Cable / Wire Size Calculator

What the calculator computes

Enter the load a cable must feed and the run it must feed it over, and the tool returns the conductor size that clears both of the checks a cable has to pass. It reads the phase and voltage, the one-way length of the run, the load as a current in amps or as a power in kW with a power factor, the conductor material, and the allowed voltage drop in percent. From these it works out the design current, sizes the conductor on the ampacity table, sizes it again on the voltage-drop limit, and reports the larger of the two as the recommended cable.

The result panel shows the design current, the size the ampacity check needs, the minimum cross-section and standard size the voltage-drop check needs, which of the two criteria governs, the real voltage drop at the recommended size in volts and percent, and the ampacity of that size. The recommended cable is a first-pass size you confirm against the local code and the install conditions, not a final specification, because a real design also weighs the install method, the ambient temperature, the number of bundled conductors, and the short-circuit withstand that this method does not model.

The two criteria that decide a cable

A conductor has to satisfy two separate limits, and they answer different questions. The first is ampacity: the cable must carry the current continuously without its insulation running hotter than its temperature rating. The second is voltage drop: the resistance of the run must not pull the voltage at the load below what the equipment tolerates. A cable can pass one and fail the other, so both are checked, and the size that satisfies both is the size that is chosen.

The rule is to size on each criterion separately and then take the larger result. Find the smallest standard size whose ampacity covers the design current. Find the smallest standard size whose cross-section holds the voltage drop inside the limit. Compare the two and keep the bigger conductor, because the bigger conductor is the only one that passes both. On a short heavy circuit the ampacity size is usually the larger. On a long feeder the voltage-drop size is usually the larger. The calculator does both checks and reports which one governs so the reason for the size is visible.

Ampacity and what derates it

Ampacity is the current a conductor can carry continuously without exceeding the temperature rating of its insulation. It is read from a table by conductor material, by the insulation temperature rating (60, 75, or 90 degrees C), and by the install method, because a cable in free air sheds heat better than the same cable buried in a full conduit. The base table value is a starting point, and several conditions cut it below the printed figure.

Three derates matter most. High ambient temperature lowers the ampacity, because the conductor starts closer to its limit before any current flows. Grouping several current-carrying conductors in one raceway lowers it, because each conductor heats the others and none can shed heat as freely. The install method sets the baseline, since a buried or bundled run runs hotter than one in open air. A cable that reads 100 A in the table can carry noticeably less once a hot attic and a crowded conduit are applied, so the field ampacity is often below the book value and the size is often one step up from the table. This tool uses a single reference ampacity and does not apply the grouping, temperature, and method tables, so treat its ampacity size as the floor and derate it for the real install.

The continuous-load 125 percent factor

A load that runs for three hours or more at a stretch is a continuous load, and the code sizes the conductor and its protection for 125 percent of that current rather than the current itself. The reason is heat. A conductor carrying a steady current for hours reaches a higher stable temperature than one carrying the same current in short bursts, so the extra 25 percent buys margin against that sustained rise and against the breaker tripping on its own long-time heat.

In practice the design current becomes the load current times 1.25 whenever the load is continuous. An 80 A continuous load is sized as 80 times 1.25, or 100 A, and the conductor and the breaker are both picked for that 100 A figure. A load that is not continuous uses the 80 A directly. The tool applies the 1.25 factor to the design current when the continuous box is ticked, and the default worked case here treats the 80 A load as not continuous, so its design current stays at 80.0 A. Check whether the load is continuous before reading the ampacity size, because the factor can push the pick up a rung.

The voltage-drop formula

Voltage drop is the voltage lost to the resistance of the run, and it grows with the length of the cable and the current in it. For a single-phase circuit the drop is Vd = 2 x rho x L x I / S, where the 2 accounts for the current flowing out and back on two conductors. For a three-phase circuit the drop is Vd = root 3 x rho x L x I / S, with the root 3 coming from the phase geometry. In both, rho is the resistivity of the conductor, about 0.0224 ohm mm2/m for hot copper, L is the one-way run length in meters, I is the current in amps, and S is the cross-section in mm2.

To size a cable rather than check one, rearrange the formula for the area. The minimum cross-section that holds the drop to a chosen limit is S = k x rho x L x I / Vd_allowed, with k = 2 for single-phase or root 3 for three-phase, and Vd_allowed the voltage times the percent limit. A larger current, a longer run, or a tighter percent all push the required area up. That minimum area is then rounded up to the next standard size, because a conductor is only made in fixed steps and the next size up is the smallest one that keeps the drop inside the limit.

