Copper Weight and Current Capacity

Section Overview

The previous sections built up the board's structure and looked at the insulating material it is made from; this closing section of the chapter turns to the copper itself and answers a very practical question: how much current can a trace carry? The answer rests on two dimensions of the copper and one limit. The first dimension is copper weight — the thickness of the copper layer, which the industry specifies in the slightly odd unit of ounces per square foot: one ounce copper is a layer about thirty-five micrometres thick, two ounce is twice as thick, and half-ounce is thinner, and heavier copper means thicker traces that carry more current and have lower resistance. The second dimension is trace width — how wide a given trace is drawn; a designer varies the width trace by trace, so a wide trace has more copper cross-section than a thin one and carries more current with less resistance, which is exactly why you can look at a board and see fat power traces and thin signal traces. Width times thickness gives the trace's cross-sectional area, and that area, together with how hot the trace is allowed to get, sets its ampacity — the amount of current it can carry continuously without overheating. The limit is heat. Current flowing through the copper's small resistance dissipates power and warms the trace, and if you push more current than the trace can shed, it overheats; a badly overloaded trace can, in the extreme, act like a fuse and burn open. Resistance also drops voltage along a trace, which is why a thin power trace sags the supply and runs warm. For a repairer, all of this converges on one rule: when you repair, bridge, or jumper a current-carrying trace, the replacement must carry at least the same current as the original — you match the cross-section, using a wire of adequate gauge or enough width on a rebuilt trace — and a trace that has burned open is telling you it was overloaded, so you find and fix the cause before you replace it. Read a trace's width and the board's copper weight, and you can judge what it carries and how to repair it without starting a fire.

Why This Matters

Knowing how much current a trace carries is not academic — it is what keeps a trace repair from overheating, failing, or catching fire, and it is what lets you read a board's power paths at a glance. This matters because a repair must carry the load: if you bridge or jumper a power trace with a wire that is too thin, it will overheat and fail exactly where you fixed it, so sizing the repair to the original current is a safety essential, not a nicety (§5; §8). This matters because the copper's size is readable: once you know that wide, heavy copper carries more current, you can look at a board and immediately tell the power traces from the signal traces, which helps you understand what the board is doing. It matters because overload leaves evidence: a trace that has discolored, lifted, or burned open is telling you it carried more current than it could handle, and treating that as a symptom — rather than just replacing the trace — is what finds the real fault (§5). It matters because resistance has real effects: a thin or long power trace drops voltage and dissipates heat, which can make a circuit misbehave or run hot, and recognizing that guides diagnosis (Volume 1). And it matters because it protects you and the device: a repair that lets too much current flow into a fault, or a jumper that overheats, is a fire risk, and understanding current capacity is what stops you from creating one. Learn to judge a trace's current capacity, and your repairs carry the load safely and your diagnoses read the board correctly; ignore it, and a "fixed" trace becomes the next thing to burn.

Required Prerequisites

• PCB Materials — FR4, Rogers, Flex — Section 1.4 covered what the board is made of and its heat limits; this section turns to the copper and how much current it carries. You should know from Section 1.1 that traces are copper, and a basic grasp of resistance, current, and power from Volume 1 (Ohm's law and the idea that current through resistance makes heat) will help. This is a knowledge and reasoning section — no hot work or power.

Recommended Consumables

• Example boards with obviously wide power traces and thin signal traces — to see the difference in copper width directly
• A board fabrication note or datasheet that states the copper weight — to see how copper thickness is actually specified
• A printed trace-width-versus-current chart, or a trace-width calculator — to estimate a trace's current capacity
• An assortment of hook-up wire of different gauges — to compare cross-sections and pick a jumper to match a trace

Recommended Practice Hardware

• A power board (a charger, a motor driver, a power supply) with visibly heavy traces — to study high-current copper
• A magnifier — to compare trace widths and see where copper narrows
• A trace-width/current calculator or an IPC-2221-style chart — to put numbers to a trace's capacity
• No iron, hot air, or powered board is needed — this section is reading and reasoning, not procedure

