Two limits, and the cable has to clear both
Almost every wrong answer about wire sizing comes from treating it as one question. It is two, and they have nothing to do with each other.
Ampacity protects the cable. Current through resistance makes heat, heat has to leave the conductor through the insulation, and if it cannot leave fast enough the conductor sits above its rated temperature. That does not blow anything up on day one. It slowly embrittles the insulation until it cracks. Ampacity depends on the metal, the insulation rating, how the cable is installed and how hot the surroundings are — and it does not care at all how long the run is.
Voltage drop protects whatever is on the far end. The resistance of the wire eats part of your supply voltage before it arrives, and that loss grows in direct proportion to distance. Voltage drop does not care in the slightest how the cable is installed.
So a short high-current run — a 40 A range circuit five metres from the panel — is a pure ampacity problem, and the voltage-drop answer will look almost comically thin. A long light run — lighting at the back of a property 150 metres out — is a pure voltage-drop problem, and the ampacity answer will be a wire you could not physically terminate. Anything in between is genuinely governed by whichever is larger, which is why this calculator refuses to show you only one number.
An undersized cable does not fail. It ages.
This is the part that makes wire sizing feel abstract until it isn't. Pull a conductor 20% over its ampacity and nothing happens. No trip, no smell, no flicker. The insulation simply runs hotter than the temperature it was rated for, and thermal ageing is cumulative: every hour above rating spends part of the material's life.
Years later the insulation has hardened. It cracks where the cable bends inside a box, at a staple, or where it passes a joist. Then you have conductor against conductor or conductor against a metal enclosure, in the wall, with a breaker that never saw a fault current big enough to care. The failure looks like a mystery. It was a decision made at install time with a cable one size too small.
This is also why "it has worked fine for six years" is not evidence. Six years of running hot is the mechanism, not a defence against it.
The breaker protects the wire, not the appliance
An overcurrent device has exactly one job: open the circuit before the conductor behind it overheats. It is not there to protect your equipment, which has its own protection, and it is not sized to the load — it is sized to the cable.
That gives you a rule with three terms. The breaker rating has to be at least the load current, or it nuisance-trips, and at most the conductor's corrected ampacity, or it fails to protect. When no standard rating fits between the two, the answer is not to pick the closest one. It is to go up a wire size, which raises the ceiling until a rating fits.
NEC 240.4(D) is the rule people miss here, because it contradicts what the ampacity table appears to say. A 12 AWG copper conductor reads 30 A in the 90 °C column. You still cannot protect it above 20 A. Same for 14 AWG at 15 A and 10 AWG at 30 A, no matter how good the insulation is. The 90 °C column exists mostly so that derating starts from a higher number, not so you can hang a bigger breaker on a small wire. The calculator applies this cap automatically, which is why selecting 90 °C sometimes changes nothing at all for small conductors.
Voltage drop is why the motor will not start
A few volts sound like nothing. For resistive loads they nearly are: a heater at 5% low just runs about 10% weaker and nobody notices. Motors are a different animal.
Starting torque in an induction motor falls with the square of the applied voltage. Ten percent low at the terminals is roughly 19% less starting torque — and the inrush current during that start is several times the running current, which makes the drop worst at the exact moment the motor most needs voltage. The symptom is a well pump or compressor that hums, draws heavily, gets hot and eventually trips on thermal overload, while the panel shows perfectly normal voltage with the motor off. Long-run electronics behave differently again: switch-mode supplies hold their output and simply draw more current as voltage sags, which makes the drop worse rather than better.
The NEC's suggested limits are 3% on a branch circuit and 5% total including the feeder. Both live in informational notes, so neither is enforceable — they are good engineering practice that an inspector cannot cite you for. Treat 3% as the number to design to, and know that going to 4% on a lightly used lighting run is a defensible trade rather than a violation.
The derating that quietly disappears from spreadsheets
Published ampacity assumes 30 °C ambient and no more than three current-carrying conductors bundled together. Real installations routinely violate both, and each violation multiplies the capacity down.
| Condition (75 °C column) | Factor | What it means |
|---|---|---|
| 40 °C attic | 0.88 | 12% of the table gone |
| 50 °C roof space | 0.75 | a quarter gone |
| 4 to 6 conductors in one raceway | 0.80 | another fifth gone |
| 7 to 9 conductors | 0.70 | nearly a third gone |
| 50 °C and 6 conductors together | 0.60 | 40% gone before install |
That last line is the one worth staring at. A 10 AWG THHN conductor reads 40 A in the 90 °C column. Put it in a 50 °C attic (0.82) sharing a conduit with two other circuits (0.80) and it is a 26 A conductor — below the 30 A that 240.4(D) would have allowed anyway, so the derating is now the binding number and not the code cap. Nothing about the cable changed; the conditions did. The calculator prints both factors and their product as visible numbers rather than folding them silently into the result, because a derate of 0.60 is information you should be uncomfortable about.
