Wire Gauge Calculator - Ampacity, Voltage Drop and Cable Size

Agarapu Ramesh — Editor and content reviewer
Editorially reviewed Reviewed by Agarapu Ramesh, science educator (chemistry). LinkedIn
Last reviewed: May 2026 | Standard electrical engineering formulas

Wire size is about heat and voltage drop. Ampacity keeps the wire from overheating. Voltage drop keeps the equipment from starving at the far end. You need both checks, especially on long runs.

Formula at a glance

  • minimum ampacity >= load current x required margin
  • voltage drop percent = Vdrop / supply V x 100
  • larger wire lowers resistance and voltage drop

Field note: A web calculator can point you in the right direction. Final conductor selection belongs to the code book, the installation conditions and a qualified electrician.

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Wire Gauge Calculator

Calculate proper wire size (AWG)

A
V
ft
Result
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NEC Ampacity (75°C)

AWGCopperAluminum
1420A—
1225A20A
1035A30A
850A40A
665A50A
485A65A
2115A90A

Common Applications

14 AWG: 15A circuits, lighting
12 AWG: 20A circuits, outlets
10 AWG: 30A circuits, dryers
8 AWG: 40A circuits, ranges
6 AWG: 50-60A, sub-panels

Related

Voltage DropConduit FillAmps to Watts

How to use the Wire Gauge Calculator

Use this for a first sizing pass, then check the actual code table, installation method, conductor material and temperature rating. A calculator can point you in the right direction. It cannot inspect the job.

Worked example

Example: a 20 A load may be fine on one gauge for ampacity, but a long run can still need the next size up to control voltage drop.

Practical checks before you trust the number

  • Copper and aluminum are not interchangeable at the same size.
  • Ambient temperature, bundling and conduit fill can force derating.
  • Local code and terminal temperature ratings decide the final answer.

Common mistake

A web calculator can point you in the right direction. Final conductor selection belongs to the code book, the installation conditions and a qualified electrician.

Sources and references

Related calculators

Frequently Asked Questions

Use a wire ampacity table from your code (NEC, IS 732, or similar). Pick gauge based on continuous load current with derating for ambient temperature, conduit fill, and bunching. For long runs, also check voltage drop. Always verify against local codes — the cable that's safe for current may still be too small for voltage drop on a long run.

For 30 A continuous on copper at 30°C ambient: 4 mm² (about 12 AWG) for short runs, 6 mm² (10 AWG) for derating margin or longer runs. Always verify in your local code and apply 125% continuous load factor — so 30 A continuous needs cable rated for 37.5 A minimum.

Longer runs need bigger wire because of voltage drop. The ampacity stays the same, but you must size up to keep voltage drop under 3% on branch circuits and 5% on feeders. Example: 30 A on 4 mm² copper drops 3% in about 25 m. For a 50 m run, jump to 6 mm² or 10 mm².

AWG (American Wire Gauge) is the US standard; mm² (cross-sectional area in square millimeters) is the international standard. Higher AWG number = smaller wire (counter-intuitive). Approximate equivalents: 14 AWG ≈ 2.5 mm², 12 AWG ≈ 4 mm², 10 AWG ≈ 6 mm². Don't mix the two on the same job.

Voltage drop = (2 × L × I × R per meter) ÷ 1000 for single-phase. Pick wire gauge so the drop is under 3% of supply voltage on branch circuits, 5% on feeders. For a 230 V circuit, 3% = 6.9 V max drop. Bigger wire reduces R and the drop accordingly.

Yes. DC battery cables often run at 12, 24, or 48 V — low voltage means high current for the same power. A 1 kW load at 12 V draws 83 A, needing 25 mm² (4 AWG) or larger. Voltage drop is critical for batteries; aim for 1 to 2% drop max to preserve runtime.

No. Calculators give a starting figure based on standard formulas, but final sizing must comply with local code, including derating for installation conditions, conductor temperature ratings, and protection device coordination. Always cross-reference with NEC, IS 732, or your applicable code before installation.