Real-World Applications

Judging current capacity comes up on every repair that touches a power path, and it is the difference between a repair that lasts and one that burns. A technician jumpering a broken power trace looks at how wide and heavy the original trace was, and chooses a jumper wire of a gauge thick enough to carry that current, rather than a thin signal wire that would overheat (§8). A repairer who finds a trace burned open does not simply bridge it — recognizing that a burned-open trace was overloaded, they look downstream for the short or failed part that caused it, fix that first, and only then repair the trace (§5). Someone reading an unfamiliar board spots the wide traces running from the power connector and immediately knows those are the high-current paths, which orients their diagnosis. A builder choosing wire for a high-current rework checks a trace-width chart or the board's copper weight to be sure the repair matches the original capacity. And anyone who once jumpered a power trace with fine wire learns, when it discolors and fails, that the repair must carry the load the original did. The failures this understanding prevents: a too-thin jumper that overheats, a burned trace replaced without finding the cause (so it burns again), a misread of which traces carry power, and a repair that becomes a fire risk — all avoided by sizing to the current and reading the copper.

Common Challenges

• Assuming any wire will do for a jumper. A power trace needs a jumper of adequate gauge — a thin wire that cannot carry the current will overheat and fail (§8).
• Replacing a burned trace without asking why. A trace that burned open was overloaded — find and fix the underlying fault before you replace it, or the repair burns too (§5).
• Not seeing that width means current. Wide, heavy copper carries more current — learn to read a board's wide traces as its power paths.

Safety Notes

Risk Level: Low. Understanding current capacity is safe desk work, but the knowledge directly governs whether the repair you later make is safe — an undersized current-carrying repair is a genuine overheating and fire risk.

Professional Tips Before Starting

• Read the copper before you cut. Note how wide and heavy a trace is before you repair it — its size tells you the current it carries and the repair it needs (§5).
• Match the gauge to the trace. For a jumper on a power trace, pick a wire whose cross-section carries the current — a chart or calculator turns trace width and copper weight into an approximate ampere figure (§8).
• Treat a burned trace as evidence. Ask why it burned before you replace it — a burned-open trace almost always means an overload with a cause worth finding (§5).

How Much Current a Trace Can Carry

The Copper Carries the Current

Everything about current capacity starts from a simple fact you already know: on a board, the wiring is copper, and it is that copper — the traces — that actually carries the current (§1.1). A trace is a thin ribbon of copper, and like any conductor it can carry current, but only so much before it gets too hot. How much depends on two things about the ribbon and one limit on the board. The two things are its dimensions: how thick the copper layer is, and how wide the trace is drawn. Together those give the trace its cross-sectional area — the amount of copper the current flows through — and a bigger cross-section carries more current, just as a wider pipe carries more water. The limit is temperature: current flowing through the copper's small resistance generates heat, and a trace can only carry current up to the point where it would get hotter than is acceptable. So the whole question of "how much current can this trace carry?" reduces to "how much copper is in its cross-section, and how hot are we willing to let it get?" This is the moment in the chapter where the copper stops being just "the wiring" and starts being something with real electrical limits set by its physical size. The next two ideas are the two dimensions — thickness, expressed as copper weight, and width — and the third pulls them together into the trace's current capacity.

Copper Weight

The first dimension, the thickness of the copper, is specified in a unit that surprises almost everyone the first time they meet it: not a length, but a weight. Copper weight is the thickness of a board's copper layer expressed as the weight of copper spread evenly over one square foot of area. So "one ounce copper" means one ounce of copper per square foot, which works out to a layer about thirty-five micrometres thick (roughly 0.035 mm). "Two ounce copper" is twice as much copper per area, so about twice as thick, and "half-ounce" is thinner. The reason for the odd unit is historical — copper foil was, and still is, sold and specified by weight per area — but the practical meaning is simply thickness: more ounces means thicker copper. One ounce is the most common weight for ordinary boards, with heavier weights (two ounce and up) used where traces must carry more current, and lighter weights on dense, low-power boards. Because the copper layer is laid down as one sheet and then etched, the copper weight is usually a single value for a whole layer — every trace on that layer is the same thickness — so it is the width, not the thickness, that a designer varies from trace to trace. For a repairer, the copper weight matters because it is half of the cross-section: a trace on a heavy-copper board carries more current for a given width than the same-width trace on a light-copper board, and a fabrication note that lists the copper weight tells you how thick the copper you are working with is. Thickness is one dimension of the current-carrying cross-section; width is the other.

