Electrical Wire Size Chart: Ampacity & Voltage Drop 2026

Voltage drop, not ampacity, sizes the wire

Two limits decide a wire’s size, and they are not the same. Ampacity is how much current the wire can carry without overheating — 12 AWG carries 20 A. Voltage drop is how much voltage the wire itself eats on the way there and back — and on a long run it is the binding limit. On the defaults — load 20 A, distance 50 ft, voltage 120 V, max drop 3% — the ampacity gauge is 12 AWG, the drop gauge is 10 AWG, the recommended gauge is 10 AWG, the voltage drop at the recommended gauge is 2.4855 V / 2.07% and the voltage drop at the ampacity gauge is 3.9510 V / 3.29% — over 3%, so 12 AWG fails on drop.

Electrical wire size chart — NEC Table 310.16, 60°C copper

At the defaults — load 20 A, distance 50 ft, voltage 120 V, max drop 3% — the ampacity gauge is 12 AWG, the drop gauge is 10 AWG, the recommended gauge is 10 AWG, the voltage drop at the recommended gauge is 2.4855 V / 2.07% and the voltage drop at the ampacity gauge is 3.9510 V / 3.29%. Check the identities: VD = 2 × 12.9 × I × L / CM; VD% = VD / V × 100; Lmax = 3.6 × CM / 516 at 20 A / 120 V / 3%; recommended = larger wire of amp gauge and drop gauge. Rounding convention: totals are computed from unrounded values, so rounded parts may differ by a cent from their rounded sum.

The worked example, line by line

Setup: load 20 A, distance 50 ft, voltage 120 V, max drop 3%.

Line 1 — the ampacity gauge. 12 AWG carries 20 A, so the ampacity gauge is 12 AWG.

Line 2 — the voltage drop on 12 AWG. VD = 2 × 12.9 × 20 × 50 / 6,530 = 3.9510 V. As a percentage: 3.9510 / 120 × 100 = 3.29% — over the 3% limit, so 12 AWG fails on drop.

Line 3 — the drop gauge. The next size up, 10 AWG, has 10,380 circular mils: VD = 2 × 12.9 × 20 × 50 / 10,380 = 2.4855 V, which is 2.07% — inside the limit, so the drop gauge is 10 AWG.

Line 4 — the recommendation. The recommended gauge is the larger wire of the two — 10 AWG. The 3% limit is the NEC informational recommendation, not a code requirement.

Reverse: the distance for exactly 3.00% drop at 12 AWG / 20 A / 120 V is 45.56 ft (exact 45.5581 ft); at 45.55 ft the drop is 2.9995%, at 45.56 ft it is 3.0001%, at 45.57 ft it is 3.0008%. The current for exactly 3.00% drop at 10 AWG / 50 ft / 120 V is 28.97 A (exact 28.9674 A); at 28.96 A the drop is 2.9992%, at 28.97 A it is 3.0003%. The distance for exactly 3.00% drop at 14 AWG / 15 A / 120 V is 38.23 ft (exact 38.2326 ft); at 38.23 ft the drop is 2.9998%, at 38.24 ft it is 3.0006%. The current for exactly 3.00% drop at 8 AWG / 100 ft / 120 V is 23.04 A (exact 23.0372 A); at 23.03 A the drop is 2.9991%, at 23.04 A it is 3.0004%. The distance for exactly 3.00% drop at 12 AWG / 20 A / 240 V is 91.12 ft (exact 91.1163 ft); at 91.11 ft the drop is 2.9998%, at 91.12 ft it is 3.0001%. The distance for exactly 3.00% drop at 10 AWG / 30 A / 120 V is 48.28 ft (exact 48.2791 ft); at 48.27 ft the drop is 2.9994%, at 48.28 ft it is 3.0001%.

Every branch, checked

Load 15 A: Amp 14 AWG, drop 12 AWG, recommended 12 AWG, VD 2.47%. 14 AWG carries 15 A, but its drop is 3.92% — over the limit — so the drop gauge is 12 AWG at 2.47%.

Load 30 A: Amp 10 AWG, drop 8 AWG, recommended 8 AWG, VD 1.95%. 10 AWG carries 30 A, but its drop is 3.11% — over the limit — so the drop gauge is 8 AWG at 1.95%.

