Choosing a conductor is two separate questions that happen to share an answer box. The first is thermal: can this conductor carry the current without its insulation running hotter than it is rated for? The second is electrical: after the resistance of the run has taken its cut, is there enough voltage left at the far end for the equipment to work properly? A conductor that passes the first and fails the second is legal and useless. The wire size calculator above runs both, tells you which one is binding, and shows you the margin on each.
Arb Digital publishes free engineering and science calculators that show their reasoning rather than handing back a bare number. This page exists because the two checks are usually presented separately, which is how people end up sizing a long run from an ampacity table and then wondering why the motor at the end of it hums and stalls.
What This Wire Size Calculator Does
It steps through standard conductor sizes from the smallest upward and stops at the first that satisfies both tests. The thermal test compares design current against the allowable ampacity of each size in your chosen insulation column and material. The electrical test computes the resistive drop over the round trip and compares it against your percentage limit.
The result panel reports the size, the design current after any continuous-load factor, the voltage drop in volts and as a percentage, and the voltage arriving at the load. The bars show how much headroom the chosen size has on each test.
Two adjacent tools on this site do neighbouring jobs and it is worth being precise about the boundaries. The voltage drop calculator evaluates a conductor you have already picked and tells you what the drop will be. This page runs the search in the other direction: it starts from the requirement and finds the conductor. The breaker size calculator sizes the overcurrent device that protects the circuit, which is a different code question with a different answer, and the two numbers do not have to line up in any obvious way.
How to Use It
- Enter the current the load actually draws. Read it from the nameplate, or divide the wattage by the voltage. Do not enter the breaker size — a 40 A breaker on a 24 A load sizes the wire for 24 A plus any code factor, not for 40 A.
- Measure the real route. One-way length is the distance the cable physically travels, around obstructions included. The calculator doubles it internally, so do not double it yourself.
- Pick the insulation column, not the wire. The 60, 75 and 90 degree columns are properties of the insulation type. THHN is a 90 degree insulation, but almost every terminal it lands on is rated 75, and the lower rating governs.
- Set the drop limit to suit the load. Three percent is the usual branch-circuit target; low-voltage and control runs often want tighter.
- Read which check was binding in the line under the headline size. If the drop limit is doing the work, shortening the run is usually cheaper than upsizing the cable.
How the Two Checks Are Calculated
The ampacity check is a comparison. Design current is the load current multiplied by 1.25 if the load is continuous, and by 1 if not, then compared against each candidate size until one is large enough.
The voltage-drop check is Ohm's law applied over the round trip. Conductor resistance is R = ρL/A, where ρ is resistivity, L is length and A is cross-sectional area. This tool uses the International Annealed Copper Standard resistivity of 0.017241 ohm-millimetre-squared per metre for copper and 0.028264 for aluminium, both quoted at 20 degrees Celsius, then corrects for the operating temperature you selected with the standard linear coefficient — 0.00393 per degree for copper and 0.00403 for aluminium. Voltage drop is then 2 × I × R for single-phase and DC, because the current makes a round trip, and √3 × I × R for a balanced three-phase circuit where the return paths cancel.
A worked example. Forty amps of continuous load on a 240 V single-phase circuit, 75 feet of copper, 75 degree column, 3 percent limit. Design current is 40 × 1.25 = 50 A, which needs 8 AWG in the 75 degree copper column. Eight AWG is 8.37 square millimetres, so its resistance at 20 degrees is 0.017241 ÷ 8.37 = 0.00206 ohms per metre, rising to 0.00251 at 75 degrees. Over 22.86 metres each way the drop is 2 × 40 × 0.00251 × 22.86 = 4.58 V, which is 1.91 percent. Both checks land on 8 AWG, and 235.4 V reaches the load.
Why This Page Does Not Publish an Ampacity Table
The calculator needs allowable ampacity figures to function, so it carries them internally, keyed to conductor material and insulation column. What it deliberately does not do is present those figures as a reference table you can read off and use directly. Our breaker size calculator takes the same position and for the same reason, so the two pages are consistent.
The reason is that a base ampacity figure is the beginning of the calculation, not the end. Before a conductor is selected it is corrected for ambient temperature, corrected again for the number of current-carrying conductors bundled in the same raceway, and then capped by the temperature rating of the terminals at both ends — which for most ordinary equipment is 75 degrees regardless of the insulation. A 90 degree conductor read from the 90 degree column and landed on a 75 degree lug is a common and real mistake. Motors, air-conditioning equipment and welders each override the general method entirely. A table stripped of all that reads like permission. Treat this number as the size to take to an electrician, not the size to buy.
Which Code Applies, and Where
The 125 percent continuous-load factor and the allowable ampacity values used here follow the general method of NFPA 70, the National Electrical Code. That describes practice in the United States and in jurisdictions that adopt the NEC. Adoption is not automatic or uniform: cities and states adopt specific editions, amend them, and can run several revision cycles behind the current one.
Outside NEC territory the method differs in structure, not just in numbers. IEC 60364-5-52, Low-voltage electrical installations, Part 5-52: Selection and erection of electrical equipment — Wiring systems sizes conductors by installation method reference codes, with its own correction factors, and works in square millimetres. The voltage-drop physics is identical either way, because resistance does not care which code book is on the shelf. The ampacity half is where the two diverge, and results should never be carried between them.