Trace Width

The second dimension is the one you can actually see and the one a designer controls trace by trace: how wide the trace is drawn. Trace width is simply the width of a copper trace on the board, and together with the copper weight (the thickness) it sets the trace's cross-sectional area. Because the thickness is fixed for a whole layer, width is the knob a designer turns to give a particular trace more or less current capacity: a wide trace has a larger copper cross-section than a thin one, so it carries more current and has lower resistance and less voltage drop. This is why a board is so readable once you know to look: the fat, obvious traces running from the power connector or a big regulator are wide because they carry significant current, while the hair-thin traces threading between fine-pitch pins are narrow because they carry tiny signal currents. Width literally shows you the current a trace was designed to carry. A designer widens a trace to carry more current (and to drop less voltage), and narrows it where space is tight and the current is small. For a repairer, trace width is the most directly useful clue on the board: before you touch a trace, its width — read together with the board's copper weight — tells you roughly how much current it carries, and therefore how substantial a repair or jumper it needs. Width and thickness together are the cross-section, and the cross-section, with the allowed temperature rise, is what sets the number the next idea names: the trace's current capacity.

Ampacity — a Trace's Current Capacity

Put the cross-section together with the heat limit and you get the single number that answers this section's question, and it has a name. Ampacity is the current-carrying capacity of a trace — the maximum current it can carry continuously without heating up beyond an acceptable temperature rise. It is set by two things: the trace's cross-sectional area (its trace width times its copper-weight thickness) and how much temperature rise above ambient is considered acceptable. The mechanism is resistive heating: the copper has a small but real resistance, and current flowing through resistance dissipates power as heat, so a trace warms up in proportion to the current it carries (more precisely, to the current squared times the resistance). A bigger cross-section has lower resistance and more surface to shed heat, so it can carry more current before reaching a given temperature rise — which is why ampacity climbs with both width and copper weight. Push a trace beyond its ampacity and it simply gets too hot: first it just runs warm, then hot enough to discolor the board or lift, and in the extreme — a gross overload, like a dead short — the trace acts exactly like a fuse and burns open, melting the copper. Because working all this out from physics is fiddly, designers use published data: standards such as IPC-2221 provide charts and formulas that relate trace width, copper weight, and allowed temperature rise to a safe current, and free online calculators do the same. You do not need to memorize the numbers, but you do need the concept: every trace has a current capacity set by its copper cross-section and the heat it can shed, and exceeding it overheats the trace. Ampacity is the property you must respect — and match — whenever a repair has to carry current.

Resistance, Heat, and Voltage Drop

The same resistance that limits ampacity has a second everyday consequence worth understanding on its own: it drops voltage along a trace and turns some power into heat. A trace is not a perfect conductor — its copper has resistance, and that resistance is larger for a trace that is thinner, narrower, or longer (Volume 1). Two things follow from that resistance whenever current flows. First, voltage drop: by Ohm's law, current through the trace's resistance produces a voltage difference along it, so the voltage arriving at the far end of a trace is slightly lower than at the start — negligible for a signal, but for a power trace carrying real current it can mean the supply "sags" by the time it reaches the load, causing a circuit to misbehave or a rail to read low. Second, heat: that same dropped voltage times the current is power dissipated in the trace as heat, which is exactly the warming that ampacity is about. So a thin, long power trace does two unwanted things at once: it wastes some voltage and it runs warm. This is another reason power traces are drawn wide and boards that carry serious current use heavy copper — lower resistance means less voltage sag and less heat. For a repairer, resistance explains real symptoms: a rail reading a little low under load, a trace or repair that runs warm, or a long thin jumper that drops more voltage than the original trace did. Keeping a repair's cross-section at least as large as the original keeps its resistance — and so its voltage drop and heating — no worse than the trace it replaces. Resistance, then, ties the whole picture together: it is why cross-section matters, for heat and for voltage alike.

