heating is what ultimately limits how much current a trace may carry. When current flows through resistance, power is dissipated as heat — and because that power grows with the square of the current, doubling the current makes four times the heat. That heat warms the trace above the temperature of its surroundings, to a temperature rise that depends on how much heat is made and how well it is carried away by the copper, the board, and the air (ampacity, §1.5). A little current makes a rise too small to matter; enough current makes a rise that can discolour the board, damage nearby parts, or eventually open the trace. This is the real meaning of a trace's ampacity: it is not a hard electrical limit but a thermal one — the current a trace may carry is the current that warms it to a temperature rise you are willing to accept. That is exactly what published current tables encode (§1.5): they take a trace's width and copper weight, assume an allowed rise — commonly 10 °C or 20 °C — and give the current that produces it, which is why the same trace has a higher rating if you accept a bigger rise. For a repairer this ties the whole section together: a repair that is too thin has more resistance, so it makes more heat for the same current and reaches a higher, possibly dangerous temperature rise. Match the cross-section and the heat stays the same; undersize it and the heat — and the risk — climb. Understand that current heats a trace to a temperature rise, and you understand why ampacity is what it is and why a repair must be sized to carry its load.

Using the Numbers for Repair

All of this exists to serve one practical end: sizing and judging a repair so a power path stays healthy after you fix it. Start by reading the original. Estimate the damaged trace's width and, from the board's copper weight, its cross-section, and count its squares for a resistance if you need one (§1.5). Then size the repair to match or beat it. A jumper must carry at least the current the original did, so choose a wire whose cross-section is at least equal to the trace's, and keep it short so it adds little length, little resistance, little voltage drop, and little heat (§8). Check the two consequences. Ask whether the repair's resistance times the current would drop a voltage the load cannot tolerate, and whether the current would heat the repair to an unacceptable temperature rise — if either is too high, use a thicker or shorter conductor (§5; §8). Use a table for the current. Rather than compute heating from scratch, look up the safe current for your wire or trace width and copper weight in a current table, noting the temperature rise it assumes (§1.5). And use the numbers to diagnose, not just to build. An unexpected voltage drop points to a high-resistance conductor; a trace that overheated carried more current than its size allowed — read both as clues to the real fault (Volume 3; §5). In every case the discipline is the same: estimate the original, match it, and check the voltage drop and the heating before you trust the repair. Put numbers on the trace, and every power-path repair becomes a judgement you can defend rather than a hope you are testing.

Common Mistakes

• Jumpering a power trace with wire that is too thin. Thin wire has more resistance, drops more voltage, and overheats — match at least the original's cross-section (§8).
• Making a power jumper long and roundabout. Length adds resistance and voltage drop — keep power jumpers short and direct (§8).
• Trusting a current table without its temperature rise. A table's current assumes an allowed rise — know whether it is 10 °C or 20 °C before you rely on it (§1.5).
• Replacing a burned trace without finding the overload. A burned trace carried too much current — fix the fault behind it first, or the repair burns too (§5).
• Measuring a voltage drop on an unpowered board. There is no drop without current — measure voltage drop on a powered board, with care (Volume 2).

Troubleshooting Guidance

Trace-number problems come down to too much resistance, too much drop, or too much heat. If a load browns out or runs weak: measure the voltage drop from supply to load on a powered board — a large drop means a thin, long, damaged, or corroded conductor between them (Volume 3). If a repaired trace runs hot: the jumper is too thin or too long for the current — replace it with a thicker, shorter conductor (§8). If a trace burned open: it was overloaded — find the short or failed part downstream before replacing it (§5). If you are not sure a wire will carry a current: look it up in a current table by cross-section and note the temperature rise (§1.5). If a resistance seems too high: count the trace's squares and check against its sheet resistance — a much higher reading suggests damage or corrosion. If a long power run drops too much voltage: widen or shorten the path, or accept it by design — a longer, thinner conductor always drops more (§8). If you must probe a powered board: keep one hand clear and use fine probes so you do not bridge conductors (Volume 2). The throughline: find whether the trouble is resistance, drop, or heat, and fix it by giving the current more copper — wider, thicker, or shorter.

Verification & Testing Methods

Use this as a check that you can put numbers on a trace, not a hot procedure:

• [ ] I can explain that a trace's resistance is copper's resistivity times its length over its cross-section, and how length, width, and thickness change it (§2.1).
• [ ] I can estimate a trace's resistance by counting its squares and multiplying by the sheet resistance for its copper weight (§1.5).
• [ ] I can explain the voltage drop along a current-carrying trace as current times resistance, and when it matters (Volume 1).
• [ ] I can relate current, temperature rise, and ampacity, and say why a current table depends on an allowed rise (§1.5).
• [ ] I can size a jumper or rebuilt trace to carry a load without excess voltage drop or heating (§5; §8).

