Wire Size Calculator — AWG from Amps & Length (Free)
Pick AWG from current, one-way length, and a 3% voltage-drop limit
Required Parameters
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Quick Answer
15 A copper branch: 14 AWG minimum. 20 A: 12 AWG. 30 A: 10 AWG. If the one-way run is long, this calculator steps up a size so voltage drop stays under 3%.
Wire Size Calculator
Pick American Wire Gauge (AWG) from load current, circuit voltage, and one-way length. The calculator returns the smallest copper or aluminum size that meets both NEC-style ampacity and your voltage-drop limit (default 3%).
Quick start: 20 A, 120 V, 50 ft copper, 3% drop → 10 AWG. Leave length at 0 to size by ampacity only (20 A → 12 AWG).
Wire size by amps (copper, short run)
NEC 240.4(D) small-conductor minimums when length is short enough that voltage drop is not the limiter:
| Load | Minimum copper AWG |
|---|---|
| 15 A | 14 AWG |
| 20 A | 12 AWG |
| 30 A | 10 AWG |
| 40 A | 8 AWG |
| 55 A | 6 AWG |
| 70 A | 4 AWG |
These are minimums. A long 20 A run at 120 V often needs 10 AWG (or thicker) so the drop stays under 3%. Use Find AWG with a real one-way length.
How the calculator chooses AWG
- Ampacity — NEC Table 310.16-style columns for three current-carrying conductors in a raceway at 30 °C. Choose the 60 °C, 75 °C, or 90 °C column to match the lowest rating of the conductor and its terminations.
- 240.4(D) — 14 / 12 / 10 AWG copper stay capped at 15 / 20 / 30 A even if the 90 °C table is higher. Leave this on for premises wiring.
- Voltage drop — single-phase and DC: VD = 2 × I × R. Three-phase: VD = √3 × I × R. R is one-way resistance at 75 °C. If drop exceeds your limit, the next larger size is tried.
Worked example — 20 A, 50 ft, 120 V
- Ampacity alone wants 12 AWG copper (20 A cap).
- 50 ft of 12 AWG at 75 °C, 20 A, single-phase → about 3.2% drop.
- 3.2% is over a 3% target → step up to 10 AWG → about 2.0%.
Ampacity was never the limiter — length was. Confirm the same run on the voltage drop calculator.
Check AWG
Already have a spool of 12 AWG? Switch to Check AWG. Enter the gauge, current, and length. You get a pass/fail on ampacity plus the computed drop, load voltage, and resistance.
Copper vs aluminum
Aluminum needs one to two sizes larger than copper for the same current. The aluminum table starts at 12 AWG. Use listed Al terminations and anti-oxidant compound — this page only sizes the conductor.
What this is not
Values follow published NEC-style columns at 30 °C with three current-carrying conductors. Ambient, bundling, rooftop sun, continuous-load 125%, and local amendments can require a larger size. 16 AWG and 18 AWG appear for chassis / hookup work and are flagged as not general branch-circuit sizes. Confirm the finished design against the adopted electrical code.
Related tools
- Voltage Drop Calculator — known AWG → volts and percent drop
- AWG Wire Size Chart — gauge to mm, mm², and circular mils
- PCB Trace Width Calculator — IPC-2221 board traces
- Ohm's Law Calculator — I = P / V before you size the wire
- Three-Phase Power Calculator — line current for 3φ loads
Design Notes
Ampacity is NEC Table 310.16 for three current-carrying conductors in a raceway at 30 °C. 240.4(D) caps 14/12/10 AWG copper at 15/20/30 A regardless of insulation. Voltage drop uses DC resistance at 75 °C: single-phase and DC use 2 × I × R; three-phase uses √3 × I × R. Always confirm the finished design against the adopted code, ambient, bundling, and termination temperature.
Common Mistakes
- 1
Sizing only for ampacity and ignoring voltage drop on a long feeder — lights dim and motors overheat.
- 2
Using the 90 °C column when terminations are only rated 60 °C or 75 °C. The lowest rating in the circuit governs.
- 3
Thinking a higher AWG number is thicker. AWG 10 is thicker than AWG 14.
Engineering Handbox
1. 20 A needs at least 12 AWG copper (240.4(D)) 2. 50 ft of 12 AWG at 75 °C, 20 A, 120 V single-phase ≈ 3.2% drop 3. 3.2% > 3% → step up to 10 AWG ≈ 2.0% drop
Knowledge Base
What wire size do I need for 20 amps?
NEC 240.4(D) requires 12 AWG copper minimum for a 20 A breaker. That is enough for a short 120 V run. At 80 ft one-way and a 3% drop target, this calculator typically steps up to 10 AWG because voltage drop—not ampacity—is the limiter.
How does a wire size calculator choose AWG?
It takes the smallest gauge whose allowed ampacity is at least the load current, then checks voltage drop. Single-phase drop is 2 × I × R. If drop exceeds your limit (default 3%), it tries the next larger conductor.
What is the difference between this and the AWG chart?
The AWG chart converts a known gauge to diameter and mm². This page starts from current and length and recommends a gauge. Use Check AWG when you already have a size and want a pass/fail.
Should I use the 60, 75, or 90 °C column?
Use the lowest temperature rating of the conductor and its terminations. Most NM-B is 60 °C for ampacity. THHN in a breaker rated 75 °C uses the 75 °C column. The 90 °C column is for derating math, not everyday residential terminations.
What is NEC 240.4(D)?
The small-conductor rule: 14 AWG copper is limited to 15 A, 12 AWG to 20 A, and 10 AWG to 30 A even if the 90 °C table is higher. Leave it on for premises wiring. Turn it off only for chassis, fixture, or motor-circuit exceptions.
Does this replace the electrical code?
No. Values follow NEC 310.16-style columns at 30 °C with three current-carrying conductors. Ambient, bundling, rooftop, and local amendments can require a larger size. Confirm the finished design against the adopted code.