The allowable voltage drop by use

The percent limit is not a single number; it depends on where in the system the run sits. A common United States practice, drawn from informational notes in the NEC, is about 3 percent on a branch circuit and about 5 percent on the whole path from the service to the load, counting the feeder and the branch together. Brazil’s NBR 5410 sets its own figures, commonly around 4 percent for the installation. The tighter the limit, the larger the conductor the voltage-drop check demands.

These limits protect the equipment at the end of the run. A motor fed at a sagging voltage draws more current to make the same torque, runs hotter, and loses starting capability. Lighting dims and electronics can misbehave below their tolerance. The 3 percent branch and 5 percent total figures are targets that keep the delivered voltage close enough to nominal for equipment to run as rated. The default case here uses a 3 percent limit on a 400 V three-phase feeder, so the allowed drop is 400 times 0.03, or 12 V, and the conductor is sized to stay under that.

Rounding up to a standard size

Conductors are made in a fixed ladder of sizes, and both the ampacity size and the voltage-drop size are rounded up to the next rung, never down. Rounding down would leave a conductor that runs past its rating or exceeds the drop limit, so the smallest standard size at or above each requirement is the one that is kept. In AWG and kcmil the ladder runs 14, 12, 10, 8, 6, 4, 3, 2, 1, 1/0, 2/0, 3/0, 4/0 AWG, then 250, 300, 350, 400, and 500 kcmil, with the cross-section rising at each step.

The voltage-drop check produces a minimum area in mm2, which is matched to the next AWG size whose area is at least as large. In the default case the minimum area is 25.9 mm2, and the next standard conductor above it is 3 AWG at 26.7 mm2, so 3 AWG is the voltage-drop size. The ampacity check produces a required current, 80 A here, and the smallest standard size whose rating covers it is 4 AWG at 85 A. The two sizes are then compared, and the larger one, 3 AWG, is the recommended cable.

Copper versus aluminum

Copper and aluminum are both used for power conductors, and the choice changes the size. Copper has the lower resistivity, so a copper conductor carries more current and drops less voltage than an aluminum one of the same cross-section. Aluminum carries roughly 61 percent of the current of copper for the same size, and it drops more voltage over the same run, so an aluminum design usually needs a larger conductor to meet both checks. Aluminum is common only at about 16 mm2, near 6 AWG, and above, where its lower cost and weight pay off on larger feeders.

Switching material moves both criteria at once. On ampacity, the aluminum size is a step or two above the copper size for the same load. On voltage drop, the higher resistivity raises the required area for the same percent limit. The calculator carries the material through both checks, so selecting aluminum raises the resistivity used in the voltage-drop formula and lowers the ampacity reference, and the recommended size grows accordingly. The default case uses copper, which is why the sizes land at 4 AWG and 3 AWG rather than larger.

Reading the governing criterion and the actual drop

The result panel names the criterion that decides the size, and it is worth reading, because it tells you where the design has slack and where it is tight. When voltage drop governs, as it does in the default case, the conductor is larger than the ampacity table alone would need, so the cable runs cool and the constraint is the length of the run. When ampacity governs, the conductor is at or near its thermal limit and the run is short enough that voltage drop is not the problem.

The actual drop at the recommended size closes the loop. Once the size is chosen, the tool puts it back into the voltage-drop formula and reports the real drop in volts and percent, which sits at or below the limit because the size was rounded up. In the default case the recommended 3 AWG conductor gives an actual drop of 11.6 V, or 2.91 percent, just inside the 3 percent limit, and the ampacity of that 3 AWG conductor is 100 A, well above the 80 A load. A shorter run or a larger cable would lower the drop further and widen that margin.

Deriving current from kW and power factor

The load is not always known as a current. When it is given as a power in kW, the current has to be worked out before the cable can be sized, and the power factor is part of that conversion. For a three-phase load the current is I = P x 1,000 / (root 3 x V x PF), and for a single-phase load it is I = P x 1,000 / (V x PF). A lower power factor means more current for the same kW, because the conductor carries the reactive current as well as the real current.

The calculator accepts either input. Enter a current directly and it is used as is. Enter a power in kW instead, with a power factor, and the tool converts it to a current with the phase-appropriate formula before running the two checks. The default case is stated as an 80 A load, so no conversion is needed, but a load quoted as, say, 48 kW three-phase at 400 V and 0.9 power factor would convert to about 77 A, close to the same size. Give the load whichever way you have it, and size the cable on the resulting current.

