Wire Size Calculator
Finds the smallest copper or aluminum wire gauge that safely carries your load (NEC ampacity) and keeps voltage drop under your target.
| Ampacity at this gauge | |
|---|---|
| Voltage drop | |
| Voltage at load |
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NEC Wire Size & Ampacity Reference (75°C column)
Copper and aluminum ampacity by AWG size, per NEC Table 310.16, for up to three current-carrying conductors. Aluminum branch-circuit wire is not rated below 12 AWG. This is the base table the calculator checks every candidate gauge against — worth a quick look before running the numbers.
| AWG | Circular mils | Copper ampacity | Aluminum ampacity |
|---|---|---|---|
| 14 | 4,110 | 20A | — |
| 12 | 6,530 | 25A | 20A |
| 10 | 10,380 | 35A | 30A |
| 8 | 16,510 | 50A | 40A |
| 6 | 26,240 | 65A | 50A |
| 4 | 41,740 | 85A | 65A |
| 2 | 66,360 | 115A | 90A |
| 1/0 | 105,600 | 150A | 120A |
| 2/0 | 133,100 | 175A | 135A |
| 4/0 | 211,600 | 230A | 180A |
| 250 kcmil | 250,000 | 255A | 205A |
| 500 kcmil | 500,000 | 380A | 310A |
How to use this calculator
Enter the load current the circuit needs to carry (in amps), the source voltage, and the one-way distance from the panel to the load. Pick copper or aluminum, and set the maximum voltage drop you're willing to accept — 3% is the NEC-recommended ceiling for a branch circuit. The calculator checks every standard wire gauge from 14 AWG up to 500 kcmil and returns the smallest one that clears both the ampacity requirement (can it safely carry the current) and your voltage-drop target (will the far end still get enough voltage) at once.
What is wire sizing and why it matters
A wire that's too small for its load runs hotter than it should, which is the actual safety mechanism NEC ampacity tables exist to prevent — insulation breaks down under sustained heat long before the metal itself is at risk. That's the ampacity side of sizing. The other side is voltage drop: even a wire rated to carry the current safely will lose some voltage to resistance over a long run, and past a certain point the load at the far end is effectively underpowered — a motor runs hot and inefficient, an LED driver flickers, a subpanel sags under load. NEC ampacity tables cover the safety minimum; voltage-drop guidance covers the performance minimum. A correctly sized wire has to satisfy both, which is why this calculator checks them together instead of just picking the smallest code-legal gauge.
Worked example
Say you're running a 30A subpanel feed, 240V, 100 feet from the main panel, in copper, and you want to stay under the standard 3% voltage-drop target. Here's the sequence the calculator runs:
| 10 AWG (35A rated) | Ampacity passes (35A ≥ 30A), but voltage drop is too high at this distance — fails the 3% target |
|---|---|
| 8 AWG (50A rated) | Ampacity passes, voltage drop comes in at 1.95% (4.69V) — clears the 3% target |
Result: 8 AWG copper. Notice the deciding factor here wasn't ampacity — a 10 AWG wire is rated for more current than this circuit needs — it was voltage drop over the 100-foot run. That's the case this calculator is built to catch: a wire that's "big enough" by the ampacity table alone but would still leave the load underpowered at the end of a long run.
Frequently asked questions
What size wire do I need for a 20A circuit?
12 AWG copper is the standard answer for a 20A branch circuit at typical residential runs (NEC ampacity: 25A, comfortably above the 20A load). But that's the ampacity answer only — if the run is unusually long, voltage drop can push you to a heavier gauge even though 12 AWG is technically code-legal for the current. Run your actual distance through the calculator above to check both.
What's the difference between 10 AWG and 12 AWG wire?
10 AWG has roughly 60% more copper cross-section than 12 AWG (10,380 circular mils vs 6,530), which gives it a higher ampacity rating (35A vs 25A) and noticeably less voltage drop over the same distance. 10 AWG is thicker, stiffer, and more expensive per foot — the right call is whichever size actually clears both the ampacity and voltage-drop requirements for your specific circuit, not automatically the bigger one.
Do I need thicker wire for a longer run?
Often yes, even when a smaller gauge is still rated to carry the current safely. Voltage drop scales with distance — the worked example above shows a 10 AWG wire that's ampacity-rated for the load still failing the voltage-drop target simply because the run is 100 feet. Longer runs are exactly when the two rules (ampacity vs. voltage drop) most often disagree, which is why checking both matters more as distance grows.
Does wire size differ between copper and aluminum?
Yes — aluminum has higher resistance than copper, so an aluminum conductor needs to be roughly one size larger than copper to carry the same current at a comparable voltage drop, and aluminum isn't rated at all below 12 AWG for branch circuits. Use the material toggle above to compare directly for your circuit.
Based on NEC Chapter 9 Table 8 (circular mils) and Table 310.16 (75°C ampacity, ≤3 current-carrying conductors). Reference only — verify against the current NEC and local code, and consult a licensed electrician before installing.