Wire Gauge Calculator

Enter your circuit’s amperage and one-way wire run length below to check both the minimum wire gauge for ampacity and whether you need to upsize further to control voltage drop.

Breaker / circuit size (amps)

15A20A30A

One-way run length (ft)

System voltage (volts)

120V240V

Minimum gauge (ampacity)

12 AWG

Recommended gauge (with voltage drop)

8 AWG

Drop at recommended gauge

2.0%

The ampacity-minimum gauge is not enough for this run.

12 AWG meets the 20A ampacity requirement, but over 75 ft one-way it would drop 4.9%, above the 3% recommendation. Upsize to 8 AWG to bring the drop down to 2.0%.

Drop at ampacity-minimum gauge: 4.9% (12 AWG). This tool covers 15A, 20A, and 30A copper branch circuits only. Always verify against current NEC and local amendments.

Three Separate Checks, Not Just One

This is the part that trips people up: a wire that’s rated for your circuit’s amperage isn’t automatically the right size. You need to check ampacity, voltage drop, and derating separately, and use whichever result calls for the thickest wire. Most guides cover the first two and stop, which is why the third is where an otherwise careful plan comes apart.

Check 1: Ampacity (Can the Wire Handle the Current?)

The NEC caps common residential branch circuits regardless of insulation temperature rating:

Wire Gauge (Copper)	Maximum Breaker Size
14 AWG	15 amps
12 AWG	20 amps
10 AWG	30 amps

These caps (NEC 240.4(D)) exist specifically to prevent overheating at standard branch-circuit terminations, and they override the wire’s theoretical higher ampacity rating under other conditions. Undersizing wire for its breaker is one of the most common causes of house fires from electrical work. The breaker won’t trip fast enough to protect an undersized wire from overheating.

Check 2: Voltage Drop (Is the Run Too Long?)

Even a correctly-ampacity-rated wire can lose too much voltage over a long distance, causing dimming lights, sluggish motors, and inefficient equipment operation. The NEC recommends keeping voltage drop under 3% for branch circuits, 5% total including feeders.

Formula: Voltage Drop = (2 × K × I × D) ÷ CM

Where K = 12.9 (copper’s resistivity constant), I = current in amps, D = one-way distance in feet, and CM = the wire’s circular mil area.

Example: A 20-amp circuit running 75 feet one-way to a detached garage, on 120V.

Using 12 AWG (6,530 circular mils, the code-minimum gauge for a 20A circuit):

(2 × 12.9 × 20 × 75) ÷ 6,530 = 38,700 ÷ 6,530 = 5.93 volts dropped = 4.9%, which exceeds the 3% recommendation

Upsizing to 10 AWG (10,380 circular mils):

(2 × 12.9 × 20 × 75) ÷ 10,380 = 3.73 volts = 3.1%, much closer but still slightly over

Upsizing to 8 AWG (16,510 circular mils):

(2 × 12.9 × 20 × 75) ÷ 16,510 = 2.35 volts = 2.0%, which clears the 3% target

This is exactly why a long detached-garage or workshop run often needs a heavier gauge than the ampacity table alone would suggest. The code-minimum wire is electrically legal for the amperage, but not for the distance.

Check 3: Derating (The One Most Guides Skip)

The ampacity table assumes favourable conditions: a small number of conductors, in open air or a lightly loaded raceway, at a moderate ambient temperature. Depart from any of those and the wire’s usable ampacity drops. Two adjustments apply, and when both do, they multiply.

More than three current-carrying conductors in a raceway or cable. Conductors bundled together cannot shed heat into the surrounding air the way a lone cable can, so each one is derated:

Adjustment factors are given in NEC Table 310.15(C)(1) (renumbered from Table 310.15(B)(3)(a) in editions before 2020):

Current-Carrying Conductors	Percent of Rated Ampacity
4–6	80%
7–9	70%
10–20	50%
21–30	45%
31–40	40%

Neutrals in a balanced multiwire circuit and equipment grounding conductors generally do not count toward the total; ungrounded conductors always do. This is where a plan to “run all the garage circuits in one conduit to save digging” quietly falls apart. Six current-carrying conductors means every one of them is derated to 80%.

High ambient temperature. The ampacity tables assume a 30°C (86°F) ambient, and NEC 310.15(B) supplies the correction factors for anything else. An attic in summer, an unconditioned garage in the southwest, or a conduit run in direct sun can sit far above that, and the correction factor drops accordingly: for 90°C-rated wire, roughly 0.91 at 40°C and 0.82 at 50°C. Attics in particular have their own elevated design temperatures in the NEC.

When both apply, multiply them. A 90°C-rated 12 AWG at 30 amps, with six conductors in a conduit at 40°C, comes out at 30 × 0.80 × 0.91 ≈ 21.8 amps, before the 240.4(D) cap of 20 amps for 12 AWG is even applied.

The Termination Trap

This one catches people who have done everything else right.

Wire insulation is rated 60°C, 75°C, or 90°C, and the higher ratings carry more current. But NEC 110.14(C) requires you to size the conductor using the temperature column matching the lowest-rated termination in the circuit (the breaker, the lug, the receptacle), not the wire.

Most residential breakers and devices are rated 60°C or 75°C. So the 90°C column that makes a wire look adequate on paper usually cannot be used for sizing at all. Its practical role is as the starting point for the derating calculation above, after which the result still has to land within the lower termination column.

Ampacities below are from NEC Table 310.16 for copper conductors:

Copper	60°C	75°C	90°C
14 AWG	15 A	20 A	25 A
12 AWG	20 A	25 A	30 A
10 AWG	30 A	35 A	40 A
8 AWG	40 A	50 A	55 A
6 AWG	55 A	65 A	75 A
4 AWG	70 A	85 A	95 A
2 AWG	95 A	115 A	130 A

Read the 90°C column as a derating starting point, not as a permission slip. And remember that 240.4(D) caps 14, 12, and 10 AWG at 15, 20, and 30 amps regardless of what any column says.

Conduit Fill

Sizing the conduit is a separate calculation from sizing the wire, and pulling more conductors into a raceway than it is rated for damages insulation during the pull.

The limits, from NEC Chapter 9, Table 1, are straightforward: one conductor may fill 53% of the conduit’s internal area, two may fill 31%, and three or more may fill 40%. The two-conductor case being the tightest is not a typo. Two round conductors pack together in a way that leaves less usable room proportionally.

Two practical notes. Fill is calculated on each conductor’s total outside area including insulation, not the copper, so the insulation type changes the answer for the same gauge. And filling to the limit makes for a very hard pull. Leaving margin is worth it on any run with bends, and future-proofs the conduit for an added circuit later.

Copper vs. Aluminum

Aluminum wire costs less and weighs about 60% less than copper, but has lower conductivity, meaning you need a larger gauge aluminum wire to match a given copper wire’s ampacity. A common rule of thumb is to go up two wire gauge sizes when substituting aluminum for copper (for example, use 8 AWG aluminum where 10 AWG copper was specified), and aluminum connections require anti-oxidant compound and connectors specifically rated for aluminum.