Five worked examples

Example 1: the design current

Start with the default case: a three-phase 400 V feeder, a load current of 80 A, a one-way run of 100 m, copper conductor, an allowed voltage drop of 3 percent, and a load that is not continuous. Because the load is not continuous, no 1.25 factor applies, so the design current equals the load current, 80.0 A. That is the figure both checks use. Had this been a continuous load running three hours or more at a stretch, the design current would instead be 80 times 1.25, or 100 A, and both the ampacity size and the breaker would be picked for that higher figure. Here it stays at 80.0 A.

Example 2: size by ampacity

Find the smallest standard conductor whose ampacity covers the 80.0 A design current. Reading the copper ampacity ladder, 6 AWG falls short of 80 A, and 4 AWG carries about 85 A, which clears it. So on ampacity alone the pick is 4 AWG (85 A). If the only concern were the current rating, the run would be wired in 4 AWG and the job would be done. The 85 A rating sits comfortably above the 80 A load, leaving a small margin. On a short run this would be the final size, because a short cable drops little voltage and the thermal limit is what binds.

Example 3: size by voltage drop

Now size the same run on the 3 percent limit. The allowed drop is the voltage times the percent, 400 times 0.03, or 12 V. The minimum cross-section is S = root 3 x rho x L x I / Vd_allowed = root 3 x 0.0224 x 100 x 80 / 12. Working the top, root 3 is about 1.732, so 1.732 x 0.0224 x 100 x 80 is about 310.4, and dividing by 12 gives 25.9 mm2. That is the minimum area the voltage-drop limit needs. Rounding up the ladder, the next standard conductor at or above 25.9 mm2 is 3 AWG at 26.7 mm2. So on voltage drop the size is 3 AWG (26.7 mm2), one step larger than the ampacity size.

Example 4: take the larger

Compare the two sizes and keep the bigger conductor. The ampacity check asked for 4 AWG. The voltage-drop check asked for 3 AWG, which is larger. Since the cable must pass both, the recommended size is the larger of the two, 3 AWG, and the governing criterion is voltage drop. The 4 AWG conductor would carry the 80 A current without overheating, but over the 100 m run it would drop more than the 12 V limit allows, so it fails the second check. The 3 AWG conductor passes both, which is why voltage drop governs and 3 AWG is the pick.

Example 5: check the actual drop

Confirm the chosen size by putting it back into the formula. The 3 AWG conductor has a cross-section of 26.7 mm2, so the real drop is Vd = root 3 x 0.0224 x 100 x 80 / 26.7. The top is about 310.4, and dividing by 26.7 gives about 11.6 V. As a percent of 400 V that is 11.6 divided by 400, or 2.91 percent, inside the 3 percent limit. The ampacity of the 3 AWG conductor is 100 A, well above the 80 A load, so the thermal check has plenty of room too. A shorter run or a larger cable would drop the 2.91 percent lower still.

Three expert tips

Always size on the larger of the two criteria

The single rule that keeps a cable honest is to size it on the larger of the ampacity result and the voltage-drop result, never on just one. A conductor that passes the ampacity table can still fail voltage drop on a long run, which is exactly what happens in the default case: 4 AWG carries the current but drops too much over 100 m, so the size steps up to 3 AWG. The reverse happens on a short heavy circuit, where the current is high but the run is too short for voltage drop to bind, so the ampacity size is the larger one. Run both checks every time and keep whichever conductor is bigger, because that is the only size that satisfies both limits.

Long feeders are usually voltage-drop governed

Voltage drop grows in direct proportion to the length of the run, so the longer the feeder, the more likely voltage drop is the criterion that sets the size. On a short branch the drop is small and ampacity usually wins; on a long feeder the drop dominates and the conductor has to grow well past its thermal need to hold the percent limit. Keeping a branch circuit near 3 percent and the whole path from the service to the load near 5 percent protects motor torque, keeps lighting at its rated output, and holds electronics inside their supply tolerance. On a long run, expect to size up for voltage drop and budget for the larger conductor from the start.

Treat the ampacity table as a starting point

The ampacity in the table is a book value under reference conditions, and the field almost always cuts it. High ambient temperature derates it, several bundled current-carrying conductors in one raceway derate it, and the install method sets a baseline that a buried or crowded run pushes lower. On top of that, a continuous load adds 25 percent to the current the conductor must be rated for. Any one of these can move the required size up a rung, and together they often do. Read the table value as a floor, apply the temperature, grouping, and method derates for the real install, add the 1.25 factor when the load is continuous, and expect the field size to land one step above what the bare table suggests.