What This Means for Repair

All of this chapter's copper theory earns its place in one practical rule that governs every trace repair: the replacement must carry the current the original did. When a current-carrying trace is broken and you repair, bridge, or jumper it, you are replacing a piece of copper of a known cross-section with something else — a wire, a blob of solder, a rebuilt strip of copper — and that something must have enough cross-section to carry the same current without overheating (§5; §8). The reasoning is now clear: the original trace's ampacity came from its cross-section, so your repair needs a comparable cross-section — a jumper wire of adequate gauge, or enough width and solder on a rebuilt trace. This scales with the trace: a heavy power trace needs a substantial repair (a thick wire), while a thin signal trace carrying almost no current can be repaired with fine wire. Under-size a power-trace repair and it becomes the weak link: it overheats, drops extra voltage, and can fail or burn right where you fixed it. And there is the other half of the rule, the diagnostic one: a trace that burned open did so because it was overloaded, which almost always means a fault — a short, or a failed part drawing too much current downstream. So a burned-open trace is a symptom: you find and fix the cause first, and only then repair the trace, because simply bridging it sends the same overload through your new copper and burns it too (§5). Never respond to a blown trace by fitting a bigger one that lets even more current into the fault — that trades a blown trace for a fire. Size to the load, and read the burn: those two habits, built on the copper theory of this chapter, are what make a trace repair carry current safely.

Common Mistakes

• Jumpering a power trace with thin wire. A thin wire cannot carry the current — match the jumper's gauge to the trace's cross-section (§8).
• Replacing a burned trace without finding the cause. A burned-open trace was overloaded — fix the underlying short or fault first (§5).
• Ignoring copper weight and width. Cross-section — width times thickness — sets the current a trace carries — read both before judging capacity.
• Making a long, thin repair on a power path. Extra length and thinness add resistance, voltage drop, and heat — keep a repair's cross-section and length reasonable.
• Fitting a bigger trace to "cure" a blown one. That just feeds more current into the fault — find the fault, do not defeat the failure.

Troubleshooting Guidance

Current-capacity questions come down to the cross-section, the heat, and the cause of a burn. If you need to know a trace's current capacity: read its trace width and the board's copper weight, and use a chart or calculator to estimate its ampacity. If a repair or jumper runs warm: it is probably undersized — its cross-section is too small for the current, so use heavier wire or more copper (§8). If a rail reads low under load: suspect resistance and voltage drop along a thin or long trace or repair (Volume 1). If a trace burned open: it was overloaded — look downstream for a short or a failed part before replacing it (§5). If you are choosing a jumper wire: pick a gauge whose cross-section carries at least the trace's current. If you cannot see which traces carry power: the wide, heavy ones are the power paths; the thin ones are signals. If a repair keeps failing at the same spot: it is under-sized or still feeding a fault — check both the gauge and the cause (§5). The throughline: read the copper's cross-section, respect the heat limit, match every repair to the current, and always ask why a trace burned.

Verification & Testing Methods

Use this as a check that you can judge current capacity, not a hot procedure:

• [ ] I can explain that a trace's current capacity depends on its copper cross-section (width times thickness) and the allowed temperature rise (§1.1).
• [ ] I can read copper weight as a copper thickness (1 oz copper is about 35 µm) and know heavier copper carries more current.
• [ ] I can explain that trace width is the per-trace dimension a designer varies, so wide traces carry power and thin traces carry signals.
• [ ] I can explain ampacity as the maximum continuous current without overheating, and that an overloaded trace warms, drops voltage, and can burn open like a fuse (IPC-2221 charts).
• [ ] I can size a trace repair or jumper to carry the original current, and I treat a burned-open trace as an overload symptom whose cause I find first (§5; §8).