AWG is a logarithmic scale, not a lookup table
American Wire Gauge is defined geometrically: 36 AWG is 0.005 inch in diameter, 4/0 is 0.46 inch, and there are 39 equal ratio steps between them. Every gauge number is the previous one multiplied by a fixed factor. Three useful consequences fall out of that definition:
- Going down three gauge numbers multiplies the cross-section by 2.005 — near enough to double.
- Going down six gauge numbers doubles the diameter, and quadruples the area.
- Nothing in the scale lands on a round metric number, ever.
Which is why the popular equivalence tables mislead. 12 AWG is 3.31 mm², not 3.5. 10 AWG is 5.26 mm², not 6 — a 6 mm² cable carries 14% more copper. 4/0 is 107 mm², not 120. Above 4/0 the scale is abandoned entirely and sizes are quoted in thousands of circular mils, where 250 kcmil is 127 mm². This tool reports every result in both systems, including the fractional AWG equivalent, so a size like 6 mm² is shown as roughly 9.4 AWG rather than pretended into a gauge it does not equal.
Copper or aluminium
Aluminium has about 61% of copper's conductivity: 0.0282 Ω·mm²/m against 0.0172. For the same voltage drop it needs 1.64 times the cross-sectional area, which usually works out to two full sizes up. It is genuinely cheaper and much lighter for long feeders and service entrances, and it is the normal choice there.
What it is not is a drop-in swap at the same gauge. Aluminium also creeps under the pressure of a terminal screw and forms a resistive oxide when a joint is disturbed, which is why terminations must be rated AL or CU-AL, torqued to spec, and generally treated with antioxidant compound. The historic problems with aluminium branch wiring were connection problems, not conductor problems. Small metric sizes below 16 mm² are excluded from the table here on purpose, since aluminium is not used at that scale in building wiring.
Reading the result
The headline number is the smallest size that clears both limits. The badge under it tells you which limit produced it, and that is the piece worth remembering: if it says voltage drop, the answer will change when you move the load; if it says ampacity, it will change when you add circuits to the conduit or the attic gets hot. The two cards show each limit's verdict separately, so you can see how much headroom you have on the side that did not govern.
The neighbouring-sizes table exists so the decision stops being binary. Going one size up often takes a 2.9% drop down to 1.8% for a modest amount of copper, which is worth knowing before you pull the cable rather than after. Going one size down shows you exactly which limit it fails and by how much.
Everything runs in your browser, and none of it replaces the datasheet for the cable you are actually buying, your local code, or a licensed electrician. Table values here are typical published figures for common insulations at 30 °C ambient; conditions of use in a real installation are for the person who signs off on it.
Frequently asked questions
Why does the calculator give me two different wire sizes?
Because a conductor has to survive two unrelated problems, and each one sets its own minimum. Ampacity is a heat limit: too much current for the copper cross-section and the conductor runs hot enough to cook its own insulation, which is a fire risk that builds over years. Voltage drop is a delivery limit: a long run wastes voltage as heat in the wire, so the appliance at the far end receives less than it was designed for. The two minimums are computed independently, and the size you actually buy is the larger of them. On a short heavy circuit ampacity wins; past roughly 25 to 30 metres on a normal branch circuit, voltage drop takes over and keeps growing while ampacity stays put.
Is voltage drop actually a code violation in the US?
No, and this trips people up. The NEC's 3% branch-circuit figure lives in an informational note to 210.19(A), and informational notes are explicitly not enforceable requirements. Ampacity, by contrast, is mandatory: 310.16 and the 240.4(D) small-conductor rule are enforced, and an inspector will fail an undersized conductor. So voltage drop is an engineering decision you own, while ampacity is a floor you cannot go below. That asymmetry is exactly why 3% is a default here and not a lock — a 4% drop on a workshop lighting run is a design choice, not a violation.
Does the length I enter include the return conductor?
No. Enter the one-way distance from the panel to the load and the calculator doubles it for you on single-phase and DC, because current has to travel out on one conductor and back on the other. On a balanced three-phase circuit the multiplier is the square root of 3 rather than 2, which comes from how the line voltages combine, not from a shorter path. Doubling the length yourself and also selecting single-phase is the most common way to get an answer two full sizes too heavy.
Can I put a bigger breaker on if I use bigger wire?
Only up to the wire's corrected ampacity, and 240.4(D) caps small conductors regardless of what the temperature column says. The breaker exists to protect the conductor, not the appliance. A 40 A breaker on a conductor good for 30 A means the wire heats indefinitely without ever tripping anything, and the wire becomes the fuse — usually at the least accessible point in the run. The calculator shows the largest standard rating that fits between your load current and the cable's corrected ampacity, which is the coordination rule in one line.
Why is 10 AWG not the same as 6 mm²?
Because AWG is a geometric scale, defined so that 36 AWG is 0.005 inch across and 4/0 is 0.46 inch, with 39 equal ratio steps between. Nothing about it lines up with round metric numbers. 10 AWG works out to 5.26 mm², about 12% less copper than a 6 mm² cable, so treating them as interchangeable quietly loses you a tenth of your conductor. The tool prints both scales for whatever size it picks, including the fractional AWG equivalent, so you can see the gap rather than assume it away.