Limits of the method

This calculator gives a first-pass cable size, not a final design. It sizes on ampacity and on voltage drop and takes the larger, which settles most of the decision, but it uses a single reference ampacity and does not apply the derating tables that a real install needs. It does not apply the install-method correction that separates a cable in free air from one in a buried conduit, and it does not apply the grouping factor that lowers the ampacity when several current-carrying conductors share a raceway. It does not apply the ambient-temperature derate that cuts the rating in a hot location.

The method also leaves out several checks a full design must carry. It does not size the conductor for short-circuit withstand, the current the cable must survive for the fraction of a second before the protection clears a fault, which can force a larger conductor than either the ampacity or the voltage-drop check. It does not account for harmonic currents from variable frequency drives and electronic loads, which add heating the base ampacity does not see. It does not apply conduit fill rules that cap how many conductors a raceway may hold, and it does not handle parallel-conductor sets for large feeders. Confirm the size against the local code, the NEC in the United States, NOM in Mexico, or NBR 5410 in Brazil, and with a licensed engineer before you buy or install.

Where this calculator fits

It suits anyone putting a first number on a cable: electricians and contractors pricing a feeder or a branch circuit, facility and plant engineers running a new machine to a distant panel, consultants scoping a job before the detailed conduit and derating work, and students learning why a cable has to pass two checks rather than one. It turns a load and a run length into a defensible conductor size in one pass, so the conversation with a supplier or an inspector starts from a real figure instead of a guess.

Because it does both checks and names the one that governs, it also makes the reasoning visible. You can see that a short run is ampacity-bound and a long run is voltage-drop bound, watch the size step up when the length grows or the percent limit tightens, and see how switching from copper to aluminum pushes the conductor larger. Use it to test a run length against a drop budget, to compare materials, or to sanity-check a size someone else proposed. Take the result to the code tables for the install-method, grouping, and temperature derates and the short-circuit check, and the first-pass size becomes a firm specification.

Common mistakes to avoid

The first mistake is sizing on ampacity alone and ignoring voltage drop, which passes a short-run habit onto a long feeder and delivers a sagging voltage the equipment cannot use. The second is the reverse, sizing on voltage drop and forgetting that a short heavy circuit can be ampacity-bound, so the conductor overheats even though the drop looks fine. The third is skipping the continuous-load factor, sizing an 80 A continuous load as 80 A instead of 100 A, so both the conductor and the breaker run hotter than the code intends.

A fourth is using the bare table ampacity without the derates, so a cable rated 100 A in the book is trusted at 100 A in a hot, crowded conduit where it carries far less. A fifth is mixing units, reading a metric cross-section against an AWG rating without converting, which lands the pick a size off. A sixth is treating the first-pass size as final and skipping the short-circuit withstand, the conduit fill, and the parallel-conductor rules a large feeder needs. Run both checks, apply the 1.25 factor when the load is continuous, derate the ampacity for the real install, keep the units straight, and confirm the short-circuit and fill rules against the code, and the size will hold up.

Frequently asked questions

Sources, disclaimer, and editorial transparency

The two-criteria method, the single-phase and three-phase voltage-drop formulas, the rearrangement for minimum area, the allowable-percent conventions, and the ampacity and derating notes described here follow recognized electrical references, including the voltage-drop formulas published in IAEI Magazine and the conductor ampacity tables set against the NEC Table 310.16 ampacity reference. Voltage drop is treated as k x rho x L x I / S, the minimum area as k x rho x L x I / Vd_allowed, and each criterion is rounded up to the next standard size with the larger of the two chosen. This calculator and guide are built and reviewed by the OpsCalculators team; see our Editorial Policy for how each tool is researched, built, and tested.

Results are accurate estimates for planning and education, not a substitute for a full electrical study or a code review. The method sizes on a single reference ampacity and on voltage drop only, and does not apply the install-method, grouping, and ambient-temperature derating tables, does not size for short-circuit withstand, and ignores harmonics, conduit fill, and parallel-conductor rules, any of which can change the conductor required, so confirm the derates, the short-circuit check, and the fill against the local code, the NEC in the United States, NOM in Mexico, or NBR 5410 in Brazil, and with a licensed engineer before you install. See our full Disclaimer. OpsCalculators.com is operated by MAFHH INTERNATIONAL LTD. Your inputs are processed in your browser and are never stored; see our Privacy Policy.