Conductor Size Calculator
A conductor has to pass two different tests: carry the current without overheating, and deliver it without losing too much voltage along the run. This conductor size calculator runs both. Enter the load current, voltage, one-way circuit length and your voltage drop limit, pick copper or aluminum, and it returns the smallest AWG size that passes the voltage drop formula and the ampacity test, with the actual drop of the chosen wire shown.
Ampacity here follows NEC Table 310.16, 60 degree C column, for the metal you select, and the ampacity test sizes for 125 percent of the load. What the tool does not do is just as clearly stated further down: no temperature correction, no bundling adjustment, and final designs still belong against the full code.
Preliminary sizing only. Ampacity uses the NEC Table 310.16 60 degree C column for the selected metal with no temperature correction or bundling adjustment, and the drop formula is the single-phase two-wire form. Verify the final size against the current NEC and your local authority before installing.
Quick Answer
Key takeaways
- Two tests decide the size: voltage drop sets a minimum area in circular mils, and ampacity, tested at 125 percent of the load, sets a minimum rating. The winner is the first size that passes both.
- Length is one-way. The factor of 2 in the formula is the round trip, so doubling your measured length double-counts the drop.
- Copper and aluminum have different resistivity constants (K = 12.9 vs 21.2) and different NEC ampacity columns. A size that passes in copper routinely fails in aluminum.
- This is a preliminary two-test check on the conservative 60 degree ampacity column. Temperature correction, bundling and terminal ratings can all push a real design one size larger.
- Quick Answer
- How to use the conductor size calculator
- The two tests, and the formulas behind them
- Worked example: 30 amps at 240 volts over 100 feet
- Copper or aluminum: the choice changes both tests
- What this calculator does not model, stated up front
- What competing calculators miss
- Frequently Asked Questions
- Add This Calculator to Your Website
- References
How to use the conductor size calculator
Start with the load in amps. Nameplate current is best; if all you have is watts, divide watts by volts to get amps (the amp watt calculator runs that conversion both ways). Enter the circuit voltage, then the one-way length of the run in feet, measured along the path the cable actually takes, not the straight-line distance.
Set the voltage drop limit next: 3 percent is the standard design target for a branch circuit, with 5 percent the usual ceiling for a feeder plus branch combined. Pick the metal, and read the result as a verdict on both tests. The actual voltage drop line is the one to check: it is the drop of the wire you would really install, recomputed from its true area, and it should sit under your limit. If the recommendation line says to consider a larger conductor, even the largest size in the table could not satisfy both tests at your inputs, which usually means the run is too long or the drop limit too tight for a single cable.
The two tests, and the formulas behind them
Voltage drop first. A conductor is a resistor, and the longer and thinner it is, the more voltage the load loses to it. Solving the drop equation for area gives the minimum size the run demands.
The 125 percent factor is the continuous-load rule: conductors feeding loads that run for three hours or more are sized at 125 percent of that load, and this tool applies the factor to every calculation so the answer is safe for continuous duty. The K constants bundle the metal’s resistivity at operating temperature into one number, which is why aluminum, with K = 21.2 against copper’s 12.9, needs roughly 1.6 times the area for the same drop. NFPA 70, the National Electrical Code, is the source document for the ampacity side: its Table 310.16 is summarized in the PNNL Building America listing for NFPA 70 and the table’s structure is walked through in ExpertCE’s guide to using NEC Table 310.16.
Worked example: 30 amps at 240 volts over 100 feet
The calculator is pre-filled with this case. The allowed drop is 240 x 3 percent = 7.2 volts, so the drop test demands 2 x 12.9 x 30 x 100 / 7.2 = 10,750 circular mils, and the ampacity test demands a rating of at least 30 x 1.25 = 37.5 amps.
| Candidate (copper) | Area test vs 10,750 cmil | Ampacity test vs 37.5 A | Verdict |
|---|---|---|---|
| 10 AWG (10,380 cmil, 30 A) | 10,380 is short of 10,750 | 30 A is short of 37.5 A | Fails both |
| 8 AWG (16,510 cmil, 40 A) | 16,510 clears it | 40 A clears it | Selected |
| Actual drop of 8 AWG | 77,400 / 16,510 = 4.69 V | 1.95% | |
Either test can be the binding one. Raise the load to 60 amps on the same run and the drop test asks for 21,500 circular mils, which 6 AWG (26,240 cmil) satisfies, but 6 AWG’s 55 amp rating fails the 75 amp ampacity requirement, and so does 4 AWG at 70 amps. The tool lands on 2 AWG (95 A, actual drop 0.97 percent): a case where the wire is far bigger than voltage drop alone demands, purely because of heat. Short runs at high current are usually decided by ampacity; long runs at modest current are usually decided by drop.
Copper or aluminum: the choice changes both tests
Run the worked example again in aluminum and every number moves. K rises to 21.2, so the drop test demands 17,667 circular mils; the ampacity column drops, so 8 AWG aluminum (30 A) fails the 37.5 amp test outright. The tool selects 6 AWG aluminum, rated 40 A, with an actual drop of 2.02 percent.
Copper
- Lower resistivity: smaller size for the same drop and current
- Higher ampacity at every AWG size
- Forgiving terminations, the default for branch circuits
- Costs more per foot, and the gap grows with size
Aluminum
- Needs roughly two AWG steps larger for the same job
- Much lighter and cheaper on feeders and service runs
- Requires aluminum-rated lugs, antioxidant, and correct torque
- Its own NEC ampacity column: never borrow copper’s numbers
That last point is a correction in this version of the calculator, and it is worth stating plainly. The ampacity figures here are now read from the NEC Table 310.16 column for the metal you selected. Aluminum’s 60 degree ratings are far below copper’s at the same gauge (8 AWG: 30 A against 40 A; 1/0: 100 A against 125 A), and any tool that applies one shared table to both metals will undersize aluminum runs. The aluminum figures used here cross-check against published NEC table reproductions such as the Wiringo aluminum ampacity chart.