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

Practice Exercises

1. Read the power paths (4 minutes, observation). On a real board, find the widest and heaviest traces and the thinnest ones, and say which carry power and which carry signals, and why.
2. Estimate a current capacity (6 minutes, reasoning). Using a trace-width chart or calculator with a board's copper weight, estimate the approximate current a given trace width can carry for a modest temperature rise.
3. Choose a jumper gauge (4 minutes, reasoning). For a broken power trace of a stated width and copper weight, choose a jumper wire gauge that will carry the current, and explain your choice (§8).
4. Read a burned trace (4 minutes, reasoning). For a trace found burned open, describe how you would investigate the cause before replacing it, and why simply bridging it would be a mistake (§5).

These core ideas — what sets current capacity, copper weight, trace width, ampacity, resistance and voltage drop, and sizing a repair to the load — are tested in the Chapter Quiz at the end of this chapter, where a score of 80% is required to continue.

Key Takeaways

• How much current a trace can carry depends on its copper cross-sectional area — its trace width times its thickness — and how much temperature rise is acceptable (§1.1).
• Copper weight measures the thickness of a board's copper as weight per area: 1 oz copper is about 35 µm, 2 oz is twice as thick, and heavier copper carries more current with lower resistance; it is usually one value for a whole layer.
• Trace width is the per-trace dimension a designer varies: wider traces have more cross-section, so they carry more current with less resistance and voltage drop — which is why power traces are wide and signal traces thin.
• Ampacity is a trace's maximum continuous current without overheating, set by cross-section and allowed temperature rise (IPC-2221 charts): push past it and the trace warms, drops voltage, and in the extreme burns open like a fuse.
• For repair, size the fix to the load: a repair or jumper must carry at least the original current (match the cross-section / use adequate wire gauge), and a trace that burned open is an overload symptom whose cause you find before replacing it (§5; §8).

Skills Learned

• You can now explain what sets how much current a trace can carry.
• You can now explain copper weight and read it as a copper thickness.
• You can now relate trace width and copper weight to current capacity and resistance.
• You can now explain ampacity and why an overloaded trace overheats or burns open.
• You can now size a repair or jumper to match a trace's original current capacity.

Glossary Additions

• copper weight — a measure of the thickness of a printed circuit board's copper layer, expressed as the weight of copper spread over one square foot of area rather than as a direct length. One ounce copper (one ounce per square foot) is a standard layer about thirty-five micrometres thick; two ounce is roughly twice as thick, and half-ounce is thinner. Heavier copper means thicker traces that carry more current and have lower resistance, so power boards use heavier weights and low-power boards lighter ones; because the copper is laid down and etched as one sheet, the copper weight is usually a single value for a whole layer, making trace width the per-trace variable.
• trace width — the width of a copper trace on a printed circuit board; together with the copper weight (the copper's thickness) it sets the trace's cross-sectional area, and therefore how much current it can carry and how much resistance it has. Because the copper thickness is fixed for a whole layer, width is the dimension a designer varies from trace to trace: a wide trace carries more current with less resistance and voltage drop, so power traces are drawn wide and low-current signal traces can be thin. Reading a trace's width, with the board's copper weight, is the quickest way to judge the current it was designed to carry.
• ampacity — the current-carrying capacity of a trace: the maximum current it can carry continuously without heating beyond an acceptable temperature rise. It is set by the trace's cross-sectional area (trace width times copper-weight thickness) and how much temperature rise above ambient is allowed, because current through the copper's resistance dissipates heat. A larger cross-section has lower resistance and sheds heat better, so it has a higher ampacity; exceeding a trace's ampacity overheats it, and a gross overload makes it burn open like a fuse. Design standards such as IPC-2221 give charts relating trace width, copper weight, temperature rise, and safe current.

Suggested Next Sections

Must read next:

• Trace Anatomy and Function — this chapter built the board from the outside in — its layers, stackup, material, and copper; the next chapter zooms into the three features you actually repair, starting with the trace: its anatomy, how it carries a signal, and how to read and follow it, before moving on to vias and pads.

Recommended:

• PCB Materials — FR4, Rogers, Flex — the board material and its heat limits, which pair with copper current capacity in deciding how much heat and current a board can take.
• What Is a PCB? — where the trace was introduced as the copper wiring whose current capacity this section quantifies.