How to Size Electrical Wire for Amps and Distance

Two checks, one answer

Every wire-sizing problem comes down to two independent requirements. First, the conductor has to handle the current without overheating — this is its ampacity, set by the NEC. Second, the conductor has to deliver the power to the far end without dropping too much voltage along the way. A wire that passes the first test can still fail the second on a long run, and vice versa. The gauge you actually install is the larger (lower AWG number) of the two results.

Beginners almost always look only at ampacity, because that is what the breaker and the wire jacket talk about. But on residential and off-grid circuits the run length is frequently the deciding factor. A 20 A circuit is fine on 12 AWG copper for a short run, yet the same 20 A over a 100-foot one-way run drops so much voltage that you need a heavier conductor. Treat both tests as mandatory.

Test 1 — ampacity: can the wire carry the load?

Ampacity is the maximum current a conductor may carry continuously without exceeding its temperature rating. For common 75°C copper the NEC 310.16 values are 20 A for 14 AWG, 25 A for 12 AWG, 35 A for 10 AWG, 50 A for 8 AWG and 65 A for 6 AWG. Small conductors also carry an overcurrent cap under NEC 240.4(D): 14 AWG is limited to a 15 A breaker, 12 AWG to 20 A and 10 AWG to 30 A regardless of the higher table value.

Ampacity is not a fixed number, either: it is derated for high ambient temperature and for bundling several current-carrying conductors together. Four to six conductors in a raceway multiply the table ampacity by 0.80, seven to nine by 0.70, and ten to twenty by 0.50. Check the derated ampacity, not the raw table value — the Ampacity calculator applies the temperature and bundling factors for you.

Test 2 — voltage drop: will the power arrive?

Voltage drop is the voltage lost in the conductor because copper and aluminum have real resistance. The industry guideline is to keep the drop at or below 3% on a branch circuit (and roughly 5% total feeder-plus-branch). The single-phase formula is:

VD = (2 × K × I × L) / CM

where K is the conductor constant (copper 12.9, aluminum 21.2, in cmil·Ω/ft), I is the current in amps, L is the one-way length in feet, and CM is the cross-section in circular mils. The factor 2 accounts for the current traveling out and back. To turn the drop limit into a required wire size, solve for CM:

CM = (2 × K × I × L) / (V × %max)

Worked example: 20 A, 100 ft, 120 V

Suppose you feed a 20 A circuit 100 feet away on a 120 V single-phase copper run, and you want to hold voltage drop to 3%. The required circular mils are:

CM = (2 × 12.9 × 20 × 100) / (120 × 0.03) = 51,600 / 3.6 = 14,333 cmil.

The smallest AWG with at least 14,333 circular mils is 8 AWG (16,510 cmil). Notice the outcome: ampacity alone would have let you use 12 AWG (good for 20–25 A), but the 100-foot run drives the size up two gauges to 8 AWG. The voltage-drop test won. Run the numbers yourself with the Wire Size calculator, and confirm the drop with the Voltage Drop calculator.

A second example: 30 A over a 50-foot one-way run at 240 V, 3% copper, needs CM = (2 × 12.9 × 30 × 50) / (240 × 0.03) = 38,700 / 7.2 = 5,375 cmil → 10 AWG. The higher voltage and shorter distance make 10 AWG sufficient here.

Why copper vs aluminum matters

Aluminum has about 1.6 times the resistance of copper for the same gauge (K of 21.2 versus 12.9), so an aluminum run either drops more voltage or needs a larger conductor. Aluminum is common and economical for service entrances and large feeders, but it requires listed terminations and anti-oxidant where specified. The calculators let you switch metal so you can compare both before buying wire.

Putting it together

The reliable workflow is: (1) determine the load in amps; (2) find the minimum gauge that meets the derated ampacity and the breaker rule; (3) compute the gauge that holds voltage drop under your target over the actual one-way distance; (4) install the larger of the two. Always round up to a real available gauge, and remember that the AHJ may require a stricter drop limit. The AWG wire size chart lists circular mils, resistance and ampacity side by side so you can sanity-check any result.

Termination temperature ratings

The ampacity column you are allowed to use depends on the lowest temperature rating anywhere in the circuit, which is almost always the terminals on the breaker or the device rather than the conductor insulation itself. Most breakers and receptacles are listed for sixty or seventy-five degree terminations even when the wire insulation is rated ninety degrees. The practical consequence is that you size the conductor from the column matching the weakest termination, typically the seventy-five degree column for larger conductors and the sixty degree column for many fifteen and twenty amp devices. The ninety degree column is generally used only as the starting point for derating, never as the final installed ampacity. Reading the highest column on the table is one of the most common ways a conductor ends up undersized on paper while looking perfectly legal at a glance.

Common wire-sizing mistakes

A handful of errors account for most undersized circuits, and recognizing them is half the battle:

• Sizing for ampacity only. The most frequent mistake is checking that the wire can carry the current and stopping there, forgetting that a long run can fail the voltage-drop test even when ampacity is satisfied.
• Measuring the wrong length. Voltage drop uses the one-way distance, but you must measure the real routed path rather than the straight-line distance between two points, because conductors follow walls and joists.
• Forgetting to derate. Bundling several conductors together or running through a hot attic reduces ampacity below the table value, so the derated number is the one that actually counts.
• Treating aluminum like copper. Aluminum needs a larger conductor for the same job, and every termination must be listed for aluminum to avoid a loose, overheating connection.
• Ignoring the small-conductor rule. Even where the ampacity table reads higher, fourteen, twelve and ten gauge are capped at fifteen, twenty and thirty amp overcurrent devices respectively.

A repeatable four-step workflow

The dependable sequence never changes, and applying it in the same order every time turns wire sizing from guesswork into a short, reliable calculation. First, establish the continuous and non-continuous load in amps, applying the continuous multiplier where the load runs three hours or more. Second, find the smallest gauge whose derated ampacity meets that load and whose overcurrent device is permitted by the small-conductor rule. Third, compute the gauge that keeps voltage drop under your chosen target over the actual one-way length, switching metals if you want to compare copper against aluminum. Fourth, install the larger of the two results and round up to a real available size. Repeat the same four steps for every circuit in the building or the off-grid system and you will never be caught out by a run that carries the current but starves the load of voltage.

These are planning estimates. The actual installed conductor, conduit, termination temperature rating and all derating must comply with the NEC edition adopted by your jurisdiction and be verified by a licensed electrician.