What this calculator does not model, stated up front
An honest sizing tool names its omissions. Conduit fill, cable bundling and direct burial change real ampacity through the NEC’s adjustment and correction factors, and none of that derating is applied here. Ambient temperature correction is not applied either, so a run through a hot attic needs a manual check against Table 310.15. The ampacity column is the 60 degree one, the conservative column: many real terminations are rated 75 degrees, which can allow a smaller size, but only when every terminal in the path carries that rating. For a metric cross-check on the drop test alone, the electrical cable size calculator returns the minimum cable cross-section in square millimetres for a stated current, length and drop limit.
The formula is also the single-phase (and DC) two-wire form. Three-phase circuits drop voltage by a different factor and start from a line current you may need to compute first; the 3-phase current calculator covers that current calculation. Treat this page as the fast, transparent two-test check it is, and the NEC, published by the NFPA and catalogued by standards libraries such as Berkeley Lab’s electrical safety standards index, as the deciding document.
What competing calculators miss
The wire sizing tools ranking for this term were reviewed before this rewrite, and they split into two camps: full NEC tools and quick calculators.
Miss 1: one ampacity table for both metals. Quick calculators commonly carry a single AWG table and swap only the resistivity constant when you pick aluminum. That is the exact error corrected here: aluminum ampacity is its own NEC column, and borrowing copper’s ratings oversells an 8 AWG aluminum conductor by 10 amps. This tool’s tables are metal-specific, and the selected metal’s column is named in the result.
Miss 2: an answer with no auditable middle. Most tools print a gauge and stop. Here you can see the required circular mils, the 125 percent ampacity requirement, and the chosen wire’s actual drop recomputed from its real area, so the result can be checked against a table by hand in a minute.
Miss 3: defaults that flatter the answer. Sizing on the 75 or 90 degree column without naming it produces smaller, cheaper-looking answers. This tool uses the conservative 60 degree column and says so in the result row, which is the safer default when terminal ratings are unknown.
The full NEC tools genuinely do more: ambient correction, bundling adjustment, terminal temperature selection. When your installation has those complications, use one, or work the tables directly. For a straight run with known load and length, the two tests here are the decision, and they are shown rather than hidden.
Frequently Asked Questions
8 AWG copper. The drop test requires 10,750 circular mils, which rules out 10 AWG, and the ampacity test requires at least 37.5 A (125 percent of 30), which 8 AWG’s 40 A rating clears. Its actual drop over the 100 feet is 1.95 percent. In aluminum the same run needs 6 AWG.
Because of the continuous-load rule: a load expected to run at maximum current for three hours or more must be fed by conductors rated at 125 percent of it, since sustained current heats conductors to their limit. This calculator applies the factor to every job, so its answers are safe for continuous duty; for a genuinely intermittent load the result may be one size conservative.
One-way: the distance from the source to the load. Current travels out and back, and the factor of 2 in the formula (2 x K x I x L) already accounts for both conductors. Entering the round-trip distance doubles the calculated drop and oversizes the wire.
K is the resistivity constant of the metal in circular-mil ohms per foot at operating temperature: 12.9 for copper, 21.2 for aluminum. Aluminum resists current flow about 1.6 times as much per unit area, so for the same drop it needs about 1.6 times the area, which is why aluminum answers land roughly two AWG sizes larger.
Because the wire is only one part of the path. Breakers, lugs and equipment terminals have their own temperature ratings, and the lowest-rated part governs. The 60 degree column is the conservative choice that stays safe when terminal ratings are unknown or mixed. A design that verifies 75 degree terminations throughout may use the higher column and sometimes land one size smaller.
Three percent for a branch circuit is the standard design target, and five percent total across feeder plus branch is the usual ceiling in NEC informational guidance. Sensitive electronics and long motor runs argue for the tighter end; motors also draw starting current several times their running current, which sags voltage further and is outside this tool’s steady-state formula.
Use it as a well-founded preliminary size. It passes the two fundamental tests with a conservative ampacity column, but it does not apply temperature correction, bundling adjustment, terminal temperature limits, or load-specific rules for motors, welders and EV charging. Verify against the current NEC and local amendments, or have a licensed electrician sign off, before installing.
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References
- Pacific Northwest National Laboratory, Building America Solution Center, “NFPA 70: National Electric Code” – catalog entry for the code whose Table 310.16 supplies the ampacity values used here. PNNL: NFPA 70 National Electric Code. Accessed October 2026.
- ExpertCE, “How to Use NEC Table 310.16 for Conductor Ampacity” – a walkthrough of the table’s temperature columns and terminal-rating limits. ExpertCE: Using NEC Table 310.16. Accessed October 2026.
- Wiringo, “Aluminum Wire Ampacity Chart” – published reproduction of the NEC aluminum ampacities used to cross-check the aluminum column in this calculator. Wiringo: Aluminum Wire Ampacity Chart. Accessed October 2026.
- Lawrence Berkeley National Laboratory, Electrical Safety Program, “NFPA Standards” – institutional index of NFPA 70 and related electrical standards. Berkeley Lab: NFPA Standards Index. Accessed October 2026.
- IEEE Technology Navigator, “National Electric Code” – overview of NFPA 70’s scope, adoption and revision cycle. IEEE TechNav: National Electric Code. Accessed October 2026.