Voltage Drop Is Not a Safety Rule, and That Confuses People
Under the NEC, the three percent branch-circuit figure appears in an informational note rather than as an enforceable requirement. That leads to a persistent misreading: because it is not mandatory, it must not matter. It matters enormously, just not to the inspector.
Consider what a low voltage actually does. A resistive heater at 5 percent drop delivers about 90 percent of its rated power, because power varies with the square of voltage. An induction motor is worse: it draws more current to make the same torque, which deepens the drop, which heats the windings. Constant-power electronic supplies behave the same way. Long low-voltage runs are the extreme case, because a 12 V system loses the same volts to the same resistance as a 240 V system, and those volts are a twentieth of the supply rather than a four-hundredth. That is why solar and automotive wiring is routinely heavier than its current alone suggests, and the Ohm's law calculator is worth a sanity check if a result looks implausible.
Where the Load Current Should Come From
Most sizing errors are upstream of the sizing. The commonest is entering the breaker rating instead of the load, which overbuilds the conductor for nothing. The next is deriving current from a power rating without power factor: for an AC load, current is real power divided by voltage and power factor, so a 3 kW load at 0.8 power factor on 240 V draws 15.6 A, not 12.5 A. The power factor calculator and the electrical power calculator handle that conversion.
Motor circuits deserve a specific warning. The code sizes motor branch circuits from published full-load current tables rather than from the nameplate, using a separate method that accounts for starting inrush many times the running current. Feeding a motor from a general-purpose calculation is not conservative, it is the wrong method. Air-conditioning equipment prints a minimum circuit ampacity on its own nameplate, and that printed figure takes precedence over anything computed here.
AWG, Square Millimetres, and Why Sizes Jump
American Wire Gauge runs backwards: the number counts drawing operations, so a larger number means a thinner wire. Three gauge steps roughly double the cross-sectional area and six steps quadruple it, which is the rule behind advice to go up two sizes. Above 4/0 the scale gives up and switches to thousands of circular mils. Metric sizing uses area directly, in a preferred series of 1.5, 2.5, 4, 6, 10, 16 and 25 square millimetres. The two systems do not align: 10 AWG is 5.26 square millimetres, sitting between the metric 4 and 6, so any substitution has to be justified against the code in force rather than read off a conversion table.
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Browse All Free Tools Suggest a ToolCommon Mistakes to Avoid
- Entering the breaker rating as the load current — the conductor is sized from what the load draws plus any code factor, and the breaker is a separate calculation with a separate answer.
- Doubling the run length by hand — enter one-way distance. The calculator already accounts for the return path, and doubling twice produces a wildly oversized result.
- Using the 90 degree column with 75 degree terminals — the lowest-rated component in the path governs, and ordinary breakers and lugs are almost always 75.
- Skipping the ambient and bundling corrections — a hot attic or a full raceway can remove a third of a conductor's usable ampacity, and neither correction is applied here.
- Sizing a motor circuit with the general method — motors use full-load current tables and their own protection percentages because of starting inrush, and equipment nameplates override any calculation.
Related Free Tools From Arb Digital
Check a conductor you have already chosen with the voltage drop calculator, size the protective device with the breaker size calculator, and work between volts, amps and ohms with the Ohm's law calculator. For load figures, the electrical power calculator converts between watts, volts and amps, and the power factor calculator handles the reactive component. If you are reading component markings, the resistor color code calculator decodes bands. The full free online tools hub lists everything else.
Frequently Asked Questions
It depends on whether the 40 amps is the load or the breaker, on the run length, and on the material. For 40 amps of continuous copper-fed load on a short run in the 75 degree column, the design current is 50 amps and 8 AWG passes both the ampacity and the 3 percent drop check. Lengthen the run and the drop limit will eventually force a larger size.
Because the voltage-drop check is binding. On long runs the resistance of the conductor removes more voltage than your percentage limit allows, so the search continues past the thermally adequate size until the drop falls within the limit.
One-way. The calculator multiplies by two internally for single-phase and DC circuits, and applies the square root of three factor for balanced three-phase. Entering a doubled length doubles the drop again and oversizes the result.
Under the National Electrical Code it appears as an informational note rather than an enforceable rule, so an inspector will not fail an installation for exceeding it. It remains a design target because low voltage reduces heater output, raises motor current and can push electronics outside their input range.
No. It uses base allowable ampacity under favourable assumptions. Ambient temperature correction, correction for bundled current-carrying conductors, and the temperature rating of the terminals all reduce the usable figure, and all must be applied before a conductor is finally selected.
Select aluminium and the calculator switches both the ampacity values and the resistivity. Aluminium carries less current and drops more voltage for the same cross-section, so it typically lands one to two sizes larger, and it requires terminations and antioxidant treatment rated for aluminium.
They do not align neatly. Ten AWG is 5.26 square millimetres, which falls between the metric 4 and 6 preferred sizes. The calculator reports both figures for the size it selects, but substituting one standard for another has to be justified against the code in force rather than by conversion alone.
The continuous-load factor and the ampacity values follow the general method of NFPA 70, the National Electrical Code, so the result describes NEC jurisdictions. IEC 60364-5-52 sizes conductors by installation method with different correction factors, and results should not be carried between the two systems.
This calculator is an educational tool that performs a general-method sizing exercise. It is not electrical design advice and it does not apply ambient, bundling or termination corrections. A licensed electrician must select, install and verify every conductor in accordance with the code edition and amendments adopted by the authority having jurisdiction over your installation.