Distance 100 ft: Amp 12 AWG, drop 8 AWG, recommended 8 AWG, VD 2.60%. Doubling the distance doubles the drop: 12 AWG is 6.58%, 10 AWG is 4.14%, and only 8 AWG at 2.60% passes.

Voltage 240 V: Amp 12 AWG, drop 14 AWG, recommended 12 AWG, VD 1.65%. At 240 V the drop halves, so the drop gauge is 14 AWG — SMALLER than the ampacity gauge — and the recommendation stays 12 AWG, the larger wire.

Load 100 A: Amp 1 AWG, drop 4 AWG, recommended 1 AWG, VD 1.28%. The drop passes on 4 AWG, but ampacity requires 1 AWG — the drop gauge is smaller than the ampacity gauge again, and the recommendation is the larger wire, 1 AWG.

Max drop 5%: Amp 12 AWG, drop 12 AWG, recommended 12 AWG, VD 3.29%. At a 5% limit 12 AWG passes with a 3.29% drop, so the ampacity and drop gauges agree at 12 AWG.

What the defaults assume

The model assumes single-phase copper conductors, K = 12.9, the NEC Table 310.16 60°C column, and a one-way distance — the wire runs there and back, so the voltage drop counts both ways. It assumes the load is continuous at the current you enter and the temperature is within the 60°C column’s range — it does not model derating for bundled cables, high ambient temperature, or aluminum wire, which needs larger gauges. With a 0 A load the ampacity gauge is 14 AWG, the recommended gauge is 14 AWG and the voltage drop is 0.00% — no load, no drop. With a 0 ft distance the recommended gauge is 12 AWG and the voltage drop is 0.00%. With a 200 A load the ampacity gauge is none — 4/0 AWG maxes at 195 A, so no single copper gauge in the table carries 200 A (parallel runs or larger service wire needed); the voltage drop at 4/0 is only 1.02% — drop is fine, ampacity is the binding limit. Rounding convention: totals are computed from unrounded values, so rounded parts may differ by a cent from their rounded sum; the reverses are nearest-value solutions, and the exact unrounded figures (45.5581, 28.9674, 38.2326, 23.0372, 91.1163, 48.2791) are stated wherever the rounded figures appear.

Output · formula · meaning

Who uses this

• The homeowner running a 20-amp circuit 50 feet: they learn the wire is 10 AWG, not 12 AWG — and they check the 100-foot branch (8 AWG) before they run the cable twice as far.
• The electrician sizing a 240 V subpanel feed: they learn the drop gauge is 14 AWG at 240 V — and they check the 100-amp branch (1 AWG) to see ampacity take over at big loads.
• The DIYer adding a 15-amp outlet: they learn 14 AWG fails on drop at 3.92% — and they check the 30-amp branch (8 AWG) to see the same pattern at higher current.
• The planner choosing a drop limit: they learn 12 AWG passes at a 5% limit — and they check the 3% table (45.56 ft for 12 AWG) to see how far the stricter limit reaches.

Accuracy & limitations

The math is first-principles electrical: VD = 2 × 12.9 × I × L / CM; VD% = VD / V × 100; Lmax = 3.6 × CM / 516 at 20 A / 120 V / 3%; recommended = larger wire of amp gauge and drop gauge. The ampacities are NEC Table 310.16, 60°C column, copper — the conservative column used for NM-cable terminations. The 4-decimal exact is shown where the rounding matters: the distance for exactly 3.00% drop at 12 AWG / 20 A / 120 V is 45.5581 ft (shown as 45.56 ft); at 45.55 ft the drop is 2.9995%, at 45.56 ft it is 3.0001%, at 45.57 ft it is 3.0008%. The current for exactly 3.00% drop at 10 AWG / 50 ft / 120 V is 28.9674 A (shown as 28.97 A); at 28.96 A the drop is 2.9992%, at 28.97 A it is 3.0003%. What the model cannot do is replace a licensed electrician: it does not model derating for bundled cables, high ambient temperature, aluminum wire, or local code amendments — and the 3% limit is an informational recommendation, not a code requirement. For the wire gauge chart itself, see the wire gauge chart; for another construction reference table, see the dryer vent length chart and the concrete mix chart. This is a calculator, not an electrical opinion.