Then try the practice exercises below — measurement and reasoning practice; scenarios differ from the quiz.

Practice Exercises

1. Count the squares (5 minutes, reasoning). Measure a power trace's length and width, work out how many squares long it is, and estimate its resistance using a per-square figure for its copper weight (§1.5).
2. Estimate a voltage drop (5 minutes, reasoning). For a current you choose, multiply it by that estimated resistance to find the voltage the trace would drop, and judge whether a load would notice.
3. Size a jumper (4 minutes, reasoning). For a broken power trace of a given width and copper weight, decide what wire gauge would carry its current, keeping the jumper short (§8).
4. Read a current table (4 minutes, applied). Using a current table or calculator, find the safe current for a trace width and copper weight, and note the temperature rise the figure assumes (§1.5).

These core ideas — trace resistance, sheet resistance and squares, voltage drop, current and temperature rise, and sizing a repair — are tested in the Chapter Quiz at the end of this chapter, where a score of 80% is required to continue.

Key Takeaways

• A trace's resistance is copper's resistivity times its length divided by its cross-section, so it rises with length and falls with width and copper weight — longer, narrower, or thinner means more resistance (§1.5).
• For a given copper weight, resistance is just the sheet resistance (about half a milliohm per square for 1 oz copper) times the number of squares — the trace's length divided by its width — which is how to estimate resistance quickly.
• A current-carrying trace drops a voltage equal to the current times its resistance; negligible for a signal trace, this voltage drop can starve a load along a thin or long power trace, and it makes a diagnostic clue and a rule for repairs (Volume 3; §8).
• That same current turns power into heat (growing with the square of the current), warming the trace to a temperature rise above ambient; a trace's ampacity is the current that produces an acceptable rise, which is what current tables encode from width and copper weight (§1.5).
• To repair a power path, estimate the original's cross-section, size a jumper to match or beat it and keep it short, and check that its voltage drop and temperature rise stay acceptable (§5; §8).

Skills Learned

• You can now explain what sets a trace's resistance and how its dimensions change it.
• You can now estimate a trace's resistance from its sheet resistance and its squares.
• You can now explain the voltage drop along a trace and when it matters.
• You can now relate current, temperature rise, and ampacity for a trace of a given size.
• You can now size a jumper or rebuilt trace to carry a load without excess drop or heat.

Glossary Additions

• resistivity — a fixed property of a material that says how strongly the material itself opposes electric current, independent of shape or size; it is the constant that, multiplied by a conductor's length and divided by its cross-sectional area, gives the conductor's resistance. Copper's resistivity is low, which is why it is used for traces and wires, but it is not zero, so every trace has some resistance that grows with its length and shrinks with its width and thickness. Resistivity is what links a trace's physical dimensions to the resistance it presents to current.
• sheet resistance — the resistance of one "square" of a conducting sheet of a given thickness, such as a board's copper layer of a given copper weight; a square is any patch as long as it is wide, and its resistance is the same whatever its size, because enlarging it widens the current path as much as it lengthens it. A trace's resistance is then its sheet resistance times the number of squares along it — its length divided by its width. For common 1 oz copper the sheet resistance is roughly half a milliohm per square, which lets a repairer estimate a trace's resistance by counting squares rather than working in microns.
• temperature rise — how much hotter a trace becomes than its surroundings when it carries current, caused by the electrical power (growing with the square of the current) that its resistance turns into heat, and set by how much heat is made versus how well the copper, board, and air carry it away. Temperature rise is the real limit on how much current a trace may carry: a trace's ampacity is the current that produces an acceptable rise — commonly 10 °C or 20 °C in published current tables — so accepting a larger rise allows a higher current, and an undersized repair reaches a higher, riskier rise for the same current.

Suggested Next Sections

Must read next:

• Identifying Traces on Multi-Layer Boards — you now know the trace inside and out — its anatomy, its role, and the numbers behind it; the final section of this chapter tackles the hardest reading problem: following traces across the hidden inner layers of a multi-layer board, using the board's stackup, its vias, X-ray, and the fabrication files to trace a connection you cannot simply see.

Recommended:

• Copper Weight and Current Capacity — the width and copper weight that set a trace's current capacity, read at a glance.
• Trace Anatomy and Function — the trace as a conductor, whose dimensions this section puts numbers to.